High-Temperature Layered Modification of Mn₂In₂Se₅

Abstract

Layered chalcogenides are interesting from the point of view of the formation of two-dimensional magnetic systems for relevant applications in spintronics. High-spin Mn²⁺ or Fe³⁺ cations with five unpaired electrons are promising in the search for compounds with interesting magnetic properties. In this study, a new layered modification of the Mn₂In₂Se₅ compound from the A₂B₂X₅ family (“225”) was synthesized and investigated. A phase transition to the polymorph with primitive trigonal lattice was recorded at a temperature of 711 °C, which was confirmed by simultaneous thermal analysis, X-ray powder diffraction at elevated temperatures, and sample annealing and quenching. The stability of Mn₂In₂Se₅ in air at high temperatures was investigated by thermal gravimetric analysis and powder X-ray diffraction. The new polymorph of Mn₂In₂Se₅ crystallizes in the Mg₂Al₂Se₅ structure type, as revealed by the Rietveld refinement against powder X-ray diffraction data. The crystal structure can be viewed as a close-packing of Se anions, in which indium and manganese cations are enclosed inside tetrahedral and octahedral voids, respectively, according to the A_MnB_InCB_InC_MnA… sequence. Magnetization measurements reveal an antiferromagnetic-like transition at a temperature of 6.3 K. The same magnetic properties are reported in the literature for the low-temperature R-centered trigonal polymorph. An approximation by the modified Curie–Weiss law yields a significant ratio of |θ|/T_N = 28, which indicates strong magnetic frustration.

1. Introduction

Layered compounds possess crystal structures, which can be represented as blocks containing several atomic layers with relatively strong chemical bonds, while at the boundaries of blocks, only low-energy Van der Waals forces are present. Layered compounds are interesting for research for three main reasons: their functional properties, their use in the production of two-dimensional and pseudo-two-dimensional materials, and the possibility of chemical composition modification while preserving the overall structure [1,2]. Two-dimensional materials are unique in terms of their electronic and magnetic properties that find applications in developing spintronics technologies [3,4].

Often, the structure of ternary and more complex layered compounds inherits the base layers of binary representatives modified into larger blocks. Such families may contain a large number of representatives, which makes them a perfect playground for finding novel functional materials. Ternary families with a layered crystal structure include compounds with the stoichiometry of AB₂X₄ and A₂B₂X₅, where A is a two-charge cation (usually Mg, Mn, Fe or Zn, Pb), B is a three-charge cation (for example, Al, Ga, In, Sb, Bi), and X is a chalcogen, S, Se or Te.

The family of layered chalcogenides with AB₂X₄ (“124”) stoichiometry is represented by the following prototypes: MgAl₂Se₄ (MgAl₂Se₄, hR21, 166), FeGa₂S₄ (FeGa₂S₄, hP7, 164), MnBi₂Te₄ (MnBi₂Te₄, hR7, 166), and FeIn₂Se₄ (FeIn₂Se₄, hR7, 166) [5,6,7,8,9]. The crystal structures of all four representatives consist of blocks, where the two-charge cations octahedrally coordinated by chalcogen are located in the center of the block, and two outer layers of a three-charge cations enclose the block (Figure 1). Three-charge cations Al and Ga prefer tetrahedral coordination by chalcogen, while bismuth—octahedral. The unit cell of the FeGa₂S₄ structure type contain one block and one formula unit, while MgAl₂Se₄ and MnBi₂Te₄—three. The A₂B₂X₅ (“225”) family includes four main types of layered structures: Mg₂Al₂Se₅ (Mg₂Al₂Se₅, hP9, 164), Fe₂Ga₂S₅ (Fe₂Ga₂S₅, hP18, 194), Mn₂In₂Se₅ ((Mn_0.5In_0.5)₄Se₅, hR27, 166), and Pb₂Bi₂Te₅ (Pb₂Bi₂Se₅, hP9, 164) [6,10,11,12]. The block in the structure of these compounds is similar to the block of the homologous “124” family, but with one additional layer of two-charge cations in the center. The “124” and “225” families demonstrate unique structural flexibility, where both the alternation of octahedral and tetrahedral sites and the mixed occupation of crystallographic positions are possible due to the close values of ionic radii of A and B cations. For example, as many as 16 layered polytypes were observed for the ZnIn₂S₄ compound [13,14]. The “124” and “225” compounds, which are based on a transition metal with a partially filled 3D shell, attract special interest due to their unique magnetic properties, including strong frustration on a triangular lattice [15,16,17], and nearly two-dimensional antiferromagnetic ordering [18].

Previously in the “124” and “225” families, we discovered new layered Mn-based compounds, MnAl₂S₄, MnAl₂Se₄, Mn₂Al₂Se₅, and Mn₂Ga₂S₅, with MgAl₂Se₄- and Mg₂Al₂Se₅-type structures [19,20]. Interestingly, MnIn₂Se₄ [21] and Mn₂In₂Se₅ [11,22,23] also possess layered crystal structures, where the latter crystallizes in the R−3m space group (in the context of this work, this polymorph is indicated as “R-phase”). This R-phase demonstrates strong magnetic frustration and a significant spin-glass contribution at low temperatures [11,22,23]. In this study, we revisited the Mn-In-Se ternary system and surprisingly found interesting phase relations, which are not in agreement with those reported recently [11,22,23]. Particularly, we found that the reaction of MnIn₂Se₄+MnSe→Mn₂In₂Se₅ yields a new polymorphic modification of Mn₂In₂Se₅, which fits the “225” family well. Here, we present the synthesis, crystal structure, oxidation resistance, and magnetic properties of the new high-temperature layered polymorph of Mn₂In₂Se₅.

2. Results and Discussion

2.1. Synthesis of Polycrystalline Mn₂In₂Se₅

Numerous syntheses of polycrystalline samples of Mn₂In₂Se₅, accompanied by careful analysis of phase composition and Le Bail fitting of powder X-ray diffraction patterns, clearly indicate the formation of non-single-phase products, where MnSe is the most probable admixture. To optimize synthetic conditions, the annealing temperature was varied in the range between 700 °C and 1100 °C, the annealing time was increased up to 240 h, and intermediate grinding was introduced. Also, samples were pressed into pellets and the use of crucibles was tested to reduce possible side reactions. All these precautions, as well as the synthesis from the binary precursors MnSe and In₂Se₃, yielded the same non-single-phase composition, indicating that either the starting ratio of elements should be non-stoichiometric for the formation of the “Mn₂In₂Se₅” phase, or this phase undergoes decomposition upon cooling. According to a recent report, Mn₂In₂Se₅ decomposes peritectically at 920 °C; however, no other reactions were observed below the decomposition temperature [22].

2.2. Elemental and Phase Composition and Thermal Analysis of Mn₂In₂Se₅

The elemental composition and its uniformity were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. Elemental maps registered across the surface of a pressed pellet clearly indicate areas with an increased fraction of manganese and a reduced content of indium, which correspond to the MnSe admixture (Figure 2). However, in the rest of the sample, the elements are distributed evenly, and the average composition of Mn_2.3(2)In_2.1(1)Se_4.7(2) is in quantitative agreement with the starting one. Obviously, the composition of the main phase does not deviate from the stoichiometric ratio; thus, we expect side reactions and more complex phase relations, which take place during the cooling of the polycrystalline sample from the annealing temperature.

Polycrystalline Mn₂In₂Se₅ was studied by differential scanning calorimetry in the mid-temperature range of 400–800 °C (Figure 3). The onset of a weak endothermic peak was observed under heating. This peak demonstrates reproducible behavior and slight temperature hysteresis. The onset temperatures of 722 °C and 699 °C during heating and cooling, respectively, correspond to the average temperature of the transition of 711 °C. To investigate this transition, we measured powder X-ray diffraction (PXRD) patterns at various elevated temperatures.

As already discussed, the as-prepared polycrystalline sample of Mn₂In₂Se₅ is not single-phase and contains the admixture of MnSe. All reflections of Mn₂In₂Se₅ at room temperature can be indexed using the R-centered trigonal unit cell with a = 4.02381(8) Å and c = 48.858(1) Å, in perfect agreement with the previous reports [11,22,23]. The main peak of MnSe is located at 2θ = 32.9° on the room-temperature PXRD pattern (Figure 4). This mixture of phases persists upon heating up to 400 °C, and then Mn₂In₂Se₅ decomposes to MnIn₂Se₄. The main peak of MnIn₂Se₄ is located at 2θ = 20.2°. In the temperature range of 600–700 °C MnIn₂Se₄ and Mn₂In₂Se₅ coexist, while at 750–800 °C the mixture of MnIn₂Se₄ and MnSe was observed. It should be noted that the low-temperature decomposition of Mn₂In₂Se₅ into MnIn₂Se₄ and MnSe is not accompanied by any peak on the DSC curve. Presumably, this decomposition goes gradually in the temperature range, where both Mn₂In₂Se₅ and MnIn₂Se₄ coexist. Finally, the reaction of MnIn₂Se₄+MnSe→Mn₂In₂Se₅ takes place at high temperatures above 800 °C, and the target compound Mn₂In₂Se₅ is formed as the main phase again at 850 °C. Surprisingly, the reflections of Mn₂In₂Se₅ at a high temperature correspond to a novel unit cell, which is primitive trigonal or hexagonal with a = 4.0742(1) Å and c = 16.4671(5) Å at 850 °C. Thus, the reaction of MnIn₂Se₄+MnSe→Mn₂In₂Se₅ yields a new polymorph of the target compound, which will be indicated as “P-phase” later in the text. Moreover, the formation of Mn₂In₂Se₅ at high temperatures is accompanied by the appearance of new peaks, which are marked by the hash symbols in Figure 4. These peaks can be indexed using a primitive trigonal or hexagonal unit cell with lattice parameters of a = 8.990(3) Å and c = 6.912(3) Å, indicating the formation of an unknown X phase in the Mn-In-Se system.

Combining the results of differential scanning calorimetry and PXRD at elevated temperatures, the observed effect at 711 °C presumably corresponds to the formation of Mn₂In₂Se₅ according to the reaction, MnIn₂Se₄+MnSe→Mn₂In₂Se₅, while its endothermic nature is connected with the fact that the Mn₂In₂Se₅ and MnIn₂Se₄ homologous compounds possess similar layered crystal structures. Thus, the formation reaction may be endothermic in this case. PXRD measurements reveal a slightly higher temperature above 800 °C for this reaction than differential scanning calorimetry.

The results of high-temperature PXRD measurements were corroborated by standard ampule synthesis. A polycrystalline stoichiometric sample of Mn₂In₂Se₅ was annealed at 900 °C for 60 h, and the ampule was subsequently quenched in cold water. As a result, the novel P-phase was stabilized at room temperature with only a tiny admixture of MnSe. No traces of the X phase were observed in this case. At the same time, the P-phase reproducibly transforms into the R-phase upon repeated annealing, as well as slowly decomposing into the mixture of MnIn₂Se₄ and MnSe under prolonged annealing at 200 °C or 300 °C.

2.3. Oxidation of Mn₂In₂Se₅

Chalcogenides, especially sulfides and selenides, may be unstable in air and react with oxygen or water vapor. Also, oxygen atoms may be present in the crystal structure replacing S or Se. To avoid possible inaccuracies in the chemical composition related to the reaction with oxygen and water, we investigated various aspects of the stability of Mn₂In₂Se₅ in air (Figure 5). Thermal gravimetric analysis in ambient air was used to investigate the oxidation process. Combined thermal analysis shows a single-stage mass loss from 500 °C to 600 °C accompanied by a weak exothermic effect. The measured mass loss of 41.4% is in good agreement with the following oxidation equation:

$$6{Mn}_2{In}_2{Se}_5+47O_2 \stackrel{T}{\to } 4{Mn}_3{O}_4+6{In}_2{O}_3+30Se{O}_2\uparrow ,$$

(1)

The reaction products were analyzed by powder X-ray diffraction (Figure 6). Phase analysis clearly indicates the presence of Mn₃O₄ and In₂O₃ in good agreement with the oxidation model proposed in Equation (1). Overall, Mn₂In₂Se₅ demonstrates good stability in ambient air, where no bulk oxidation occurs at temperatures at least below 500 °C.

2.4. Crystal Structure of the New Polymorph of Mn₂In₂Se₅

The crystal structure of the new layered polymorph of Mn₂In₂Se₅ was obtained by the Rietveld refinement against the high-temperature PXRD data, as well as by using the room-temperature PXRD pattern of polycrystalline sample quenched from 900 °C. Both PXRD patterns can be indexed in the primitive trigonal unit cell. For example, the high-temperature data yield unit cell parameters of a = 4.0742(1) Å and c = 16.4671(5) Å, which are close to those of Mg₂Al₂Se₅. The latter is a prototype structure for a number of layered “225” compounds, including Mn₂Ga₂S₅ and Mn₂Al₂Se₅ [6,20]. Therefore, the unit cell parameters of the crystal structure of Mg₂Al₂Se₅ were used as a starting model for the refinement. Mn and In atoms were placed on the Mg and Al sites, respectively. Data collection and refinement details are given in

Table 1. Details of data collection and structure refinement for the P-phase of Mn₂In₂Se₅.

Parameter	Value
sample	high-temperature	quenched from 900 °C
composition	Mn2In2Se5
formula weight (g/mol)	734.31
diffractometer	Bruker D8 Advance	Huber G670
detector	LYNXEYE	image plate
radiation	Cu Kα1,2	Cu Kα1
wavelength (Å)	1.5419	1.5406
crystal system	trigonal
space group	P$\overline{3}$m1
Z	1
unit cell parameters
a (Å)	4.0742(1)	4.02535(5)
c (Å)	16.4671(5)	16.3503(3)
V (Å3)	236.72(1)	229.438(6)
temperature (K)	1123	298
ρcalc (g/cm3)	5.15	5.31
μ (cm−1)	81.0	83.6
2θ range (deg)	5–80	3–100
Rp	0.0404	0.0257
wRp	0.0545	0.0379
Robs	0.1124	0.0923
wRobs	0.0937	0.1158
Rall	0.1278	0.0987
wRall	0.0967	0.1159
GOF	1.19	3.17
parameters	33	31
constraints	7	7
residual peaks (e−/Å3)	2.81/−3.30	4.71/−3.29

. The obtained parameters of atomic positions and selected interatomic distances are provided in

Table 2. Parameters of atomic positions for the crystal structure of P-phase of Mn₂In₂Se₅.

(a)	P-Phase of Mn2In2Se5 (850 °C)
Label	Symmetry	x	y	z	Occupancy	Uiso (Å2)
Mn1	3m	1/3	2/3	0.1048(1)	0.5Mn + 0.5In	0.150(2)
In1	3m	1/3	2/3	0.6631(2)	0.5In + 0.5Mn	0.032(2)
Se1	$\overline{3}$m	0	0	0	1	0.069(6)
Se2	3m	1/3	2/3	0.4014(3)	1	0.062(4)
Se3	3m	1/3	2/3	0.8145(3)	1	0.023(3)
(b)	P-Phase of Mn2In2Se5 (room temperature)
Label	Symmetry	x	y	z	Occupancy	Uiso (Å2)
Mn1	3m	1/3	2/3	0.1025(1)	0.704(2)Mn + 0.296In	0.0186(9)
In1	3m	1/3	2/3	0.66222(8)	0.704(2)In + 0.296Mn	0.0147(4)
Se1	$\overline{3}$m	0	0	0	1	0.0108(8)
Se2	3m	1/3	2/3	0.4017(1)	1	0.0171(7)
Se3	3m	1/3	2/3	0.8201(1)	1	0.0193(8)

and

Table 3. Selected interatomic distances in the crystal structure of Mn₂In₂Se₅.

P-Phase of Mn2In2Se5 (850 °C)
Central Atom	Neighbor Atom	Distance (Å)
Mn1	Se1 (×3)	2.917(1)
	Se3 (×3)	2.702(2)
In1	Se2 (×3)	2.581(2)
	Se3 (×1)	2.492(5)
P-phase of Mn2In2Se5 (room temperature)
Central atom	Neighbor atom	Distance (Å)
Mn1	Se1 (×3)	2.865(1)
	Se3 (×3)	2.647(1)
In1	Se2 (×3)	2.5483(9)
	Se3 (×1)	2.581(2)
R-phase of Mn2In2Se5 [11]
Central atom	Neighbor atom	Distance (Å)
Mn1	Se1 (×3)	2.8508(7)
	Se3 (×3)	2.6496(8)
In1	Se2 (×3)	2.5840(6)
	Se3 (×1)	2.516(1)

, respectively. The experimental and calculated PXRD patterns are shown in Figure 7.

According to the refinement, the new modification of Mn₂In₂Se₅ possesses a layered crystal structure, which is shown in Figure 8a. The structure can be described by blocks with the Se atoms on boundaries, and the adjacent blocks are separated by van der Waals gaps. Each block is formed by an alternation of closely packed layers of Se anions, where Mn and In cations populate octahedral and tetrahedral voids, respectively. The following sequence of layers describes a single structural block, A_MnB_InCB_InC_MnA..., with two-layered packing of Mn and In between the CBC... and BCB... layers, respectively. The crystal structure of the R-phase, which is shown in Figure 8b, is constructed in a similar way, but with a more complex arrangement of anionic layers: A_MnB_InAC_InB_MnC_MnA_InCB_InA_MnB_MnC_InBA_InC_MnA… with three-layered packing CABC… for Mn and a mixed BBAACCB… for In. Nevertheless, both the P- and R-phases possess similar atomic environments regarding Mn and In cations, as revealed by the observed interatomic distances (Table 3). If the occupancy is not taken into account, this new structure is identical to Mg₂Al₂Se₅ with the corresponding substitution of cations to Mn and In. The crystal structure of the P-phase of Mn₂In₂Se₅ is closely related to the Mg₂Al₂Se₅ structure type. It has the same coordination environment of atoms, content of the unit cell content, and connection of the adjacent blocks. At the same time, the Pb₂Bi₂Te₅ structure type has different environments of cations, while Fe₂Ga₂S₅ and the R-phase of Mn₂In₂Se₅ possess larger unit cells.

The refinement of displacement parameters clearly indicates that both Mn and In are mixed in the octahedral and tetrahedral positions. At room temperature, this mixing is moderate: the octahedral sites contain 70% of Mn, while tetrahedral—70% of In (Table 2). At high temperatures, antisite defects are largely present in the crystal structure, yielding the uniform distribution of Mn and In across octahedral and tetrahedral sites. Remarkably, the octahedral position has a large value displacement parameter at 850 °C, indicating possible structural instability in the vicinity of peritectic decomposition. The interatomic distances, which are listed in Table 3, point at the pronounced temperature expansion at 850 °C, too.

2.5. Magnetic Properties of Mn₂In₂Se₅

Given the investigated phase transformations and established synthesis protocols, two polymorphs of Mn₂In₂Se₅ were obtained independently with a minimal amount of admixtures. The magnetic properties of the P-phase and R-phase of Mn₂In₂Se₅ were probed by DC magnetization measurements, which are shown in Figure 9. According to the magnetic susceptibility data, both samples demonstrate Curie–Weiss-type paramagnetic behavior at elevated temperatures. An onset of antiferromagnetic-like transition is visible at a temperature of 6.3 K for the P-phase and at 6 K for the R-phase, which is suppressed by an external magnetic field. This transition is accompanied by a slight hysteresis of magnetization at 2 K, while at higher temperatures the magnetization follows simple paramagnetic behavior.

The magnetic susceptibility of the R-phase measured in 100 Oe magnetic field reveals the presence of impurity in the temperature range of 100–300 K, which is extremely characteristic of MnSe [24]. The contribution of impurity is suppressed by the increasing magnetic field. MnSe is always formed as a result of the partial decomposition of the main phase when attempting to obtain the R-phase at low temperatures, and it is observed by PXRD phase analysis, as indicated previously in the text. Otherwise, the magnetic properties of the P-phase and R-phase perfectly coincide, in agreement with the fact that both polymorphs are based on the same structural block (Figure 8).

An approximation of the high-temperature inverse magnetic susceptibility of the P-phase by the Curie–Weiss law yields a Weiss temperature of θ = −180 K with a ratio of |θ|/T_N = 28 (Figure 10). The observed negative value of θ indicates the strong exchange interaction of the antiferromagnetic nature between the neighboring paramagnetic centers, while large |θ|/T_N points at the significant frustration in the system. Thus, the low dimensionality and large frustration index of the P-phase are similar to those reported recently for the R-phase [23]. The effective magnetic moment of µ= 4.12 µ_B per Mn atom turns out to be significantly lower than expected for Mn²⁺ = 5.92 µ_B. The same deviation was observed in the isostructural compound, Mn₂Ga₂S₅ [20].

3. Materials and Methods

For the synthesis of Mn₂In₂Se₅, Mn (plates, 99%, Sigma-Aldrich, Steinheim, Germany), In (lump, 99.99%, Sigma-Aldrich, Steinheim, Germany), and Se (granules, 99.999%, Sigma-Aldrich, Steinheim, Germany) were used. All operations with precursors and samples were performed in an argon-filled glove box (Spectro-systems, p(H₂O, O₂) < 1 ppm). The precursors were weighed in a stoichiometric molar ratio with an accuracy of 0.1 mg. Before annealing, they were placed in a quartz ampule, which was evacuated to a residual pressure of 5 × 10⁻³ mbar and flame-sealed. The series of samples were prepared using annealing temperatures of 973 K, 1073 K, 1173 K, and 1273 K, each for 5 days with intermediate grinding. Additionally, pellet pressing and the use of a crucible inside an ampoule were tested. Samples were pressed into a pellet with a diameter of 6 mm at a pressure of 1200 kgf/cm². However, these samples exhibited multiphase composition based on the R-phase. To obtain the P-phase, the stoichiometric mixture of precursors enclosed inside an evacuated quartz ampule was annealed at 1173 K for 60 h and subsequently quenched in water.

Samples were studied by differential scanning calorimetry using a simultaneous thermal analyzer STA 449 F3 Jupiter in a high-purity argon flow of 240 mL/min. During the first measurement, the sample was heated twice to 800 °C and cooled to 400 °C at a rate of 10 °C/min. Secondly, the data between 600 °C and 900 °C were acquired at a lower rate of 3 °C/min. TG-DTA data were obtained on a Derivatograph Q-1500 D in air. The oxidation process was studied by heating the sample to 900 °C at a rate of 10 °C/min.

A phase composition study and crystal structure refinements were performed by powder X-ray diffraction. Room-temperature data were obtained on a Huber G670 Guinier camera (Cu Kα₁ radiation, Ge₁₁₁ monochromator, image plate detector). The samples were enclosed between two mylar films and fixed in a sample holder in an argon-filled glove box. High-temperature data were acquired using a Bruker D8 Advance diffractometer (Cu X-ray source, no monochromator, LYNXEYE detector) equipped with the XRK900 temperature chamber. Measurements were performed in a flow of high-purity argon at temperatures of 200, 400, 600, 650, 700, 750, 800 and 850 °C. The experimental and refinement details, parameters of atomic positions, and selected interatomic distances are given in Table 1, Table 2 and Table 3, respectively. Indexing was performed using the VISSER algorithm in the WinXPow program (version 2.25). The crystal structure was refined using the Jana2006 program [25].

Images of pellets, mapping, and analysis of the elemental composition were obtained using a scanning electron microscope (SEM) JSM JEOL6490-LV with an energy-dispersive X-ray (EDX) detection system INCA x-Sight at an accelerating voltage of 20 kV.

The magnetization of polycrystalline samples was measured using a Magnetic Properties Measurement System (Quantum Design, MPMS-XL5 SQUID). The measurements were performed in zero-field-cooling and field-cooling conditions. Temperature dependences were measured from 2 K to 300 K in magnetic fields of 0.01 T and 5 T and field dependences were measured at temperatures of 2, 30, and 300 K, while sweeping the magnetic field from −5 T to 5 T.

4. Conclusions

Mn₂In₂Se₅ is a layered van der Waals compound based on the magnetic Mn²⁺ species. Its layered crystal structure is formed by alternating hexagonal close-packing layers of Se. As a result, the Mn atoms feature a perfect triangular arrangement within the two adjacent layers of octahedral voids in the center of the van der Waals block. This triangular magnetic system can be easily prepared in a two-dimensional fashion due to low-energy van der Waals bonds between the structural blocks. Thus, Mn₂In₂Se₅ is a perfect system to study strong frustration on a triangular lattice in two dimensions. In this study, we carefully developed synthesis protocols to obtain high-quality polycrystalline samples of the new high-temperature polymorph of Mn₂In₂Se₅. This polymorph is stable against oxidation up to 400 °C; thus, stability in two-dimensional nanomaterials is expected, too. The bulk material demonstrates strong antiferromagnetic coupling between the paramagnetic centers, two-dimensionality, and large frustration, as revealed by magnetization measurements.