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Review

(Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties

1
Max Planck Institute for Solid State Research, D-70569 Stuttgart, Germany
2
Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
*
Author to whom correspondence should be addressed.
Crystals 2017, 7(6), 167; https://doi.org/10.3390/cryst7060167
Submission received: 28 March 2017 / Revised: 22 May 2017 / Accepted: 3 June 2017 / Published: 6 June 2017
(This article belongs to the Special Issue Correlated Electron Crystals)

Abstract

:
This review focuses on the growth of high-quality (Li1−xFex)OHFeSe single crystals by a hydrothermal method using floating-zone-grown AxFe2−ySe2 (A = K, Rb, and Cs) as precursors. The structure, superconductivity, and magnetic behavior of the obtained crystals are highly influenced by the growth conditions, such as time, temperature, and composition. A phase diagram with temperature against the c-lattice constant is summarized including the antiferromagnetic spin density wave, superconducting, and paramagnetic phases.

1. Introduction

Iron-based superconductors (FeSCs) have attracted much attention because of their diverse structures, complex phases, and unconventional superconductivity [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. FeSCs have the highest superconducting (SC) transition temperatures (Tc) after cuprate SCs, above the generally accepted McMillan limit of 39 K predicted by Bardeen–Cooper–Schrieffer (BCS) theory, opening up new opportunities for exploring novel high-Tc SCs [17].
The crystal structure of FeSCs characteristically consists of some kind of stacking layers which conduct the SC current [18]. In iron-arsenide and iron-selenide, compounds that have been most widely studied, the alternately stacked Fe2X2 (X = As and Se) layers are formed by edge-sharing disordered FeX4 tetrahedra, which leads to them sharing many similar structures and properties [19]. Up to now, several types of iron-arsenide SCs have been discovered, including Fe2As-type AFeAs (111-system, A is Li or Na) [20,21,22], ThCr2Si2-type AeFe2As2 (122-system, Ae is an alkaline or alkaline-earth metal element) [23,24], ZrCuSiAs-type ReFeAsO, Co doped AeFFeAs (1111-system, Re is a rare-earth element) [25,26,27], and the perovskite-based materials such as (Sr4M2O6)(Fe2Pn2) (42622-system; M = Sc, Ti, and V; Pn = P and As) [28,29], and (Ca3Al2O5-y)(Fe2Pn2) (32522-system) [30]. At ambient pressure, Tc reaches as high as 55 K and 56 K in SmO1−xFxFeAs and Sr1−xSmxFFeAs, respectively [31,32]. Since their discovery, iron-selenide SCs have been found to provide a strong platform for the investigation of their high-Tc superconductivity, since they possess a simple structure, are nontoxic, and may be fabricated via numerous synthesis methods [2,18,33,34]. The simplest tetragonal β-Fe1+δSe (11-system) has Tc ≈ 8.5 K at ambient pressure and is sensitive to chemical and physical pressure effects [35]. It was observed that Tc can be increased up to 15 K in the β-Fe1+δSe system by partially replacing Se with Te and up to 36.7 K under the application of a hydrostatic pressure of 8.9 GPa [36,37]. The positive pressure coefficient dTc/dP reaches as high as 9.1 K/GPa for this system, although smaller than |−(60 ± 3)| K/GPa for Ca(Fe1−xCox)2As2 [38,39]. In contrast to the negatively charged FeAs layers in iron arsenide systems, the anti-PbO-type FeSe layers are charge neutral, and the excess Fe is important to the phase diagram and structural stability of the β-Fe1+δSe system [40]. The two adjacent FeSe layers are weakly coupled and are susceptible to intercalation, which can improve the superconductivity of the system by modulating the crystal lattice constants or even by changing the crystal structure [40,41]. A series of new SCs with Tc about 30 K have been fabricated by the intercalation of metal ions and small molecules, including AxFe2−ySe2 [A = K, Rb, Cs, (Tl,K), and (Tl,Rb)] [42], Mx(NH3)yFe2Se2 (M: metal elements) [43,44], Lix(NH2)y(NH3)1−yFe2Se2 [45], and Lix(C5H5N)yFe2−zSe2 [46]. Besides the coexistence of superconductivity and ferromagnetism, a variety of phase separation phenomena have been observed in these SCs [18]. It was reported that two SC phases with Tc of 44 K and 30 K coexist in K0.4Fe2Se2(NH3)0.5 [47]. In polycrystalline Fe(Se1−xTex)0.82 (0.15 ≤ x ≤ 0.3), there are also two different SC phases, arising from two tetragonal structures with the same space group but different lattice parameters [48]. In the AxFe2−ySe2 system, intrinsic phase separation in coexisting crystallographic phases is a common feature [18]. The majority phase A0.8Fe1.6Se2 (the so-called ‘245’ phase) is antiferromagnetic (AFM) with large magnetic moments of 3.31 μB per Fe and a Néel transition temperature (TN) up to 560 K. It shows an insulating/semiconducting behavior accompanied by Fe-vacancy orders with a 5 × 5 superstructure. Embedded in the 245 phase is the minority metallic-SC phase which is Fe vacancy free with the formula A1−xFe2Se2 (0 ≤ x ≤ 0.7) [49]. Direct observation shows that the phase separation in a KxFe2−ySe2 single crystal at temperatures below 520 K is characterized by the coexistence of the majority tetragonal magnetic phase, minority orthorhombic metallic phase, and an interfacial tetragonal phase which appears below ~300 K [50]. However, the coexistence of these complex microstructures makes it difficult to grow bulk SC AxFe2Se2 single crystals and hinders the determination of their intrinsic electrical and magnetic properties [51].
Recently, (Li0.8Fe0.2)OHFeSe (11111-system) polycrystalline SCs with Tc ~40 K were synthesized using a hydrothermal method [52]. This new FeSe-derived SC material has an alternate stacking of anti-PbO-type FeSe layers and (Li0.8Fe0.2)OH layers, with a weak interlayer hydrogen bonding interaction. Compared with β-Fe1+δSe, the FeSe4 tetrahedron in the ab plane in (Li0.8Fe0.2)OHFeSe is highly compressed, which is believed to play a key role in enhancing superconductivity. In contrast, the ideal FeAs4 tetrahedron is favorable for superconductivity in FeAs-based SCs. In (Li0.8Fe0.2)OHFeSe, the electron-type carriers dominate the conduction, and a canted AFM order occurs at ~8.5 K, coexisting with superconductivity at ~40 K. Moreover, (Li0.8Fe0.2)OHFeSe is stable in air, unlike other FeSe SCs intercalated with alkali metal ions, NH3 molecules, or organic molecules, which are extremely air sensitive. Soon after the synthesis of polycrystalline (Li1−xFex)OHFeSe via a hydrothermal method, (Li1−xFex)OHFeSe single crystals with Tc above 44 K were successfully synthesized via a hydrothermal ion-exchange process using AxFe2−ySe2 (A = K, Rb, and Cs) as precursors, in which the 245 phase is absent [53]. They have a narrow SC transition and 100% shield volume fractions, indicating their excellent bulk superconductivity; in contrast, the shield fractions of AxFe2−ySe2 are only 10–20% [47]. These advantages make the (Li1−xFex)OHFeSe crystal an ideal system for investigating the intrinsic mechanism behind its superconductivity and the effect of anisotropy in its crystal structure on its superconducting properties. In this article, we review the growth technology, structure, and physical properties of (Li1−xFex)OHFeSe single crystals.

2. Crystal Growth

2.1. AxFe2−ySe2 (A = K, Rb, and Cs) Precursor

AxFe2−ySe2 (A = K, Rb, and Cs) single crystals used as precursors can be obtained using self-flux, Bridgman, and optical floating-zone (OFZ) crystal growth methods [54,55,56,57,58,59,60,61,62,63]. Figure 1a,b show the schematic illustrations of the self-flux and Bridgman growth methods, respectively. A typical procedure for the self-flux growth method is as follows [56,63]. First, the FeSe precursor was prepared by reacting Fe and Se powders at 700 °C for 4 h in an evacuated quartz tube. Then, the obtained FeSe precursor was ground into powder and mixed with A (A = K, Rb, and Cs) pieces at a ratio of A:FeSe = 0.8:2. The mixtures were put into a double-sealed quartz ampule which would protect the raw materials from exposure to air, in case the inner tube broke during growth. The samples were heated to 1030 °C in 4 h and kept at this temperature for 2 h. Before the furnace was shut down at 750 °C, either a cooling rate of 6 °C/h or a quenching method was applied. The actual compositions of KxFe2Se2 and CsxFe2Se2 were determined to be K:Fe:Se = 0.75:1.66:2 and Cs:Fe:Se = 0.81:1.61:2, respectively [56]. In addition, it was reported that using a Bridgman method, AxFe2−ySe2 single crystals were grown when the melt was soaked at 1070 °C for 5 h and a slow traveling speed of 3 mm/h was applied [62]. Using the starting mixture Rb:FeSe = 1:2.5, the actual compositions of the obtained crystal were determined to be Rb:Fe:Se = 0.740(36):1.600(6):2.000(25) [62].
The OFZ crystal growth method is unique in growing both congruent and incongruent melt compounds without contamination of crucible materials. This also allows the production of large crystals that cannot be obtained using solution methods. The relatively high thermal gradient at the crystallization front can be used to control the extent of the constitutional supercooling and has the advantage of a rapid growth of crystals from an incongruent melt. The OFZ method has been successfully used to grow large and high-quality single crystals of high-Tc superconductors, including FeSCs such as AxFe2−ySe2 (A = K, Rb, and Cs) [2,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77]. Figure 2 shows a schematic illustration of the OFZ growth setup and snapshot of the real growth process. The feed rod was prepared via a one-step conventional solid-state reaction as reported in Ref. [63]. Elemental A, Fe, and Se were mixed with the nominal molar ratio of A:Fe:Se = 0.8:2:2 and sintered at 850 °C for 10 h. After sintering, the A0.8Fe2Se2 mixture was uniformly ground into powder, which was pressed into a feed rod of ~70–80 mm in length and ~6–7 mm in diameter using a hydraulic press under an isostatic pressure of 600 bar. The feed rod was used to grow the crystal directly, without the conventional sintering or pre-melting process. A seed rod of 2 cm in length was obtained by cutting the feed rod. The AxFe2−ySe2 single crystals were grown in a four-mirror OFZ furnace (FZ–T–10000–H–III–VPR, Crystal System Inc., Salem, MA, USA) having four 300 W halogen lamps as the heating source. Rotation rates of 20 rpm were employed for both the feed and seed shafts in opposite directions. Growths were carried out at a travelling velocity rate of 0.8 mm/h under an 8 bar argon atmosphere. The obtained crystals were large and homogeneous with a mass of up to 1.8 g. Figure 3a shows a schematic illustration of the OFZ growth process for the AxFe2−ySe2 single crystals. It was found that as the molten zone was slowly moved up along the length of the rotating feed rod, the grown crystal continuously passed through a special temperature zone, Td, where it began to decompose. Therefore, to protect the grown crystal, the four halogen lamps were turned off towards the end of the crystal growth, resulting in un-destroyed crystal phase at the upper portion of the grown ingot. Figure 3b–d shows the typical as-grown single-crystal ingots of K0.80Fe1.81Se2, Rb0.80Fe1.81Se2, and Cs0.80Fe1.81Se2, respectively. It can be seen in the inset of Figure 3b that the cleaved ingot displays a large crystal grain with a flat shiny surface. It is interesting to find that although the A ions are different, the iron content in all the AxFe2−ySe2 crystals is approximately 1.81, which is higher than for the ones grown by the self-flux and Bridgman methods. In addition, it was observed that the Fe content in the crystals can be adjusted by changing the growth atmospheres.

2.2. Hydrothermal Growth

The hydrothermal method is an important method of crystal growth that is complementary to the classical melt-based or vapor transport methods, being widely used to synthesize new materials, new structures, and new phases for various applications [78]. This method is available for preparing novel SCs, such as Ba1−xKxBiO3 [79], Ba1−xKxBi1−yNayO3 [79], BaPd1−xBixO3 [80], and La2CuO4+y [81]. Recently, the hydrothermal growth of (Li1−xFex)OHFeSe single crystals has been reported [53]. The crystal precursors were prepared by cleaving the OFZ-grown single crystals of AxFe2−ySe2 (A = K, Rb, and Cs) [63]. C(NH2)2Se (99.95%), Fe powder (99.995%), LiOH (99.95%), and 9 mL H2O with a mol ratio of Se:Fe:Li = 1:4.38:51.47–154.41 were used for preparing the mixtures. Both the crystal precursors and the mixtures were tightly sealed in a Teflon-lined steel autoclave (30 mL) prior to being heated in a box furnace from room temperature to 120–180 °C at a rate of 60 °C/h. The crystal precursors and mixtures were held at the temperature for up to 4 d, before cooling down naturally to room temperature. The large as-grown (Li1−xFex)OHFeSe single crystals were separated and washed several times with methanol solution to obtain clean surfaces. Figure 4a,b show the Teflon-lined hydrothermal synthesis autoclave reactor and the growth procedure, respectively. Similar hydrothermal growth methods have been used to prepare (Li1−xFex)OHFeSe materials, but, the precursor chosen has significant effects on the final products. Using tetragonal FeSe as the precursor, small (~10 × 10 × 1 μm) crystals were grown [82]. Without using any precursor, polycrystalline samples were obtained [52].

2.3. Crystal Characterization Techniques

The (Li1−xFex)OHFeSe and AxFe2−ySe2 (A = K, Rb, and Cs) single crystals have been characterized by different kinds of techniques [2,52,53,63]. The actual composition of the crystals is determined by energy-dispersive X-ray (EDX) spectroscopy or more precise inductively coupled plasma atomic-emission spectroscopy (ICP-AES) analysis. The crystal structures are characterized by X-ray diffraction (XRD) and neutron powder diffraction (NPD). Electrical resistivity is measured by a standard four-probe technique to exclude the resistance of the leads using a physical properties measuring system (PPMS, Quantum Design). The contact resistance is smaller than 100 μΩ. Measurements of the magnetic susceptibility are investigated by a superconducting quantum interface device (SQUID) magnetometer (Quantum Design MPMS).

3. Crystal Structure

The final shape and size of the obtained (Li1−xFex)OHFeSe single crystals depends on the precursor crystals used. Thus, the large OFZ-grown AxFe2−ySe2 (A = K, Rb, and Cs) crystal precursors used in the hydrothermal growth allows the preparation of large, high-quality (Li1−xFex)OHFeSe single crystals ~1 cm in diameter, as shown in Figure 5a. Due to the hydrothermal reaction, a thin layer made from [(Li1−xFex)OH](Fe1−yLiy)Se is usually deposited on the surface of the as-grown crystals, which shows Tc ~ 30–40 K with a broad transition [83], as shown in Figure 5b. When the deposited materials are carefully removed with Q-cotton in methanol, the black and plate-like crystal surface appears, as shown in Figure 5a. The as-grown crystals can be easily cleaved along the (001) direction, as shown in Figure 5c. This because that the c-axis is increased by 31%, since the adjacent edge-sharing FeSe4 tetrahedra are linked to the intercalated (Li1−xFex)OH layer via a much weaker hydrogen bonding interaction [52,53,65], however, the (001) plane, i.e., the surface of the crystal, is robust during the hydrothermal growth, as shown in Figure 5d. The as-grown crystals of (Li1−xFex)OHFeSe are easily decomposed at room temperature, however, they are stable and can be stored in low-temperature environments like [(Li1−xFex)OH](Fe1−yLiy)Se [83].
Figure 6 shows the layered structure of the (Li1−xFex)OHFeSe crystal. During the hydrothermal growth, intercalated (Li1−xFex)OH layers are formed due to the ion exchange of A (K, Rb, and Cs). Along the c-axis anti-PbO type layers of (Li1−xFex)OH alternate with anti-PbO type FeSe layers. In the hydroxide crystal, positively polarized hydrogen atoms of the (Li1−xFex)OH layer point towards the negatively polarized selenium of the FeSe layer. The (Li1−xFex)OH layer has a similar structure to LiOH itself, which likewise crystallizes in the anti-PbO-type [52,83].
The hydrothermal growth process induces a complete structural change. The obtained (Li1−xFex)OHFeSe single crystals show an intergrowth of the ion-exchanged (Li1−xFex)OH layers and the FeSe layers along the c-axis, which is greatly different from the structure of the AxFe2−ySe2 (A = K, Rb, and Cs) crystals, representing an intergrowth of the iron-vacancy-ordered and -disordered states along the c-axis. Figure 7a–c show the XRD patterns of the A0.80Fe1.81Se2 (A = K, Rb, and Cs) crystal precursors. In the three figures, the (00l) reflections demonstrating a tetragonal structure with space group I4/m are related to the 245 insulating phase. The ordered and disordered iron vacancies along the c-axis in the AxFe2−ySe2 crystals are characterized by slightly different lattice constants. It can be seen that there are three weak shoulders beside the (008), (0010), and (0012) reflections, as marked with the asterisks. The second set of the (00l) reflections with lower intensity is attributed to phase separation in the crystals [60]. The stronger reflections come from the superstructure, indicating that they are related to the iron vacancy-ordered insulating phase. Thus, the second phase marked with asterisks must come from the minor metallic phase which is free of iron vacancy ordering, indicating that the crystal precursor only has a small volume (10%) of the SC phase [84]. However, only one set of the 00l (l = 2n) peaks is measured in (Li1−xFex)OHFeSe single crystals, indicating all of them are in a pure phase, as shown in Figure 7a–c.
Due to the hydrothermal ion exchange, it can be seen that all reflections related to the 245 insulating phase are absent, indicating that all A0.80Fe1.81Se2 (A = K, Rb, and Cs) single crystals have been changed into (Li1−xFex)OHFeSe with its P4/nmm space group, regardless of kind of A ion. The lattice parameters of the (Li1−xFex)OHFeSe produced using the A0.80Fe1.81Se2 precursors are a = b = 3.7862(7) Å, 3.779(1) Å, and 3.7693(0) Å, and c = 9.255(1) Å, 9.268(1) Å, and 9.281(7) Å, for A = K, Rb, and Cs, respectively. The c-axis lattice parameter increases with K, Rb, and Cs, which indicates that the larger ionic radius (R) provides more space, since RK + (1.51 Å) < RRb + (1.61 Å) < RCs + (1.74 Å). It is reported that the lattice parameters from the NPD refinement of polycrystalline (Li0.8Fe0.2)OHFeSe at 2.5 K are a = b = 3.77871(4) Å and c = 9.1604(1) Å [52]. For polycrystalline (Li1−xFex)ODFeSe with x = 0.183(6), the Rietveld refinement of the neutron diffraction pattern collected at 4 K gave a = 3.7827(1) Å and c = 9.1277(3) Å [85]. The XRD measurement carried out at 298 K shows that a = 3.7865(2) Å and c = 9.2802(6) Å for polycrystalline (Li0.8Fe0.2)OHFeSe [86]. These results show the lattice parameters are small at low temperatures.
Hydrothermal growth parameters greatly influence the structure and properties of the (Li1−xFex)OHFeSe single crystals. In order to find the optimum synthesis conditions, it is necessary to study the effects of the important growth parameters such as growth time (t), growth temperature (Tg), and the molar ratio of lithium ions (cm) on c. Figure 8a–c show t, Tg, and cm dependent XRD patterns, respectively [53]. The insets of the figures show the parameter dependence of c. In the experiments t, Tg, and cm vary in the ranges 20–120 h, 120–200 °C, and 51.47–154.41 respectively, and c has the maximum value (cmax) as each parameter varies. For t and Tg, cmax is obtained in the medium parameter ranges, and cmax = 9.278(3) Å and 9.282(0) Å for t = 72 h and Tg = 160°C, respectively. For cm, however, the cmax = 9.278(8) Å is obtained at the lowest cm = 51.47. The maximum cmax = 9.282(0) Å is obtained as Tg = 160 °C. It is noted that as Tg, t, and cm are adjusted the c values vary in the ranges 9.246(3)–9.282(0) Å, 9.255(1)–9.278(3) Å, and 9.262(3)–9.278(8) Å respectively, such that the range of variation in c due to changes in Tg spans the ranges of variation in c due to changes in t and cm. Therefore, compared with t and cm, Tg is more preferable for investigating the structural changes of the crystals, because by varying Tg both the highest values and the broadest distribution of c can be achieved at the same time. In addition, it is reported that reductive lithiation of the hydrothermally synthesized samples using lithium/ammonia solution can increase the c lattice parameter to obtain the SC phase [82].

4. Superconductivity, Spin Density Wave, Antiferromagnetism and Ferromagnetism

Surprisingly, although the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors have very different properties, all the synthesized (Li1−xFex)OHFeSe crystals show enhanced SC properties, indicating that the intercalating layers play important roles in the high-Tc superconductivity mechanism [53].
Figure 9a–f show the electrical and magnetic properties of the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors and the (Li1−xFex)OHFeSe crystals. It can be clearly seen that the properties of the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors are different despite their similar structures.
The K0.80Fe1.81Se2 precursor is a good superconductor, with a Tc of 31.76 (29.31) K derived from magnetic (electrical) measurements. After the hydrothermal ion-exchange process, its Tc is increased by 6.08 (13.38) K. Its zero field cooled (ZFC) susceptibility is nearly temperature independent before the onset of superconductivity, indicating that the normal state is Pauli paramagnetic, which can be found in the SC samples of (Li1−xFex)OHFeSe, AxFe2−ySe2 (A = K, Rb, and Cs), and FeSe [3,7,56,62,87,88]. The hump peak of the normalized resistance versus temperature (Rab/RpT) curve shifts from 220.79 K to 266.06 K and the resistivity of the hump peak is decreased from 560 mΩcm to 0.13 mΩcm due to the hydrothermal reaction. The peak-shift effect indicates that the SC/metallic property is improved. The SC transition width is defined as ΔT = Tc(onset) − Tc(0), where Tc(onset) is the temperature where the resistivity starts to deviate from the normal-state resistivity, and Tc(0) is the temperature where zero resistivity state is achieved. For the K0.80Fe1.81Se2 crystal precursor ΔT is 4.19 K, which is smaller than those for all the (Li1−xFex)OHFeSe crystals, indicating that some inhomogeneous structures resulting from the intercalating layers broaden the transition width.
The Rb0.80Fe1.81Se2 precursor is non-conductive. As the temperature increases its resistance decreases monotonically and sharply, indicating that it acts as an electrical insulator.
The Cs0.80Fe1.81Se2 precursor (Tc = 28.67 K) exhibits poor superconducting properties. Its resistance does not reach zero even at T = 2.53 K, but its Rab/Rp is as high as 7.80%. Its electrical transport properties are complex. It can be seen that its Rab/RpT curve shows two hump peaks, P1(61.31 K, 0.991) and P2(31.46 K, 1.000), dividing the curve into three regions: high-, medium-, and low-temperature regions, which are denoted as ‘I’, ‘II’, and ‘III’, respectively. In region ‘I’, it shows semiconducting behavior. Additionally, the first peak P1 is reached at T = 61.31 K, which is much smaller than those for the K0.80Fe1.81Se2 crystal precursor and all the (Li1−xFex)OHFeSe crystals, indicating that an insulating/semiconducting phase is dominant in the high-temperature region. In region ‘II’, Rab/Rp decreases with temperature because the metallic phase starts to play a role, and at the valley point ‘V’ (T = 47.32 K) the balance between the competitive conductive and non-conductive phases happens. After the ‘V’ point an unbalance occurs and thus the resistance increases again before the second hump peak ‘P2’ is reached, where the SC state arises. In region ‘III’, the SC phase plays a leading role, and a sharp drop in resistance occurs after ‘P2’ under the co-effects of the SC and metallic phases.
Despite the significant variation in the properties of the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors, high-quality (Li1−xFex)OHFeSe crystals with high Tc were obtained from all of them. The susceptibility measurements show that all the (Li1−xFex)OHFeSe crystals have high-temperature SC transitions, with Tc = 37.84 K, 38.69 K, and 39.03 K measured from samples produced from the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors, respectively. The superconductive shielding fraction estimated from the zero-field-cooling magnetization at T = 3 K is 97%, 100%, and 100% respectively, indicating the bulk superconductivity nature and high quality of the crystals. The electrical measurements show onset Tc = 42.69 K, 43.87 K, and 44.66 K respectively, confirming the SC transitions. Zero resistance of the (Li1−xFex)OHFeSe crystals grown using the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors is measured below 35.12, 32.79, and 38.18 K respectively, giving associated transition widths ΔT of 7.57 K, 11.08 K, and 6.48 K. In addition, it is found that the ionic radius of A+ (RA+) influences greatly the structures and properties of the (Li1−xFex)OHFeSe crystals. Both Tc and c increase with RA+, suggesting that Tc depends on c. Among the (Li1−xFex)OHFeSe crystals, the crystal using the Cs0.80Fe1.81Se2 precursor shows the best superconductivity: the highest Tc, the highest Tc(0), and the narrowest ΔT. In addition, the Rab/RpT curves of all the obtained crystals show a hump peak near room temperature, which is attributed to the doping effect of the (Li1−xFex)OH layers [82]. For the crystals made from the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors, the hump temperature Th = 266.06, 276.18, and 281.34 K respectively. It shows that as RA+ (or c) increases, Th shifts toward high temperatures, indicating that the SC/metallic property is enhanced obviously in the crystals with the largest c-axis length, but the non-conductive property is weakened. Therefore, Tc increases due to the enhanced doping.
The (Li1−xFex)OHFeSe and the AxFe2−ySe2 (A = K, Rb and Cs) crystals demonstrate complex electrical transport properties, which could be attributed to the doping effects of the (Li1−xFex)OH layer or A ions or the vacant sites in the FeSe layer [60,82,89]. However, using the same hydrothermal synthesis route, all (Li1−xFex)OHFeSe crystals have been synthesized with high superconductive shielding fraction and high Tc, despite the A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors having significantly different properties that vary from being a good superconductor, to an insulator, and finally to a poor superconductor.
It is interesting to point out that SC and non-SC (Li1−xFex)OHFeSe single crystals can be selectively grown by controlling the growing parameters. SC crystals with large c-axis length can be prepared by a low-temperature hydrothermal reaction (e.g., 120 °C) or a high-temperature hydrothermal reaction for a short time (e.g., 160 °C for 40 h). In contrast, non-SC crystals with small c-axis length can be grown at high hydrothermal reaction temperatures for a long time (e.g., 180 °C and 72 h). Non-SC single crystals exhibit AFM spin density wave (SDW) transitions. Figure 10 shows the XRD patterns for a non-SC crystal prepared using a Cs0.80Fe1.81Se2 crystal precursor. The inset shows that the AFM SDW transition temperature Ts = 125.41 K [53]. For polycrystalline (Li1−xFex)OHFeSe, the SC sample with Tc = 40 K was grown at 120 °C for 96 h, while the non-SC sample with Ts = 125 (127) K was grown at 160 (180) °C for 72 h [90].
In addition, the (Li1−xFex)OHFeSe provides a new platform to study the interplay between superconductivity and magnetism. It was reported that in polycrystalline (Li0.8Fe0.2)OHFeSe superconductivity coexists with a field-induced ferromagnetism under external magnetic field and antiferromagnetism at zero field [52,91]. In [(Li1−xFex)OH](Fe1−yLiy)Se, superconductivity below Tc = 43 K coexists with ferromagnetism below 10 K [83]. The study of SC and non-SC (Li1−xFex)OHFe1−ySe suggests a glassy magnetic state, probably comprising clusters of iron ions of varying cluster sizes distributed within the lithium hydroxide layer [92].

5. Phase Diagram

The phase diagram of the (Li1−xFex)OHFeSe single crystals using the A0.80Fe1.81Se2 (A = K, Rb, and Cs) crystal precursors is established based on c dependence of Ts and Tc, as shown in Figure 11. It can be seen that the crystals show AFM SDW and SC transitions in low and high c regions, respectively. As T > Ts (Tc), Curie–Weiss (Pauli paramagnetic) phase forms. The phase diagram shows a step behavior of the AFM SDW and SC transitions between 9.254(3) and 9.255(1) Å, which is similar to that for polycrystalline samples of LaO1−xFxFeAs [93], but different from those for cuprate SCs [94] and other FeSCs [95], such as CeFeAsO1−xFx, (K,Tl)FexSe2 [96], NaFe1−xCoxAs [97], Ba(Fe1−xCox)2As2, SmFeAsO1−xFx, Ba1−xKxFe2As2 [98], BaFe2−xRhxAs2, BaFe2−xPdxAs2, SrFe2−xNixAs2, SrFe2−xRhxAs2, SrFe2−xIrxAs2, SrFe2−xPdxAs2. In addition, as c increases the Tc-c relationship experiences three regions successively: region ‘I’, region ‘II’, and region ‘III’. In region ‘I’ (‘II’), Tc gradually decreases (increases) stably as c increases. However, in region ‘III’, the Tc-c relationship is not stable, indicating that a large c-axis weakens the hydrogen bonding interaction between the layers which makes the structure and property unstable. The Tc-c relationship has a V-shape, which is in sharp contrast to the inverse V-shaped one in cuprate SCs [94] and other FeSCs [93,95,96,97,98]. The complex SC phase diagram can be attributed to the doping effect of the (Li1−xFex)OH layer (the Fe/Li ratio, the migration of Fe and the charge transfer from the intercalated layer to the FeSe layer) as well as the complex competing effects between the several different states related to phase separation [18,52,82,89,99,100,101]. Comparison of this phase diagram to that for (Li1−xFex)OHFeSe polycrystals shows some similarities [90]. They both have AFM SDW and SC phases, which lie in the low and high c regions, respectively. However, for the single crystals, the AFM SDW phase exists in the region where c ≤ 9.254(3) Å, whereas for the polycrystals, the SC phase forms for c ≥ 9.2152(8) Å [90]. Therefore, in the region where 9.2152(8) Å ≤ c ≤ 9.254(3) Å, the phase of the single crystal system differs from that of the polycrystalline system, even if both systems have the same c value. During the hydrothermal growth of (Li1−xFex)OHFeSe, the chemical reactions are different for synthesizing these two different kinds of samples: For the polycrystals it is a three-dimensional (3D)-diffusion-3D-growth process, but for growing the single crystals, it is a 2D-diffusion-1D-growth process, as shown in Figure 4. These different growth mechanisms, along with the size effect, are likely to lead to the significant differences in the characteristics observed between the two types of samples.

6. Anisotropic Behavior

The anisotropic properties of a high-quality (Li1−xFex)OHFeSe single crystal have been systematically investigated by performing electrical and magnetic measurements [102].
Figure 12 shows the temperature dependence of the Hall coefficient RH. It can be seen that RH is negative in the whole temperature range from 2–300 K, indicating that electron-like charge carriers dominate in the crystal. RH decreases as T decreases from room temperature, and reaches a minimum value at T* ≈ 150 K, below which it demonstrates a remarkable upturn. The strong temperature dependence of RH may suggest a strong multiband-effect in the crystal: the electron scattering rate of each band varies with temperature differently, and thus a combined contribution of multiple bands may result in a strongly temperature dependent RH. Another possible reason can be due to the magnetic skew scattering mechanism: scattering of conduction electrons from local moments is not symmetric because of spin-orbital coupling [103]. However, it is hard to distinguish between the different contributions from different mechanisms. A rough estimation based on the simple relation RH = −1/ne indicates that the carrier density n is low, i.e., n is estimated to be 3.0 × 1021 cm−3 at 225 K, which is similar to other anisotropic SCs, such as NdFeAsO0.82F0.18, polycrystalline LaFeAsO0.9F0.1−δ, and cuprate SCs [103,104].
Figure 13 shows ρab-T curves measured near Tc, where ρab is the ab-plane resistivity. The onset Tc = 43 K and the residual resistance ratio RRR = ρab(300 K)/ρab(43 K) = 4.95. For both H || ab and H || c, Tc(0) decreases faster than Tc(onset) as μ0H increases, resulting in a broadening effect of ΔT. Figure 13c shows that both ΔT(H || ab) and ΔT(H || c) increase monotonically with μ0H. A quantitative fit gives ΔTHα, where α is a constant. For H || ab, α is 0.177 in the range of 0–9 T. For H || c, α is 0.076, 0.155, and 0.195 in region “I” (≤1 T), region “II” (1–3 T), and region “III” (3–9 T), respectively. The broad SC transition at 0 T can be attributed to minority impurities, which are not observable in XRD measurements but may influence ΔT. Such minorities could be a normal state and/or SC phase. Furthermore, it can be seen that ΔT(H || c) is always larger than ΔT(H || ab), indicating that the broadening effect is anisotropic. The anisotropy ratio of ΔT is defined as η = ΔT(H || c)/ΔT(H || ab). Figure 13c shows that in region “I”, η increases rapidly with μ0H. In region “II”, the increase of η is very slow, and the maximum value ηmax = 1.67 is obtained at 3 T. In region “III”, η decreases steadily as μ0H increases, and η = 1.64 is reached at 9 T, showing that high fields can suppress the anisotropy ratio. The tunability of η as a function of μ0H shows great potential toward practical applications like magnetic sensors and switching devices.
The broadening behavior of ΔT can be analyzed based on the thermally activated flux flow (TAFF) model, and the TA resistivity ρ(T,H) = ρ0exp(-U/T), where ρ0 is a constant and U is the activation energy [105]. Assuming U = U0(1 − t), where t = T/Tc is the reduced temperature and U0 is apparent activation energy, the Arrhenius relation can be derived as lnρ(T, H) = lnρ0 + U0/TcU0/T. Figure 14a,b show that the experimental data can be fitted very well using the Arrhenius relation (solid lines). Figure 14c shows the μ0H dependence of U0. Inset of Figure 14c displays the same data on a double-logarithmic scale. It is seen that U0 for H || ab is much higher than for H || c, indicating a much stronger flux pinning for H || ab. In addition, for both configurations, it is found that U0 decreases with increasing μ0H according to the power law of U0 ∝ (1/H)α, where α is a constant. In region “I” (≤1 T) and region “II” (1–9 T), α = 0.364 (0.615) and 0.645 (0.436) for H || ab (H || c), respectively. For H || ab, the weak power-law decrease of U0 in region “I” suggests that single-vortex pinning is dominant in low-field, while a more rapidly decrease of U0 in region “II” can be related to the crossover to a collective pinning regime in high-field. However, it is reversed for H || c, suggesting that in region “I” (“II”) collective (single-vortex) pinning is dominant. Furthermore, it can be seen that the anisotropy ratio σ = U0(H || ab)/U0(H || c) also shows a crossover at 1 T. In region “I”, σ increases rapidly with μ0H, from σ = 3.91 at 0.25 T to the maximum value σmax = 5.53 at 1 T. However, in region “II”, σ decreases slowly as μ0H increases, to a minimum value σmin = 3.52 at 9 T, indicating that high field weakens the anisotropy. In addition, σ is higher than that for a Bi2.2Sr2Ca0.8Cu2O8+δ single crystal, which typically lies in the range of 1.5–3 [105]. Since σ can be tuned by the applied field, it shows great potential for device applications, such as in the development of magnetic field and temperature sensors.
Figure 15a shows temperature dependence of μ0Hc2, where μ0Hc2 is determined from the 10% and 50% criteria of the normal resistivity at the onset temperature. For both configurations, the curves show an upward curvature and the values of d(μ0 H c 2 a b )/dT and d(μ0 H c 2 c )/dT are calculated to be −12.30 (−4.01) and −1.61 (−1.27) T/K under the 50% (10%) criterion, respectively. For both criteria, the more positive curvature leads to much higher μ0Hc2 for H || ab than for H || c. It is observed that in some FeSCs, μ0Hc2 depends linearly on T due to the orbital limiting effect [106]. Based on this, using linear extrapolation (LE), under the 50% (10%) criterion, the zero-temperature μ0Hc2, μ0Hc2(0), is estimated to be 457.66 (144.74) T and 57.55 (39.17) T for H || ab and H || c, respectively, as shown in Figure 15b. The anisotropic ratio γ = H c 2 a b ( 0 ) / H c 2 c ( 0 )   = 7.95 (3.70) is obtained for the 50% (10%) criterion. In addition, μ0Hc2(0) can also be estimated using the Werthamer-Helfand-Hohenberg (WHH) equation μ 0 H c 2 ( 0 ) = 0.693 T c [ d ( μ 0 H c 2 ) / d T ] T = T c [107]. For the 50% (10%) criterion, μ0Hc2(0) = 324.28 (102.84) T and 42.45 (32.77) T are obtained from the WHH equation for H || ab and H || c, respectively, and thus γ = 7.64 (3.14) is obtained for the 50% (10%) criterion. The values of μ0Hc2(0) and γ obtained by the WHH model are all smaller than by the LE method. Based on the WHH theory and using the parameter values under the 50% criterion, the zero-temperature coherence lengths ξab(0) and ξc(0) are estimated to be 2.7858 nm and 0.3647 nm, respectively, from Ginzburg-Landau theory. Furthermore, using γ =   ( m c / m ab ) 1 / 2 , a value of mc/mab = 2.76 is obtained, where mc and mab are the effective mass tensors when the electrons are moving perpendicular and parallel to the FeSe layers, respectively.
The magnetization hysteresis loops of the crystal indicated anisotropic behavior and type-II superconductivity, as shown in Figure 16. The critical current density J c ab ( J c c ) for H || c (H || ab) can be calculated using the extended Bean model [108]. Figure 17a,b show that for a given T (μ0H), as μ0H (T) increases both J c ab and J c c decrease. When T ≤ 5 K, J c ab ( J c c ) exceeds 1.44 × 104 (8.29 × 103) A/cm2 in high fields up to 4 T. Figure 17c shows the temperature dependence of zero-field Jc, whose values are extracted by extrapolation to 0 T. J c ab is higher than J c c for a given T. The anisotropic ratio δ = J c ab / J c c increases from 2.90 to the maximum value of 3.48 as T increases from 2–7 K, but decreases as T increases further, and reaches its lowest value of 2.62 at 30 K. These results show that δ can be adjusted by varying temperature, providing a new basis for designing power-control or power-switching devices.
In order to study the vortex pinning mechanism, the authors plot the normalized pinning force fp = Fp/ F p max as a function of the reduced field h = H/Hirr, where the pinning force density Fp = μ0HJc and the irreversibility field μ0Hirr is obtained by extrapolating the Jc1/2(μ0H)1/4 versus μ0H curve to the horizontal axis [109]. Values of μ0Hirr = 20.39 T and 5.19 T are obtained for H || ab and H || c, respectively. The anisotropy of μ0Hirr is 20.39/5.19 = 3.93. Figure 18 shows that the formula fphp(1 − h)q fits the experimental data well for each configuration at 15 K. Flux-pinning parameters of p = 0.686 (0.365) and q = 3.242 (2.452) are obtained for H || ab (H || c) by fitting. The position of the maximum of Fp/ F p max is hmaxp/(p + q). For H || ab (H || c), the fitting value h max fit = 0.175 (0.130) is consistent with the peak position h max exp = 0.174 (0.129) of the experimental curve. These values are close to hmax = 0.2 responsible for normal surface pinning (NSP) [110], indicating that the NSP mechanism is dominant in the crystal for each configuration. The ratio 0.175/0.130 = 1.35 shows a small anisotropy between the two orientations. However, for the quenched KxFe2−ySe2 single crystals hmax = 0.32 or 0.34, indicating that the normal point pinning (NPP) is dominant [111,112]. For the Mn doped KxFe2−ySe2 single crystal hmax = 0.27 is smaller than for the KxFe2−ySe2 quenched crystals, implying that the NPP may coexist with NSP [112]. In the Mn doped KxFe2−ySe2 single crystals, Mn atoms can form non-SC K-Fe-Mn inclusion phases, and such large normal inclusions may serve as the NSP centers in the crystals [112]. In comparison with the quenched and the Mn doped KxFe2−ySe2 single crystals, hmax for the (Li1−xFex)OHFeSe single crystal is much smaller, indicating that the hydrothermal ion-exchange process is more effective for obtaining NSP centers and changing the pinning mechanism.

7. Superconducting Mechanism

Density functional theory (DFT) calculations were used to investigate the dominant roles of the (Li0.8Fe0.2)OH layers in the high-Tc superconductivity of (Li0.8Fe0.2)OHFeSe, and it was found that substitution of Li by Fe can enhance the structural stability both in the ab plane and along the c axis. The Fe0.2 atoms can be the origin of significant electron injection into FeSe. The (Li0.8Fe0.2)OH layers can be either AFM or FM depending on the spatial distribution of Fe0.2 atoms. The stable structure with large electron injection leads to high-Tc superconductivity [113]. In addition, low temperature scanning tunneling microscopy (STM) suggests that the (Li1−xFex)OHFeSe is a plain s-wave superconductor with strong coupling mechanism [114,115].

8. Conclusions

FeSCs have been attracting a great deal of research interest for the development of new high-Tc SCs. Their diverse structures, complex phases, and exotic SC properties are important for both fundamental studies and technical applications. In general, Tc can be enhanced by chemical and physical methods, such as ion doping and the application of external pressure. In addition, many measures have also been taken to improve other SC properties in FeSC systems. In this paper, a hydrothermal method which can be used for inducing intercalation to increase Tc and improve other SC properties is reviewed, taking the (Li1−xFex)OHFeSe single crystal system as an example.
The hydrothermal method has been successfully applied to grow high-quality (Li1−xFex)OHFeSe single crystals using OFZ-grown AxFe2−ySe2 (A = K, Rb, and Cs) precursors. A stacking layer of (Li1−xFex)OH sandwiched between the FeSe layers is formed by the hydrothermal ion exchange of Li/Fe–O–H for K, Rb, and Cs. The structure of the ion-exchanged crystal belongs to the P4/nmm space group, which is different from the I4/m space group of the AxFe2−ySe2 precursor. In the (Li1−xFex)OHFeSe single crystals the space between two adjacent FeSe layers is enlarged by the intercalated (Li1−xFex)OH layers, resulting in both larger c-axis lattice constants and a higher Tc by weakening the interlayer coupling, compared to FeSe materials. In addition, the Tc of (Li1−xFex)OHFeSe crystals increases from 29.31 K to 42.69 K and from 28.67 K to 44.66 K compared to the SC K0.80Fe1.81Se2 and poor SC Cs0.80Fe1.81Se2 crystal precursors, respectively. For the insulating Rb0.80Fe1.81Se2 crystal precursor, a significant change from insulator to superconductor occurs after the ion-exchange process, and Tc = 43.87 K is obtained in the synthesized (Li1−xFex)OHFeSe crystal. The sharp transitions of resistivity at Tc ~ 42 K with 100% SC shielding ratio confirm the bulk superconductivity of the (Li1−xFex)OHFeSe single crystals.
By optimizing the growth parameters, such as time, temperature, and composition, SC (Li1−xFex)OHFeSe single crystals have been obtained, regardless of the SC phase of the precursor such as SC K0.80Fe1.81Se2 (Tc = 29.31 K), non-SC Rb0.80Fe1.81Se2, or poor-SC Cs0.80Fe1.81Se2 (Tc = 28.67 K). Furthermore, by adjusting the growth parameters, non-SC (Li1−xFex)OHFeSe single crystals showing AFM SDW have also been synthesized regardless of the SC phase of the precursor. For SC crystals, Tc > 42 K is achieved. Anisotropic properties including magnetoresistance broadening, upper critical field, coherence length, activation energy, magnetization hysteresis loops, critical current density, irreversibility field, and flux pinning were systematically reviewed. Crystal anisotropies are tunable through adjustments in the external magnetic field and temperature. In non-SC crystals, the AFM SDW transition occurs at ~125 K. The phase diagram including AFM SDW, SC and paramagnetic phases was summarized using the reviewed data. The results show that the (Li1−xFex)OHFeSe single crystal system provides a new research platform for both fundamental research and device applications.

Acknowledgment

The authors thank Alexander Blair for his assistance in refining the language used throughout this review.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Johrendt, D. Structure–property relationships of iron arsenide superconductors. J. Mater. Chem. 2011, 21, 13726–13736. [Google Scholar] [CrossRef]
  2. Chen, D.P.; Lin, C.T. The growth of 122 and 11 iron-based superconductor single crystals and the influence of doping. Supercond. Sci. Technol. 2014, 27, 103002. [Google Scholar] [CrossRef]
  3. Guo, J.G.; Jin, S.F.; Wang, G.; Wang, S.C.; Zhu, K.X.; Zhou, T.T.; He, M.; Chen, X.L. Superconductivity in the iron selenide KxFe2Se2 (0 ≤ x ≤ 1.0). Phys. Rev. B 2010, 82, 180520. [Google Scholar] [CrossRef]
  4. Li, M.T.; Chen, L.; Feng, Z.J.; Deng, D.; Kang, B.J.; Cao, S.X.; Lin, C.T.; Zhang, J.C. Anisotropic characteristics and critical behaviors in Mn doped K0.8Fe2Se2 single crystal. Physica C 2014, 506, 40–46. [Google Scholar] [CrossRef]
  5. Friemel, G.; Liu, W.P.; Goremychkin, E.A.; Liu, Y.; Park, J.T.; Sobolev, O.; Lin, C.T.; Keimer, B.; Inosov, D.S. Conformity of spin fluctuations in alkali-metal iron selenide superconductors inferred from the observation of a magnetic resonant mode in KxFe2−ySe2. Europhys. Lett. 2012, 99, 67004. [Google Scholar] [CrossRef]
  6. Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H. Iron-based layered superconductor La[O1−xFx]FeAs (x = 0.05–0.12) with Tc = 26 K. J. Am. Chem. Soc. 2008, 130, 3296–3297. [Google Scholar] [CrossRef] [PubMed]
  7. Hsu, F.C.; Luo, J.Y.; Yeh, K.W.; Chen, T.K.; Huang, T.W.; Wu, P.M.; Lee, Y.C.; Huang, Y.L.; Chu, Y.Y.; Yan, D.C.; et al. Superconductivity in the PbO-type structure α-FeSe. Proc. Natl. Acad. Sci. USA 2008, 105, 14262–14264. [Google Scholar] [CrossRef] [PubMed]
  8. Mizuguchi, Y.; Hara, Y.; Deguchi, K.; Tsuda, S.; Yamaguchi, T.; Takeda, K.; Kotegawa, H.; Tou, H.; Takano, Y. Anion height dependence of Tc for the Fe-based superconductor. Supercond. Sci. Technol. 2010, 23, 054013. [Google Scholar] [CrossRef]
  9. Hirschfeld, P.J.; Korshunov, M.M.; Mazin, I.I. Gap symmetry and structure of Fe-based superconductors. Rep. Prog. Phys. 2011, 74, 124508. [Google Scholar] [CrossRef]
  10. Hanaguri, T.; Niitaka, S.; Kuroki, K.; Takagi, H. Unconventional s-wave superconductivity in Fe(Se,Te). Science 2010, 328, 474–476. [Google Scholar] [CrossRef] [PubMed]
  11. Gasparov, V.A.; Audouard, A.; Drigo, L.; Rodigin, A.I.; Lin, C.T.; Liu, W.P.; Zhang, M.; Wang, A.F.; Chen, X.H.; Jeevan, H.S.; et al. Upper critical magnetic field of KxFe2−ySe2 and Eu0.5K0.5Fe2As2 single crystals. Phys. Rev. B 2013, 87, 094508. [Google Scholar] [CrossRef]
  12. Rahlenbeck, M.; Sun, G.L.; Sun, D.L.; Lin, C.T.; Keimer, B.; Ulrich, C. Phonon anomalies in pure and underdoped R1−xKxFe2As2 (R = Ba, Sr) investigated by Raman light scattering. Phys. Rev. B 2009, 80, 064509. [Google Scholar] [CrossRef]
  13. Inosov, D.S.; Leineweber, A.; Yang, X.; Park, J.T.; Christensen, N.B.; Dinnebier, R.; Sun, G.L.; Niedermayer, C.; Haug, D.; Stephens, P.W.; et al. Suppression of the structural phase transition and lattice softening in slightly underdoped Ba1−xKxFe2As2 with electronic phase separation. Phys. Rev. B 2009, 79, 224503. [Google Scholar] [CrossRef]
  14. Park, J.T.; Inosov, D.S.; Niedermayer, C.; Sun, G.L.; Haug, D.; Christensen, N.B.; Dinnebier, R.; Boris, A.V.; Drew, A.J.; Schulz, L.; et al. Electronic phase separation in the slightly underdoped iron pnictide superconductor Ba1−xKxFe2As2. Phys. Rev. Lett. 2009, 102, 117006. [Google Scholar] [CrossRef] [PubMed]
  15. Zabolotnyy, V.B.; Inosov, D.S.; Evtushinsky, D.V.; Koitzsch, A.; Kordyuk, A.A.; Sun, G.L.; Park, J.T.; Haug, D.; Hinkov, V.; Boris, A.V.; et al. (π, π) electronic order in iron arsenide superconductors. Nature 2009, 457, 569–572. [Google Scholar] [CrossRef] [PubMed]
  16. Liu, Y.; Lin, C.T. A comparative study of Fe1+δTe1−xSex single crystals grown by Bridgman and self-flux techniques. J. Supercond. Nov. Magn. 2011, 24, 183–187. [Google Scholar] [CrossRef]
  17. Liu, R.H.; Wu, T.; Wu, G.; Chen, H.; Wang, X.F.; Xie, Y.L.; Yin, J.J.; Yan, Y.J.; Li, Q.J.; Shi, B.C.; et al. A large iron isotope effect in SmFeAsO1−xFx and Ba1−xKxFe2As2. Nature 2009, 459, 64–67. [Google Scholar] [CrossRef] [PubMed]
  18. Dagotto, E. Colloquium: The unexpected properties of alkali metal iron selenide superconductors. Rev. Mod. Phys. 2013, 85, 849–867. [Google Scholar] [CrossRef]
  19. Büchner, B.; Hess, C. Iron-based superconductors: Vital clues from a basic compound. Nat. Mater. 2009, 8, 615–616. [Google Scholar] [CrossRef] [PubMed]
  20. Tapp, J.H.; Tang, Z.; Lv, B.; Sasmal, K.; Lorenz, B.; Chu, P.C.W.; Guloy, A.M. LiFeAs: An intrinsic FeAs-based superconductor with Tc = 18 K. Phys. Rev. B 2008, 78, 060505. [Google Scholar] [CrossRef]
  21. Wang, X.C.; Liu, Q.; Lv, Y.; Gao, W.; Yang, L.X.; Yu, R.C.; Li, F.Y.; Jin, C.Q. The superconductivity at 18 K in LiFeAs system. Solid. State Commun. 2008, 148, 538–540. [Google Scholar] [CrossRef]
  22. Pitcher, M.J.; Parker, D.R.; Adamson, P.; Herkelrath, S.J.C.; Boothroyd, A.T.; Ibberson, R.M.; Brunelli, M.; Clarke, S.J. Structure and superconductivity of LiFeAs. Chem. Commun. 2008, 45, 5918–5920. [Google Scholar] [CrossRef] [PubMed]
  23. Rotter, M.; Tegel, M.; Johrendt, D. Superconductivity at 38 K in the iron arsenide (Ba1−xKx)Fe2As2. Phys. Rev. Lett. 2008, 101, 107006. [Google Scholar] [CrossRef] [PubMed]
  24. Sasmal, K.; Lv, B.; Lorenz, B.; Guloy, A.M.; Chen, F.; Xue, Y.Y.; Chu, C.W. Superconducting Fe-based compounds (A1−xSrx)Fe2As2 with A = K and Cs with transition temperatures up to 37 K. Phys. Rev. Lett. 2008, 101, 107007. [Google Scholar] [CrossRef] [PubMed]
  25. Chen, X.H.; Wu, T.; Wu, G.; Liu, R.H.; Chen, H.; Fang, D.F. Superconductivity at 43 K in SmFeAsO1−xFx. Nature 2008, 453, 761–762. [Google Scholar] [CrossRef] [PubMed]
  26. Ren, Z.A.; Che, G.C.; Dong, X.L.; Yang, J.; Lu, W.; Yi, W.; Shen, X.L.; Li, Z.C.; Sun, L.L.; Zhou, F.; Zhao, Z.X. Superconductivity and phase diagram in iron-based arsenic-oxides ReFeAsO1−δ (Re = rare-earth metal) without fluorine doping. Europhys. Lett. 2008, 83, 17002. [Google Scholar] [CrossRef]
  27. Matsuishi, S.; Inoue, Y.; Nomura, T.; Yanagi, H.; Hirano, M.; Hosono, H. Superconductivity induced by Co-doping in quaternary fluoroarsenide CaFeAsF. J. Am. Chem. Soc. 2008, 130, 14428–14429. [Google Scholar] [CrossRef] [PubMed]
  28. Ogino, H.; Matsumura, Y.; Katsura, Y.; Ushiyama, K.; Horii, S.; Kishio, K.; Shimoyama, J. Superconductivity at 17 K in (Fe2P2)(Sr4Sc2O6): A new superconducting layered pnictide oxide with a thick perovskite oxide layer. Supercond. Sci. Technol. 2009, 22, 075008. [Google Scholar] [CrossRef]
  29. Zhu, X.Y.; Han, F.; Mu, G.; Cheng, P.; Shen, B.; Zeng, B.; Wen, H.H. Transition of stoichiometric Sr2VO3FeAs to a superconducting state at 37.2 K. Phys. Rev. B 2009, 79, 220512. [Google Scholar] [CrossRef]
  30. Shirage, P.M.; Kihou, K.; Lee, C.H.; Kito, H.; Eisaki, H.; Lyo, A. Emergence of superconductivity in “32522” structure of (Ca3Al2O5–y)(Fe2Pn2) (Pn = As and P). J. Am. Chem. Soc. 2011, 133, 9630–9633. [Google Scholar] [CrossRef] [PubMed]
  31. Ren, Z.-A.; Lu, W.; Yang, J.; Yi, W.; Shen, X.-L.; Li, Z.-C.; Che, G.-C.; Dong, X.-L.; Sun, L.-L.; Zhou, F.; et al. Superconductivity at 55 K in iron-based F-doped layered quaternary compound Sm[O1−xFx]FeAs. Chin. Phys. Lett. 2008, 25, 2215–2216. [Google Scholar]
  32. Wu, G.; Xie, Y.L.; Chen, H.; Zhong, M.; Liu, R.H.; Shi, B.C.; Li, Q.J.; Wang, X.F.; Wu, T.; Yan, Y.J.; et al. Superconductivity at 56 K in samarium-doped SrFeAsF. J. Phys. Condens. Matter 2009, 21, 142203. [Google Scholar] [CrossRef] [PubMed]
  33. Chen, X.H.; Dai, P.C.; Feng, D.L.; Xiang, T.; Zhang, F.-C. Iron-based high transition temperature superconductors. Natl. Sci. Rev. 2014, 1, 371–395. [Google Scholar] [CrossRef]
  34. Lei, H.; Wang, K.; Hu, R.; Ryu, H.; Abeykoon, M.; Bozin, E.S.; Petrovic, C. Iron chalcogenide superconductors at high magnetic fields. Sci. Technol. Adv. Mater. 2012, 13, 054305. [Google Scholar] [CrossRef] [PubMed]
  35. Chen, T.-K.; Chang, C.-C.; Chang, H.-H.; Fang, A.-H.; Wang, C.-H.; Chao, W.-H.; Tseng, C.-M.; Lee, Y.-C.; Wu, Y.-R.; Wen, M.-H.; et al. Fe-vacancy order and superconductivity in tetragonal β-Fe1−xSe. Proc. Natl. Acad. Sci. USA 2014, 111, 63–68. [Google Scholar] [CrossRef] [PubMed]
  36. Yeh, K.-W.; Huang, T.-W.; Huang, Y.-L.; Chen, T.-K.; Hsu, F.-C.; Wu, P.M.; Lee, Y.-C.; Chu, Y.-Y.; Chen, C.-L.; Luo, J.-Y.; et al. Tellurium substitution effect on superconductivity of the α-phase iron selenide. Europhys. Lett. 2008, 84, 37002. [Google Scholar] [CrossRef]
  37. Medvedev, S.; McQueen, T.M.; Troyan, I.A.; Palasyuk, T.; Eremets, M.I.; Cava, R.J.; Naghavi, S.; Casper, F.; Ksenofontov, V.; Wortmann, G.; et al. Electronic and magnetic phase diagram of bold italic β-Fe1.01Se with superconductivity at 36.7 K under pressure. Nat. Mater. 2009, 8, 630–633. [Google Scholar] [CrossRef] [PubMed]
  38. Mizuguchi, Y.; Tomioka, F.; Tsuda, S.; Yamaguchi, T.; Takano, Y. Superconductivity at 27 K in tetragonal FeSe under high pressure. Appl. Phys. Lett. 2008, 93, 152505. [Google Scholar] [CrossRef]
  39. Gati, E.; Köhler, S.; Guterding, D.; Wolf, B.; Knöner, S.; Ran, S.; Bud’ko, S.L.; Canfield, P.C.; Lang, M. Hydrostatic-pressure tuning of magnetic, nonmagnetic, and superconducting states in annealed Ca(Fe1−xCox)2As2. Phys. Rev. B 2012, 86, 220511. [Google Scholar] [CrossRef]
  40. Bao, W. Structure, magnetic order and excitations in the 245 family of Fe-based superconductors. J. Phys. Condens. Matter 2015, 27, 023201. [Google Scholar] [CrossRef] [PubMed]
  41. Stewart, G.R. Superconductivity in iron compounds. Rev. Mod. Phys. 2011, 83, 1589–1652. [Google Scholar] [CrossRef]
  42. Liu, R.H.; Luo, X.G.; Zhang, M.; Wang, A.F.; Ying, J.J.; Wang, X.F.; Yan, Y.J.; Xiang, Z.J.; Cheng, P.; Ye, G.J.; et al. Coexistence of superconductivity and antiferromagnetism in single crystals A0.8Fe2−ySe2 (A=K, Rb, Cs, Tl/K and Tl/Rb): Evidence from magnetization and resistivity. Europhys. Lett. 2011, 94, 27008. [Google Scholar] [CrossRef]
  43. Scheidt, E.-W.; Hathwar, V.R.; Schmitz, D.; Dunbar, A.; Scherer, W.; Mayr, F.; Tsurkan, V.; Deisenhofer, J.; Loidl, A. Superconductivity at Tc = 44 K in LixFe2Se2 (NH3)y. Eur. Phys. J. B 2012, 85, 279. [Google Scholar] [CrossRef]
  44. Zheng, L.; Izumi, M.; Sakai, Y.; Eguchi, R.; Goto, H.; Takabayashi, Y.; Kambe, T.; Onji, T.; Araki, S.; Kobayashi, T.C.; et al. Superconductivity in (NH3)yCs0.4FeSe. Phys. Rev. B 2013, 88, 094521. [Google Scholar] [CrossRef]
  45. Burrard-Lucas, M.; Free, D.G.; Sedlmaier, S.J.; Wright, J.D.; Cassidy, S.J.; Hara, Y.; Corkett, A.J.; Lancaster, T.; Baker, P.J.; Blundell, S.J.; et al. Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer. Nat. Mater. 2013, 12, 15. [Google Scholar] [CrossRef] [PubMed]
  46. Krzton-Maziopa, A.; Pomjakushina, E.V.; Pomjakushin, V.Y.; von Rohr, F.; Schilling, A.; Conder, K. Synthesis of a new alkali meta–organic solvent intercalated iron selenide superconductor with Tc ≈ 45 K. J. Phys. Condens. Matter 2012, 24, 382202. [Google Scholar] [CrossRef] [PubMed]
  47. Ying, T.; Chen, X.; Wang, G.; Jin, S.; Lai, X.; Zhou, T.; Zhang, H.; Shen, S.; Wang, W. Superconducting phases in potassium-intercalated iron selenides. J. Am. Chem. Soc. 2013, 135, 2951–2954. [Google Scholar] [CrossRef] [PubMed]
  48. Fang, M.H.; Pham, H.M.; Qian, B.; Liu, T.J.; Vehstedt, E.K.; Liu, Y.; Spinu, L.; Mao, Z.Q. Superconductivity close to magnetic instability in Fe(Se1−xTex)0.82. Phys. Rev. B 2008, 78, 224503. [Google Scholar] [CrossRef]
  49. Long, Y.J.; Wang, D.M.; Wang, Z.; Yang, H.X.; He, J.B.; Zhao, L.X.; Wang, P.P.; Xue, M.Q.; Li, J.Q.; Ren, Z.A.; et al. Synthesis and characterization of the layered iron-selenide Na0.8Fe1.6Se2. Phys. Rev. B 2014, 90, 144519. [Google Scholar] [CrossRef]
  50. Ricci, A.; Poccia, N.; Joseph, B.; Innocenti, D.; Campi, G.; Zozulya, A.; Westermeier, F.; Schavkan, A.; Coneri, F.; Bianconi, A.; et al. Direct observation of nanoscale interface phase in the superconducting chalcogenide KxFe2−ySe2 with intrinsic phase separation. Phys. Rev. B 2015, 91, 020503. [Google Scholar] [CrossRef]
  51. Shoemaker, D.P.; Chung, D.Y.; Claus, H.; Francisco, M.C.; Avci, S.; Llobet, A.; Kanatzidis, M.G. Phase relations in KxFe2−ySe2 and the structure of superconducting KxFe2Se2 via high-resolution synchrotron diffraction. Phys. Rev. B 2012, 86, 184511. [Google Scholar] [CrossRef]
  52. Lu, X.F.; Wang, N.Z.; Wu, H.; Wu, Y.P.; Zhao, D.; Zeng, X.Z.; Luo, X.G.; Wu, T.; Bao, W.; Zhang, G.H.; et al. Coexistence of superconductivity and antiferromagnetism in (Li0.8Fe0.2)OHFeSe. Nat. Mater. 2015, 14, 325–329. [Google Scholar] [CrossRef] [PubMed]
  53. Yu, G.; Zhang, G.Y.; Ryu, G.H.; Lin, C.T. Structure and superconductivity of (Li1−xFex)OHFeSe single crystals grown using AxFe2−ySe2 (A = K, Rb, and Cs) as precursors. J. Phys. Condens. Matter 2016, 28, 015701. [Google Scholar] [CrossRef] [PubMed]
  54. Liu, Y.; Xing, Q.; Dennis, K.W.; McCallum, R.W.; Lograsso, T.A. Evolution of precipitate morphology during heat treatment and its implications for the superconductivity in KxFe1.6+ySe2 single crystals. Phys. Rev. B 2012, 86, 144507. [Google Scholar] [CrossRef]
  55. Mizuguchi, Y.; Takeya, H.; Kawasaki, Y.; Ozaki, T.; Tsuda, S.; Yamaguchi, T.; Takano, Y. Transport properties of the new Fe-based superconductor KxFe2Se2 (Tc = 33 K). Appl. Phys. Lett. 2011, 98, 042511. [Google Scholar] [CrossRef]
  56. Ying, J.J.; Wang, X.F.; Luo, X.G.; Wang, A.F.; Zhang, M.; Yan, Y.J.; Xiang, Z.J.; Liu, R.H.; Cheng, P.; Ye, G.J.; et al. Superconductivity and magnetic properties of single crystals of K0.75Fe1.66Se2 and Cs0.81Fe1.61Se2. Phys. Rev. B 2011, 83, 212502. [Google Scholar] [CrossRef]
  57. Wang, A.F.; Ying, J.J.; Yan, Y.J.; Liu, R.H.; Luo, X.G.; Li, Z.Y.; Wang, X.F.; Zhang, M.; Ye, G.J.; Cheng, P.; et al. Superconductivity at 32 K in single-crystalline RbxFe2−ySe2. Phys. Rev. B 2011, 83, 060512. [Google Scholar] [CrossRef]
  58. Krzton-Maziopa, A.; Shermadini, Z.; Pomjakushina, E.; Pomjakushin, V.; Bendele, M.; Amato, A.; Khasanov, R.; Luetkens, H.; Conder, K. Synthesis and crystal growth of Cs0.8(FeSe0.98)2: A new iron-based superconductor with Tc = 27 K. J. Phys. Condens. Matter 2011, 23, 052203. [Google Scholar] [CrossRef] [PubMed]
  59. Hu, R.; Cho, K.; Kim, H.; Hodovanets, H.; Straszheim, W.E.; Tanatar, M.A.; Prozorov, R.; Bud’ko, S.L.; Canfield, P.C. Anisotropic magnetism, resistivity, London penetration depth and magneto-optical imaging of superconducting K0.80Fe1.76Se2 single crystals. Supercond. Sci. Technol. 2011, 24, 065006. [Google Scholar] [CrossRef]
  60. Luo, X.G.; Wang, X.F.; Ying, J.J.; Yan, Y.J.; Li, Z.Y.; Zhang, M.; Wang, A.F.; Cheng, P.; Xiang, Z.J.; Ye, G.J.; et al. Crystal structure, physical properties and superconductivity in AxFe2Se2 single crystals. New J. Phys. 2011, 13, 053011. [Google Scholar] [CrossRef]
  61. Wang, D.M.; He, J.B.; Xia, T.-L.; Chen, G.F. Effect of varying iron content on the transport properties of the potassium-intercalated iron selenide KxFe2−ySe2. Phys. Rev. B 2011, 83, 132502. [Google Scholar] [CrossRef]
  62. Tsurkan, V.; Deisenhofer, J.; Günther, A.; Krug von Nidder, H.-A.; Widmann, S.; Loidl, A. Anisotropic magnetism, superconductivity and the phase diagram of Rb1−xFe2−ySe2. Phys. Rev. B 2011, 84, 144520. [Google Scholar] [CrossRef]
  63. Liu, Y.; Li, Z.C.; Liu, W.P.; Friemel, G.; Inosov, D.S.; Dinnebier, R.E.; Li, Z.J.; Lin, C.T. KxFe2−ySe2 single crystals: Floating-zone growth, transport and structural properties. Supercond. Sci. Technol. 2012, 25, 075001. [Google Scholar] [CrossRef]
  64. Maljuk, A.; Watauchi, S.; Tanakab, I.; Kojima, H. The effect of B2O3 addition on La2−xSrxCuO4 single-crystal growth. J. Cryst. Growth 2000, 212, 138–141. [Google Scholar] [CrossRef]
  65. Lin, C.T.; Maljuk, A.; Liang, B. The seeding effect of floating zone growth on Nd1.85Ce0.15CuO4 and Bi2Sr2CaCu2O8-δ single crystals. Supercond. Sci. Technol. 2002, 15, 1736–1740. [Google Scholar] [CrossRef]
  66. Liang, B.; Lin, C.T. On the growth of underdoped Bi2Sr2CaCu2O8+δ single crystals by traveling solvent floating zone method. J. Cryst. Growth 2002, 237, 756–761. [Google Scholar] [CrossRef]
  67. Maljuk, A.; Liang, B.; Lin, C.T.; Emelchenko, G.A. On the growth of overdoped Bi-2212 single crystals under high oxygen pressure. Physica C 2001, 355, 140–146. [Google Scholar] [CrossRef]
  68. Gu, G.D.; Takamuku, K.; Koshizuka, N.; Tanaka, S. Large single crystal Bi-2212 along the c-axis prepared by floating zone method. J. Cryst. Growth 1993, 130, 325–329. [Google Scholar] [CrossRef]
  69. Lin, C.T.; Freiberg, M.; Schöenherr, E. Growth and oxygenating studies of Bi2+xSr2−xCan-1CunO2n+4+δ single crystals. Physica C 2000, 337, 270–276. [Google Scholar] [CrossRef]
  70. Takekawa, S.; Nozaki, H.; Umezono, A.; Kosuda, K.; Kobayashi, M. Single crystal growth of the superconductor Bi2.0(Bi0.2Sr1.8Ca1.0)Cu2.0O8. J. Cryst. Growth 1988, 92, 687–690. [Google Scholar] [CrossRef]
  71. Gu, G.D.; Lin, Z.W. Single crystal growth of high-temperature superconductor Bi2.1Sr1.9Ca1.0Cu2.0AlyOx. Supercond. Sci. Technol. 2000, 13, 1197–1201. [Google Scholar] [CrossRef]
  72. Maljuk, A.; Lin, C.T. Floating zone growth of Bi2Sr2Ca2Cu3Oy superconductor. Crystals 2016, 6, 62. [Google Scholar] [CrossRef]
  73. Liang, B.; Lin, C.T.; Shang, P.; Yang, G. Single crystals of triple-layered cuprates Bi2Sr2Ca2Cu3O10+δ: Growth, annealing and characterization. Physica C 2002, 383, 75–88. [Google Scholar] [CrossRef]
  74. Kulakov, A.B.; Maier, D.; Maljuk, A.; Bdikin, I.K.; Lin, C.T. Study of growth/intergrowth behavior and structural analyses of Bi2Sr2Ca2Cu3O10+δ single crystals. J. Cryst. Growth 2006, 296, 69–74. [Google Scholar] [CrossRef]
  75. Fujii, T.; Watanabe, T.; Matsuda, A. Single-crystal growth of Bi2Sr2Ca2Cu3O10+δ (Bi-2223) by TSFZ method. J. Cryst. Growth 2001, 223, 175–180. [Google Scholar] [CrossRef]
  76. Peng, F.; Liu, W.P.; Lin, C.T. Study of thermal behavior and single crystal growth of A0.8Fe1.81Se2 (A = K, Rb, and, Cs). J. Supercond. Nov. Magn. 2013, 26, 1205–1211. [Google Scholar] [CrossRef]
  77. Liu, W.P.; Li, M.T.; Lin, C.T. Effect of Te doping on the structure and superconductivity of KxFe2−ySe2−zTez single crystals. J. Supercond. Nov. Magn. 2014, 27, 2419–2426. [Google Scholar] [CrossRef]
  78. McMillen, C.D.; Kolis, J.W. Bulk single crystal growth from hydrothermal solutions. Philos. Mag. 2012, 92, 2686–2711. [Google Scholar] [CrossRef]
  79. Zhang, G.; Li, G.; Huang, F.; Liao, F.; Li, K.; Wang, Y.; Lin, J. Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3. J. Alloys Compd. 2011, 509, 9804–9808. [Google Scholar] [CrossRef]
  80. Hirano, S.; Takahashi, S. Hydrothermal synthesis and properties of BaPb1−xBixO3. J. Cryst. Growth 1986, 79, 219–222. [Google Scholar] [CrossRef]
  81. Lan, Y.C.; Chen, X.L.; Cao, Y.G.; Huang, J.K.; Che, G.C.; Liu, G.D.; Xu, Y.P.; Xu, T.; Li, J.Y. Structure and superconducting properties of chemically oxidized La2CuO4+y under hydrothermal conditions. Physica C 2000, 336, 151–156. [Google Scholar] [CrossRef]
  82. Sun, H.; Woodru, D.N.; Cassidy, S.J.; Allcroft, G.M.; Sedlmaier, S.J.; Thompson, A.L.; Bingham, P.A.; Forder, S.D.; Cartenet, S.; Mary, N.; et al. Soft Chemical control of superconductivity in lithium iron selenide hydroxides Li1−xFex(OH)Fe1−ySe. Inorg. Chem. 2015, 54, 1958–1964. [Google Scholar] [CrossRef] [PubMed]
  83. Pachmayr, U.; Nitsche, F.; Luetkens, H.; Kamusella, S.; Brückner, F.; Sarkar, R.; Klauss, H.-H.; Johrendt, D. Coexistence of 3d-ferromagnetism and superconductivity in [(Li1−xFex)OH](Fe1−yLiy)Se. Angew. Chem. Int. Ed. Engl. 2015, 54, 293–297. [Google Scholar] [CrossRef] [PubMed]
  84. Shermadini, Z.; Luetkens, H.; Khasanov, R.; Krzton-Maziopa, A.; Conder, K.; Pomjakushina, E.; Klauss, H.-H.; Amato, A. Superconducting properties of single-crystalline AxFe2−ySe2 (A=Rb, K) studied using muon spin spectroscopy. Phys. Rev. B 2012, 85, 100501. [Google Scholar] [CrossRef]
  85. Lynn, J.W.; Zhou, X.; Borg, C.K.H.; Saha, S.R.; Paglione, J.; Rodriguez, E.E. Neutron investigation of the magnetic scattering in an iron-based ferromagnetic superconductor. Phys. Rev. B 2015, 92, 060510. [Google Scholar] [CrossRef]
  86. Nejasattari, F.; Stadnik, Z.M. Search for Fe magnetic ordering in the 40 K superconductor (Li0.8Fe0.2)OHFeSe. J. Alloy Compd. 2015, 652, 470. [Google Scholar] [CrossRef]
  87. McQueen, T.M.; Huang, Q.; Ksenofontov, V.; Felser, C.; Xu, Q.; Zandbergen, H.; Hor, Y.S.; Allred, J.; Williams, A.J.; Qu, D.; et al. Extreme sensitivity of superconductivity to stoichiometry in Fe1+δSe. Phys. Rev. B 2009, 79, 014522. [Google Scholar] [CrossRef]
  88. Sun, Y.; Park, A.; Pyon, S.; Tamegai, T.; Kambara, T.; Ichinose, A. Effects of heavy-ion irradiation on FeSe. Phys. Rev. B 2017, 95, 104514. [Google Scholar] [CrossRef]
  89. Ying, J.J.; Wang, X.F.; Luo, X.G.; Li, Z.Y.; Yan, Y.J.; Zhang, M.; Wang, A.F.; Cheng, P.; Ye, G.J.; Xiang, Z.J.; et al. Pressure effect on superconductivity of AxFe2Se2 (A = K and Cs). New J. Phys. 2011, 13, 033008. [Google Scholar] [CrossRef]
  90. Dong, X.; Zhou, H.; Yang, H.; Yuan, J.; Jin, K.; Zhou, F.; Yuan, D.; Wei, L.; Li, J.; Wang, X.; et al. Phase Diagram of (Li1–xFex)OHFeSe: A bridge between iron selenide and arsenide superconductors. J. Am. Chem. Soc. 2014, 137, 66–69. [Google Scholar] [CrossRef] [PubMed]
  91. Wu, Y.P.; Zhao, D.; Lian, X.R.; Lu, X.F.; Wang, N.Z.; Luo, X.G.; Chen, X.H.; Wu, T. NMR evidence for field-induced ferromagnetism in (Li0.8Fe0.2)OHFeSe superconductor. Phys. Rev. B 2015, 91, 125107. [Google Scholar] [CrossRef]
  92. Topping, C.V.; Kirschner, F.K.; Blundell, S.J.; Baker, P.J.; Woodruff, D.N.; Schild, F.; Sun, H.; Clarke, S.J. Coexistence of magnetism and superconductivity in separate layers of the iron-based superconductor (Li1−xFex)OHFe1−ySe. Phys. Rev. B 2017, 95, 134419. [Google Scholar] [CrossRef]
  93. Luetkens, H.; Klauss, H.-H.; Kraken, M.; Litterst, F.J.; Dellmann, T.; Klingeler, R.; Hess, C.; Khasanov, R.; Amato, A.; Baines, C.; et al. The electronic phase diagram of the LaO1−xFxFeAs superconductor. Nat. Mater. 2009, 8, 305–309. [Google Scholar] [CrossRef] [PubMed]
  94. Keller, H.; Bussmann-Holder, A.; Müller, K.A. Jahn–Teller physics and high-Tc superconductivity. Mater. Today 2008, 11, 38–46. [Google Scholar] [CrossRef]
  95. Johnston, D.C. The puzzle of high temperature superconductivity in layered iron pnictides and chalcogenides. Adv. Phys. 2010, 59, 803–1061. [Google Scholar] [CrossRef]
  96. Fang, M.-H.; Wang, H.-D.; Dong, C.-H.; Li, Z.-J.; Feng, C.-M.; Chen, J.; Yuan, H.Q. Fe-based superconductivity with Tc = 31 K bordering an antiferromagnetic insulator in (Tl,K) FexSe2. Europhys. Lett. 2011, 94, 27009. [Google Scholar] [CrossRef]
  97. Wang, A.F.; Ying, J.J.; Luo, X.G.; Yan, Y.J.; Liu, D.Y.; Xiang, Z.J.; Cheng, P.; Ye, G.J.; Zou, L.J.; Sun, Z.; et al. A crossover in the phase diagram of NaFe1− xCoxAs determined by electronic transport measurements. New J. Phys. 2013, 15, 043048. [Google Scholar] [CrossRef]
  98. Chen, H.; Ren, Y.; Bao, W.; Liu, R.H.; Wu, G.; Wu, T.; Xie, Y.L.; Wang, X.F.; Huang, Q.; Chen, X.H. Coexistence of the spin-density wave and superconductivity in Ba1–xKxFe2As2. Europhys. Lett. 2009, 85, 17006. [Google Scholar] [CrossRef]
  99. Zhang, A.-M.; Xia, T.-L.; Liu, K.; Tong, W.; Yang, Z.-R.; Zhang, Q.-M. Superconductivity at 44 K in K intercalated FeSe system with excess Fe. Sci. Rep. 2013, 3, 1216. [Google Scholar] [CrossRef] [PubMed]
  100. Yan, Y.J.; Zhang, M.; Wang, A.F.; Ying, J.J.; Li, Z.Y.; Qin, W.; Luo, X.G.; Li, J.Q.; Hu, J.; Chen, X.H. Electronic and magnetic phase diagram in KxFe2−ySe2 superconductors. Sci. Rep. 2012, 2, 212. [Google Scholar] [CrossRef] [PubMed]
  101. Li, W.; Ding, H.; Deng, P.; Chang, K.; Song, C.; He, K.; Wang, L.; Ma, X.; Hu, J.-P.; Chen, X.; et al. Phase separation and magnetic order in K-doped iron selenide superconductor. Nat. Phys. 2012, 8, 126–130. [Google Scholar] [CrossRef]
  102. Ryu, G.H.; Zhang, G.Y.; Yu, G.; Chou, M.C.; Lin, C.T. Anisotropic behavior in (Li1−xFex)OHFeSe superconducting single crystal. (Unpublished).
  103. Chen, G.F.; Li, Z.; Li, G.; Zhou, J.; Wu, D.; Dong, J.; Hu, W.Z.; Zheng, P.; Chen, Z.J.; Yuan, H.Q.; et al. Superconducting properties of the Fe-based layered superconductor LaFeAsO0.9F0.1−δ. Phys. Rev. Lett. 2008, 101, 057007. [Google Scholar] [CrossRef] [PubMed]
  104. Cheng, P.; Yang, H.; Jia, Y.; Fang, L.; Zhu, X.; Mu, G.; Wen, H.-H. Hall effect and magnetoresistance in single crystals of NdFeAsO1−xFx (x = 0 and 0.18). Phys. Rev. B 2008, 78, 134508. [Google Scholar] [CrossRef]
  105. Palstra, T.T.M.; Batlogg, B.; Schneemeyer, L.F.; Waszczak, J.V. Thermally activated dissipation in Bi2.2Sr2Ca0.8Cu2O8+δ. Phys. Rev. Lett. 1988, 61, 1662–1665. [Google Scholar] [CrossRef] [PubMed]
  106. Yuan, H.Q.; Singleton, J.; Balakirev, F.F.; Baily, S.A.; Chen, G.F.; Luo, J.L.; Wang, N.L. Nearly isotropic superconductivity in (Ba,K)Fe2As2. Nature 2008, 457, 565–568. [Google Scholar] [CrossRef] [PubMed]
  107. Werthamer, N.R.; Helfand, E.; Hohenberg, P.C. Temperature and purity dependence of the superconducting critical field, Hc2. III. electron spin and spin-orbit effects. Phys. Rev. 1966, 147, 295–302. [Google Scholar] [CrossRef]
  108. Gyorgy, E.M.; van Dover, R.B.; Jackson, K.A.; Schneemeyer, L.F.; Waszczak, J.V. Anisotropic critical currents in Ba2YCu3O7 analyzed using an extended Bean model. Appl. Phys. Lett. 1989, 55, 283–285. [Google Scholar] [CrossRef]
  109. Larbalestier, D.C.; Cooley, L.D.; Rikel, M.O.; Polyanskii, A.A.; Jiang, J.; Patnaik, S.; Cai, X.Y.; Feldmann, D.M.; Gurevich, A.; Squitieri, A.A.; et al. Strongly linked current flow in polycrystalline forms of the superconductor MgB2. Nature 2001, 410, 186–189. [Google Scholar] [CrossRef] [PubMed]
  110. Dew-Hughes, D. Flux pinning mechanisms in type II superconductors. Philos. Mag. 1974, 30, 293–305. [Google Scholar] [CrossRef]
  111. Lei, H.C.; Petrovic, C. Giant increase in critical current density of KxFe2−ySe2 single crystals. Phys. Rev. B 2011, 84, 212502. [Google Scholar] [CrossRef]
  112. Li, M.; Chen, L.; You, W.L.; Ge, J.; Zhang, J. Giant increase of critical current density and vortex pinning in Mn doped KxFe2−ySe2 single crystals. Appl. Phys. Lett. 2014, 105, 192602. [Google Scholar] [CrossRef]
  113. Chen, W.; Zeng, C.; Kaxiras, E.; Zhang, Z. Dual role of Fe dopants in enhancing stability and charge transfer in (Li0.8Fe0.2)OHFeSe superconductors. Phys. Rev. B 2016, 93, 064517. [Google Scholar] [CrossRef]
  114. Yan, Y.J.; Zhang, W.H.; Ren, M.Q.; Liu, X.; Lu, X.F.; Wang, N.Z.; Niu, X.H.; Fan, Q.; Miao, J.; Tao, R.; et al. Surface electronic structure and evidence of plain s-wave superconductivity in (Li0.8Fe0.2)OHFeSe. Phys. Rev. B 2016, 94, 134502. [Google Scholar] [CrossRef]
  115. Du, Z.; Yang, X.; Fang, D.; Du, G.; Xing, J.; Yang, H.; Zhu, X.; Wen, H.H. Scrutinizing the double superconducting gaps and strong coupling pairing in (Li1−xFex)OHFeSe. Nat. Commun. 2016, 7, 10565. [Google Scholar] [CrossRef] [PubMed]
Figure 1. Schematic illustrations of the (a) self-flux; and (b) Bridgman growth methods.
Figure 1. Schematic illustrations of the (a) self-flux; and (b) Bridgman growth methods.
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Figure 2. Optical floating-zone (OFZ) growth setup (left); and snapshot (right) of the real iron-based superconductor growth process.
Figure 2. Optical floating-zone (OFZ) growth setup (left); and snapshot (right) of the real iron-based superconductor growth process.
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Figure 3. (a) Schematic drawing illustrates the OFZ growth process of the AxFe2−ySe2 single crystals, which form between the solid–liquid interface temperature (Ti) and the decomposition temperature (Td), below which the crystals undergo a continuous decomposition during the molten zone traveling; (b) a typical as-grown K0.80Fe1.81Se2 single-crystal ingot. Inset shows a crystal cleaved along the growing direction; (c,d) single crystals cut off from ingots of Rb0.80Fe1.81Se2 and Cs0.80Fe1.81Se2, respectively.
Figure 3. (a) Schematic drawing illustrates the OFZ growth process of the AxFe2−ySe2 single crystals, which form between the solid–liquid interface temperature (Ti) and the decomposition temperature (Td), below which the crystals undergo a continuous decomposition during the molten zone traveling; (b) a typical as-grown K0.80Fe1.81Se2 single-crystal ingot. Inset shows a crystal cleaved along the growing direction; (c,d) single crystals cut off from ingots of Rb0.80Fe1.81Se2 and Cs0.80Fe1.81Se2, respectively.
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Figure 4. Growth of (Li1−xFex)OHFeSe single crystals via hydrothermal ion exchange. (a) Schematic apparatus of the autoclave used for the crystal growth; (b) illustration of the Li/Fe–O–H ions diffusing in between the FeSe layers and the A ions diffusing out from the AxFe2−ySe2 (A = K, Rb, and Cs) precursors during the hydrothermal growth process [53]. Reprinted with permission from IOP. All rights reserved.
Figure 4. Growth of (Li1−xFex)OHFeSe single crystals via hydrothermal ion exchange. (a) Schematic apparatus of the autoclave used for the crystal growth; (b) illustration of the Li/Fe–O–H ions diffusing in between the FeSe layers and the A ions diffusing out from the AxFe2−ySe2 (A = K, Rb, and Cs) precursors during the hydrothermal growth process [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 5. (a) As-grown ion-exchanged (Li1−xFex)OHFeSe single crystals; (b) [(Li1−xFex)OH]( Fe1−yLiy)]Se by-products; (c) the (100)/(010) plane showing cracked layers along the (001) after ion exchange process; (d) the as-cleaned (001) surface after ion exchange process [53]. Reprinted with permission from IOP. All rights reserved.
Figure 5. (a) As-grown ion-exchanged (Li1−xFex)OHFeSe single crystals; (b) [(Li1−xFex)OH]( Fe1−yLiy)]Se by-products; (c) the (100)/(010) plane showing cracked layers along the (001) after ion exchange process; (d) the as-cleaned (001) surface after ion exchange process [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 6. Crystal structure of (Li1−xFex)OHFeSe [53]. Reprinted with permission from IOP. All rights reserved.
Figure 6. Crystal structure of (Li1−xFex)OHFeSe [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 7. (ac) The X-ray diffraction (XRD) patterns of the cleaved parent A0.80Fe1.81Se2 (A = K, Rb, and Cs) and the ion-exchanged (Li1−xFex)OHFeSe crystals, respectively [53]. Reprinted with permission from IOP. All rights reserved.
Figure 7. (ac) The X-ray diffraction (XRD) patterns of the cleaved parent A0.80Fe1.81Se2 (A = K, Rb, and Cs) and the ion-exchanged (Li1−xFex)OHFeSe crystals, respectively [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 8. Dependence of the XRD patterns of the obtained (Li1−xFex)OHFeSe single crystals using the K0.80Fe1.81Se2 precursors on (a) t, (b) Tg, (c) cm. The insets show the effects of these three parameters on c, respectively [53]. Reprinted with permission from IOP. All rights reserved.
Figure 8. Dependence of the XRD patterns of the obtained (Li1−xFex)OHFeSe single crystals using the K0.80Fe1.81Se2 precursors on (a) t, (b) Tg, (c) cm. The insets show the effects of these three parameters on c, respectively [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 9.χT curves for the (a) K0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; (b) (Li1−xFex)OHFeSe single crystal using the Rb0.80Fe1.81Se2 precursor; (c) (Li1−xFex)OHFeSe single crystal using the Cs0.80Fe1.81Se2 precursor; Rab/RpT curves for the (d) K0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; the (e) Rb0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; the (f) Cs0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal. Rab is the the ab-plane resistance, and Rp is the maximum value of Rab [53]. Reprinted with permission from IOP. All rights reserved.
Figure 9.χT curves for the (a) K0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; (b) (Li1−xFex)OHFeSe single crystal using the Rb0.80Fe1.81Se2 precursor; (c) (Li1−xFex)OHFeSe single crystal using the Cs0.80Fe1.81Se2 precursor; Rab/RpT curves for the (d) K0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; the (e) Rb0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal; the (f) Cs0.80Fe1.81Se2 precursor and (Li1−xFex)OHFeSe single crystal. Rab is the the ab-plane resistance, and Rp is the maximum value of Rab [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 10. XRD patterns for the (Li1−xFex)OHFeSe single crystal which was synthesized at 180 °C for 72 h using the Cs0.80Fe1.81Se2 precursor. The inset is the 4πχT curve, showing the antiferromagnetic (AFM) spin density wave (SDW) transition occurs at Ts = 125.41 K [53]. Reprinted with permission from IOP. All rights reserved.
Figure 10. XRD patterns for the (Li1−xFex)OHFeSe single crystal which was synthesized at 180 °C for 72 h using the Cs0.80Fe1.81Se2 precursor. The inset is the 4πχT curve, showing the antiferromagnetic (AFM) spin density wave (SDW) transition occurs at Ts = 125.41 K [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 11. Phase diagram of (Li1−xFex)OHFeSe single crystals grown using A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors. The values of Ts and Tc were derived from the 4πχT curves [53]. Reprinted with permission from IOP. All rights reserved.
Figure 11. Phase diagram of (Li1−xFex)OHFeSe single crystals grown using A0.80Fe1.81Se2 (A = K, Rb, and Cs) precursors. The values of Ts and Tc were derived from the 4πχT curves [53]. Reprinted with permission from IOP. All rights reserved.
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Figure 12. Temperature-dependent RH of single-crystal (Li1−xFex)OHFeSe. The point-dashed curve is a guide for the eyes.
Figure 12. Temperature-dependent RH of single-crystal (Li1−xFex)OHFeSe. The point-dashed curve is a guide for the eyes.
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Figure 13. ρab-T relations for (a) H || ab and (b) H || c. (c) ΔT-μ0H and η-μ0H relations. ηmax = 1.67 at μ0H = 3 T.
Figure 13. ρab-T relations for (a) H || ab and (b) H || c. (c) ΔT-μ0H and η-μ0H relations. ηmax = 1.67 at μ0H = 3 T.
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Figure 14. Arrhenius plot of ρab for (a) H || ab and (b) H || c. (c) μ0H dependence of U0 and σ. Inset of (c) shows the same data on a double-logarithmic scale.
Figure 14. Arrhenius plot of ρab for (a) H || ab and (b) H || c. (c) μ0H dependence of U0 and σ. Inset of (c) shows the same data on a double-logarithmic scale.
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Figure 15. (a) μ0Hc2-T relations; (b) the zero-temperature upper critical fields are extrapolated using Werthamer-Helfand-Hohenberg (WHH) formula and linear extrapolation (LE) method.
Figure 15. (a) μ0Hc2-T relations; (b) the zero-temperature upper critical fields are extrapolated using Werthamer-Helfand-Hohenberg (WHH) formula and linear extrapolation (LE) method.
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Figure 16. Magnetization hysteresis loops for (a) H || ab and (b) H || c.
Figure 16. Magnetization hysteresis loops for (a) H || ab and (b) H || c.
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Figure 17. μ0H dependence of (a) J c c and (b) J c ab at different temperatures; (c) Jc-T and δ-T relations in the temperature range 2–30 K, 2.62 ≤ δ ≤ 3.48.
Figure 17. μ0H dependence of (a) J c c and (b) J c ab at different temperatures; (c) Jc-T and δ-T relations in the temperature range 2–30 K, 2.62 ≤ δ ≤ 3.48.
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Figure 18. Fp/ F p max as a function of h = H/Hirr for (a) H || ab and (b) H || c. The solid curves are fits of the data to the formula Fp/ F p max hp(1−h)q.
Figure 18. Fp/ F p max as a function of h = H/Hirr for (a) H || ab and (b) H || c. The solid curves are fits of the data to the formula Fp/ F p max hp(1−h)q.
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Zhang, G.-Y.; Chou, M.M.-C.; Lin, C.-T. (Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties. Crystals 2017, 7, 167. https://doi.org/10.3390/cryst7060167

AMA Style

Zhang G-Y, Chou MM-C, Lin C-T. (Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties. Crystals. 2017; 7(6):167. https://doi.org/10.3390/cryst7060167

Chicago/Turabian Style

Zhang, Guo-Yong, Mitch Ming-Chi Chou, and Cheng-Tian Lin. 2017. "(Li1−xFex)OHFeSe Superconductors: Crystal Growth, Structure, and Electromagnetic Properties" Crystals 7, no. 6: 167. https://doi.org/10.3390/cryst7060167

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