Synthesis, Structure, and Properties of EuLnCuSe₃ (Ln = Nd, Sm, Gd, Er)

Abstract

EuLnCuSe₃ (Ln = Nd, Sm, Gd, Er), due to their complex composition, should be considered new materials with the ability to purposefully change the properties. Samples of the EuLnCuSe₃ were prepared using Cu, rare earth metal, Se (99.99%) by the ampoule method. The samples were obtained by the crystallization from a melt and annealed at temperatures 1073 and 1273 K. The EuErCuSe3 crystal structure was established using the single-crystal particle. EuErCuSe₃ crystallizes in the orthorhombic system, space group Cmcm, KCuZrS₃ structure type, with cell parameters a = 4.0555 (3), b = 13.3570 (9), and c = 10.4602 (7) Å, V = 566.62 (6) ų. In structure EuErCuSe₃, erbium ions are coordinated by selenium ions in the octahedral polyhedron, copper ions are in the tetrahedral coordination, europium ions are between copper and erbium polyhedra layers and are coordinated by selenium ions as two-cap trigonal prisms. The optical band gap is 1.79 eV. At 4.7 K, a transition from the ferrimagnetic state to the paramagnetic state was detected in EuErCuSe₃. At 85 and 293 K, the compound is in a paramagnetic state. According to XRPD data, EuLnCuSe₃ (Ln = Nd, Sm, Gd) compounds have a Pnma orthorhombic space group of the Eu₂CuS₃ structure type. For EuSmCuSe₃, a = 10.75704 (15) Å, b = 4.11120 (5) Å, c = 13.37778 (22) Å. In the series of EuLnCuSe₃ compounds, the optical band gap increases 1.58 eV (Nd), 1.58 eV (Sm), 1.72 eV (Gd), 1.79 eV (Er), the microhardness of the 205 (Nd), 210 (Sm), 225 (Gd) 235 ± 4 HV (Er) phases increases, and the thermal stability of the phases increases significantly. According to the measurement data of differential scanning calorimetry, the EuNdCuSe₃ decomposes, according to the solid-phase reaction T = 1296 K, ΔH = 8.2 ± 0.8 kJ/mol. EuSmCuSe₃ melts incongruently T = 1449 K, ΔH = 18.8 ± 1.9 kJ/mol. For the EuGdCuSe_3, two (T_α↔β = 1494 K, ΔH_α↔β = 14.8 kJ/mol, T_β↔γ = 1530 K, ΔH_β↔γ = 4.8 kJ/mol) and for EuErCuSe₃ three polymorphic transitions (T_α↔β = 1561 K, ΔH_α↔β = 30.3 kJ/mol, T_β↔γ = 1579 K, ΔH_β↔γ = 4.4 kJ/mol, and T_γ↔δ = 1600 K, ΔH_γ↔δ = 10.1 kJ/mol). The compounds melt incongruently at the temperature of 1588 K, ΔH_melt = 17.9 ± 1.8 kJ/mol and 1664 K, ΔH_melt = 25.6 ± 2.5 kJ/mol, respectively. Incongruent melting of the phases proceeds with the formation of a solid solution of EuSe and a liquid phase.

1. Introduction

Starting with the works of James A. Ibers [1], compounds of general composition A^IILn^IIIBX₃, where A is an alkaline earth metal or europium, Ln is an element of the 4f subgroup, B is copper or silver, and X is a chalcogen, increasingly attract researcher’s attention [2,3,4,5]. The presence of 3d, 4f elements in the composition of quaternary compounds makes them promising materials in the field of catalysis [6,7], nonlinear optics [8,9,10,11,12,13,14], and thermoelectricity [15]. EuLnCuS₃ exhibit ferro- and ferrimagnetic transitions at temperatures 4.5–5.4 K [16]. The compounds belong to semiconductors and have a band gap of 1.63–2.61 eV, according to the UV spectroscopy data [3,17].

In the A^IILn^IIIBX₃ family, the formation of more than 200 complex chalcogenide compounds was predicted. More than 100 sulfide compounds were synthesized and investigated, and only 23 selenide compounds are known [8,18,19,20]. ALnCuSe₃ crystallized in the following structural types (ST): SrLuCuSe₃ type KCuZrS₃ Cmcm [21], SrGdCuSe₃, type Eu₂CuS₃ Pnma, SrLaCuSe₃ type Ba₂MnS₃ [20] Pnma, SrCeCuSe₃ type Ba₂MnS₃ [20], PbLuCuSe₃ PbTmCuSe₃ PbErCuSe₃ PbHoCuSe₃ PbDyCuSe₃ PbTbCuSe₃, PbGdCuSe₃ type Eu₂CuS₃ Pnma [3].

Complex sulfides EuLnCuS₃ crystallize in ST Ba₂MnS₃ SG Pnma (Nd), ST KZrCuS₃ SG Cmcm (Sm, Gd, Er). The optical band gap of EuErCuS₃ is 1.934 eV [22]. Ferromagnets are EuNdCuS₃ and EuSmCuS₃ with magnetic phase transition temperatures of 3.1 K and 3.1 K, respectively [23]. Ferrimagnets are EuGdCuS₃ (Tc = 5.2 K) [16] and EuErCuS₃ (Tc = 4.8 K) [22]. Above the temperatures of magnetic transitions, the compounds exhibit paramagnetic properties [16,23]. EuLnCuS₃ exhibit polymorphism (Sm, Gd, Er), melt incongruently at temperatures of 1470 K (Nd), 1574 K (Sm), 1720 K (Gd), and 1735 K (Er). Data on the structure and properties of EuLnCuSe₃ were not found in the literature.

For the synthesis of EuLnCuSe₃, rare earth elements were selected, which represent four tetrads of lanthanides, which would make it possible to trace the main regularities in the structure and properties of the compounds. Nd is in the first tetrad (La-Nd), Sm and Gd in the second tetrad (Sm-Gd), Gd simultaneously represents the third tetrad (Gd-Ho), and Er from the fourth tetrad (Er-Lu).

It is valuable to observe the structural parameters of several simple selenides related to the present study. The EuSe has a cubic NaCl-type structure, space group (SG) Fm$\overline{3}$m, a = 6.185 Å [24], and melts congruently at 2488 K [25]. The Nd₂Se₃ compound has a cubic structure, ST Th₃P₄ a = 8.871 Å. Sm₂Se₃, Gd₂Se₃ exist in the form of low-temperature modifications of the rhombic form α-La₂S₃ and high-temperature modifications of the Th₃P₄ type, a = 8.785 Å, a = 8.718 Å. Er₂Se₃ crystallized in SG Fddd, ST Sc₂S3, a = 8.085, b = 11.346 and c = 24.140 Å [26] All compounds melt congruently at temperatures T(Nd₂Se₃) = 2200 K, T(Sm₂Se₃) = 2150 ± 40 K [27], T(Gd₂Se₃) = 1943 K [28] T (Er₂Se₃) = 1520 K [29].

As known, copper semi-selenide forms a solid solution region, described by the formula Cu_2−xSe [30,31]. The Cu_1.999Se composition was chosen as the initial one, due to the precipitation of copper during the synthesis of the Cu₂Se composition [30]. Cu₂Se can exist in two polymorphic modifications: α-Cu₂Se, monoclinic system, a = 14.083, b = 20.481 and c = 4.145 Å, β = 90.4°, SG P2₁/n [32] and β-Cu₂Se, cubic system, a = 5.765 Å, SG F4$\overline{3}$m [32]. The α-Cu₂Se → β-Cu₂Se phase transition occurs at a temperature of 396 ± 15 K. The Cu_2−xSe phase melts congruently at 1403 K [33].

The aim of the article is to synthesize EuLnCuSe₃, to establish the structure, regularities in changes in crystal chemical parameters, optical band gap, thermal characteristics of compounds, and magnetic properties (Er).

2. Methods and Materials

2.1. Synthesis

Four types of samples of the EuLnCuSe₃ compound were obtained: polycrystalline ingots, powder, single crystal, and technical ceramics.

The following simple substances were used as starting reagents: Cu 99.99 at. % (OJSC «Uralredmet», Sverdlovsk, Russia), Se 99.99 at. % (JSC «Khimreaktiv», Yekaterinburg, Russia), and rare earth metals 99.99 at. % (TOPLUS, LTD, Guangzhou, China). Metal surfaces have been thoroughly brushed. The Cu plate was broken into pieces of 0.1–0.5 g. Each rare earth metal is ground with its own DeWALT DT5048 HSS-G steel drill. The surface condition of the drills did not change during the grinding process. The chip thickness was 2–10 microns. The Eu metal fibers have been mechanically separated. To reduce oxidation, metals are stored and crushed in an argon atmosphere. Weighed portions were taken, with an accuracy of 0.0001 g, on a METLER TOLEDO ME 204 balance. The stoichiometric batch weight was 5 g. Three parallel syntheses were performed. The mixture of metals was placed in a graphite crucible, with an inner diameter of 13 mm and a height of 35 mm, located in a quartz ampoule. Then, the ampoule was evacuated to a residual pressure of 0.1 Pa and sealed. The ampoules were placed in a muffle and heated from 473 to 1273 K, at a rate of 50 K per day. Then, the ampoules were kept at 1273 K for 50 h. The samples were melted in a high-frequency conversion unit. It was heated by increasing the power supplied to the inductor to the temperature at which the sample passed into the melt. The melting point was well established by visual inspection.

The samples crystallized from the melt had the shape of a cylinder, with a dense grain structure. Each of the samples is divided into two parts, which are placed in quartz ampoules; the ampoules are vacuumized and sealed.

Two series of annealed samples were obtained. At 1273 K, the samples were annealed for 50 h, at 1073 K for 500 h.

Powders of EuErCuSe₃ were obtained by grinding annealed samples in an agate mortar to particles with a linear size of 1–100 microns.

Monocrystal of EuErCuSe₃ with linear dimensions from 0.08 to 0.10 mm was picked out with a scalpel from a sample drained at 1070 K to study the crystal structure.

Technical ceramics of EuErCuSe₃ were obtained for measuring the magnetic properties. The compound powder was pressed into a cylinder, with a diameter of 4 mm and height of 4 mm, under standard conditions in a cylindrical mold with a punch pressure of 0.8 tons. The mold is made of steel C80W1, the punch and matrix have mirror surfaces. There is no interaction between the matrix and substance to be pressed. The cylinder of the joint was placed in a quartz ampoule, which was evacuated and sealed. After annealing at 1073 K for 500 h, the sample acquired the necessary mechanical properties for the manufacture of a cylinder of the sample with parallel planes.

Impurity phases.

It was found that the impurity oxide phases Er₂O₃ and EuO, contained on the surface of metal particles during heat treatment, form Er₂O₂Se and Er₂Si₂O₇. There is a clear relationship between the content of oxide phases in the starting materials and impurities in the samples after synthesis. According to the XRD data and microstructure analysis, EuErCuSe₃ contained of 1.9 mass (m.) % Er₂O₂Se (ICDD PDF Entry No 01-074-0352) [34], of 4.1 m. % Er₂Si₂O₇ (01-073-3003) [35].

The samples of EuLnCuSe3, after annealing, contained the following impurity phases: EuNdCuSe₃: 8.4 m. % NdCuSeO (ICDD PDF Entry No. 00-046-0437) [36], 3.3 m. % EuNdSe₂, 0.9 m. % EuNd₂Se₄ (01-077-7873) [37], EuSmCuSe₃: 7.4 m. % SmCuSeO (00-051-0065, 01-082-1480) [38], 1.4 m. % EuSmSe₂ (01-075-6214, 03-065-4615) [39], 0.9 m. % EuSm₂Se₄ (01-077-7899) [37], EuGdCuSe₃: 3.1 m. % GdCuSeO (00-051-0068; 01-085-2207) [40], and 1.9 m. % EuGdSe₂.

2.2. Analysis Methods

The X-ray powder diffraction (XRPD) patterns of samples, obtained by crystallization from the melt and annealed at 1073 K (XRPD), were collected at room temperature on a STADI-P (STOE) diffractometer in the transmission geometry, with a linear mini-PSD detector, using CuK_α1 radiation in the 2θ range from 5 to 120°, with the step of 0.02°. Polycrystalline silicon (a = 5.43075 (5) Å) was used as an external standard. Possible impurity phases were checked by comparing X-ray diffraction (XRPD) patterns with those given in the PDF2 database (ICDD, USA, release 2016). The crystal structure refinement was carried out with the GSAS program suite, using XRPD data [41,42]. The peak profiles were fitted with a pseudo-Voigt function, I(2θ) = x × L(2θ) + (1 − x) × G(2θ) (where L and G are the Lorentzian and Gaussian parts, respectively). The peak width angular dependence was described by the relation (FWHM)² = Utg²θ + Vtgθ + W. The background level was described by a combination of 36-order Chebyshev polynomials. The absorption correction function for a flat plate sample in the transmission geometry was applied.

A single-crystal XRD study of EuErCuSe₃ was performed, using a STOE IPDS II diffractometer with MoKα radiation and an image-plate detector. A sample, with dimensions of 0.08–0.10 mm, was isolated from a sample annealed at 1273 K. The parameters, characterizing the data collection and crystal structure refinement, are summarized in

Table 1. Main parameters of data collection and crystal structure refinement.

Crystal Data
Chemical Formula	EuErCuSe3
Mr	619.64
Crystal system, space group	Orthorhombic, Cmcm
Temperature (K)	298
a, b, c (Å)	4.0555 (3), 13.3570 (9), 10.4602 (7)
V (Å3)	566.62 (6)
Z	4
Radiation type	Mo Kα
No. of reflections for cell measurement	863
μ (mm−1)	48.44
Crystal size (mm) and shape	0.10 × 0.08 × 0.03, block
Data Collection
Absorption correction	Multi-scan (CrysAlis PRO 1.171.40.53)
Tmin, Tmax	0.047, 0.068
No. of measured, independent and observed (I > 2σ(I)) reflections	1336, 313, 270
Rint	0.080
(sin θ/λ)max (Å−1)	0.602
Range of h, k, l	h = −4→4, k = −12→16, l = −12→12
Refinement
R(F2 > 2σ(F2)), wR(F2), S	0.031, 0.070, 1.05
No. of reflections	313
No. of parameters	24
∆⟩max, ∆⟩min (e Å−3)	1.78, −2.08

. X-AREA [43] was used for the data collection and CrysAlis Pro [44], for data reduction and cell refinement. The crystal structure was solved with SHELXT [45] and refined using SHELXL-2018 [46] with ShelXle as a GUI [47]. The atomic parameters for all atoms were refined in the anisotropic approximation. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service [48] https://www.ccdc.cam.ac.uk/structures/ by quoting the deposition number CSD 2122151 (accessed on 20 October 2021).

The particle micromorphology and contents of chemical elements in samples was studied by scanning electron microscopy (SEM) Tescan Mira 3 LMU (Brno, Czech Republic). Microhardness was measured on an Shimadzu HMV-G21DT microhardness tester, at the load of 490.3 mN (50 g), using the HMV-G instrument software (Kyoto, Japan), and it was measured on a polycrystalline sample. Reflection spectra were recorded on a Shimadzu UV-3600 spectrophotometer (Kyoto, Japan). The studied sample was in the powder form. The studies were carried out on samples annealed at 1073 K.

Magnetic measurements were performed on a polycrystalline ceramic EuErCuSe₃ sample, shaped as a cylinder of 4 mm in diameter and 4 mm high. The sample mass M was 0.2888 g. To impart a mechanical strength to the sample, it was preliminarily pressed and annealed in an oven at the temperature of 1070 K for 250 h. The magnetic field dependences of the magnetic moment m of the sample, in the range of magnetic fields H = 0–±15 kOe and temperature range from liquid nitrogen to room temperature, were recorded on a vibrating sample magnetometer [49] with an electromagnet, designed by I.M. Puzey [50]. The study of the EuErCuSe₃ magnetic susceptibility in weak magnetic fields, in the temperature range from liquid helium to room temperature, was carried out with the use of an original SQUID (superconducting quantum interference device) magnetometer [51,52]. The temperature dependences of the sample magnetic moment m were measured in two modes: (1) during cooling, in the absence of magnetic field (ZFC—zero-field cooling), and (2) when cooling in a magnetic field (FC—cooling in a nonzero magnetic field).

Thermal analysis was performed on two independent units. The NETZSCH Jupiter STA 449 F3 device is equipped with a W-3% Re-W-25% Re thermocouple. A sample obtained by crystallization from a melt and annealed at 1073 K was used. A cylinder weighing 70–110 mg was cut out of the sample. The sample adhered tightly to the walls of a conical graphite crucible, with the volume of 0.1 mL, grade G-8. The survey was carried out in a helium atmosphere (99.99999 at. % Cryogen Russia). The results were processed using the Proteus analysis software [53]. The thermal analysis on a SETARAM SETSYS Evolution device, a PtRh-6%–PtRh-30%. A powder sample, weighing EuErCuSe3 100 mg, was taken from the same EuErCuSe3 sample. The compound crystals were placed in a quartz conical ampoule, evacuated, and sealed. The ampoule had a flat bottom, and the ampoule volume was 0.1 mL. The DSC results were processed using the SETSOFT 2000 program.

3. Results and Discussion

As shown in Figure 1, the EuErCuSe₃ samples were obtained by the crystallization from a melt (Figure 1b), as a powder (Figure 1c), and technical ceramics (Figure 1d). The phase compositions of the samples, obtained in the parallel synthesis, were qualitatively similar. The samples contained more than 94 mass % EuErCuSe₃, and the representative (XRPD) pattern is shown in Figure 1a (the sample annealed at 1073 K). The XRPD pattern of EuErCuSe₃ was fitted using the starting model, obtained from the single-crystal structure determination given below, and the results are in perfect agreement for both cell parameters and atom coordinates (See ESI). The diffractometry data of the EuErCuSe₃ compound are identified in the orthorhombic system SG Cmcm.

The crystal structure of EuErCuSe₃ is shown in Figure 2, and it is very similar to that of EuErCuS₃ [22], despite that the compounds belong to different structural types. The EuErCuSe₃ crystal structure can be described as the layers parallel to the (010) crystallographic plane. Each layer is built of ErSe₆ octahedra and CuSe₄ tetrahedra. Eu cations are located between these layers. The atom coordinates determined for the EuErCuSe₃ structure are presented in Table S1 (Supplementary Materials). The bond lengths and angles are summarized in Table S2 (Supplementary Materials). An increase in the metal-chalcogen bond lengths correlate with an increase in the ionic radius of selenium ion rSe²⁻ = 1.98 Å, as compared to that of the sulphur ion rS²⁻ = 1.84 Å [54]. The unit cell parameters of EuErCuSe₃ (Table 1) increase proportionally, in comparison with EuErCuS₃ a = 10.1005(2) Å, b = 3.91255(4) Å, c = 12.8480(2) Å; V = 507.737(14) ų, Z = 4, and ρ_x = 6.266 g/cm³ [22]. Complete structural data were deposited via the joint CCDC/FIZ Karlsruhe deposition service, with refcode CSD 2122151. In the series of EuLnCuSe₃ compounds, the structural type changes, in proportion to a decrease in the ionic radii of rare earth elements rNd³⁺ (coordination number = 6) = 0.983 Å, rSm³⁺ = 0.958 Å, rGd³⁺ = 0.938 Å [55], the values of the unit cell parameters monotonically decrease (Figure 3,

Table 2. Unit cell parameters of EuLnCuSe₃ compounds (the sample annealed at 1073 K).

Ln	Nd	Sm	Gd
Space group	Pnma	Pnma	Pnma
Structure type	Eu2CuS3 (Gd3NiSi2)	Eu2CuS3 (Gd3NiSi2)	Eu2CuS3 (Gd3NiSi2)
a, Å	10.87487 (18)	10.75704 (15)	10.67493 (22)
b, Å	4.13258 (6)	4.11120 (5)	4.09671 (7)
c, Å	13.36404 (22)	13.37778 (22)	13.39231 (31)
V, Å3	600.597 (16)	591.624 (14)	585.673 (21)
ρxrd, gm/cm3	6.589	6.767	6.914
Z	4	4	4

).

The EuErCuSe₃ sample was obtained by sintering ground metal selenides at 1073 K for 500 h. As seen in Figure 4, the EuErCuSe₃ phase crystals are predominantly oval with faceting elements. The average grain size is 5–30 μm. In some grains, the layered structure is clearly traced (Figure 4a). At individual points of the sintered powder, there are Er₂O₂Se grains, in which the oxygen content is detected (Figure 4b). A thin section was prepared from the cast annealed sample of EuErCuSe₃. According to the SEM data, chemical elements are evenly distributed in the sample (Figure 4c,

Table 3. Theoretical (B) and experimental (C) (according to the SEM data) contents of chemical elements in the EuErCuSe₃ samples.

Element	Theoretical Content Mass. %	Sample B Mass. %	Sample C Mass. %
Eu	24.5	25.3	25.3 ± 0.3
Er	27.0	27.5	26.4 ± 0.3
Cu	10.3	10.3	10.4 ± 0.3
Se	38.2	36.9	37.9 ± 0.3

). The constituent element ratio, within the limits of determination errors, is consistent with the nominal composition of the sample.

Several tens of imprints of a Vickers diamond indenter were obtained on a thin section of a polycrystalline sample of EuLnCuSe₃. The regular geometric imprints without cracks, or with one limited crack, were selected. The EuLnCuSe₃ polycrystal microhardness was determined as H = 205 (Nd), H = 210 (Sm), H = 225 (Gd) H = 235 ± 4 HV (Er), 235 ± 4 HV.

The reflection spectra were recorded in the range of 200–1400 nm. The corresponding Kubelka–Munk function for EuErCuSe₃ is shown in Figure 5, in comparison to the recently studied orthorhombic crystal EuErCuS₃ [22]. The bandgap of the latter is equal to 1.94 eV, and it was explained [22] by the presence of 5d electronic states of the Eu²⁺ ion at the conduction band bottom. The EuErCuSe₃ bandgap is a bit narrower, and it is equal to 1.79 eV. EuErCuSe₃ crystallizes in the space group different from that of EuErCuS₃. However, both crystal structures belong to the same orthorhombic symmetry class. Therefore, the EuErCuSe₃ bandgap narrowing, with respect to that of EuErCuS₃, could be ascribed mainly to the lower position of the subbands originating from the 5d states of the Eu²⁺ ion in the field of Se ion potential. An increase in the ionic radius of Ln³⁺ for elements of the beginning of the series further lowers the position of the 5d level, which causes a decrease in the bandgap of compounds: EuNdCuSe₃ 1.58 eV, EuSmCuSe₃ 1.58 eV, and EuGdCuSe₃ 1.72 eV. However, it should be noted that the behavior of the Kubelka–Munk function, for all complex selenenides in the regions below the values of the bandgap, is very peculiar.

While, in the case of EuErCuS₃, the behavior close to that typical for Urbach tail is observed below 1.94 eV; in the case of EuErCuSe₃, something like the formation of the additional narrower bangap, with a width below 1 eV, can be suspected.

This second bandgap may correspond to additional bands originating from the main composition of the material and positioned within the main bandgap or to the crystal structure, possibly a complex effect, including impurities. A further study with the help of ab initio simulations is desirable.

At temperatures 293 and 85 K, the EuErCuSe₃ mass magnetization:

σ = m/M

is directly proportional to the strength of magnetic field H (Figure 6a), which indicates that the substance is in a paramagnetic state.

At a temperature of several kelvin, a magnetically-ordered state is realized in the substance under study. There is a difference in the temperature dependences of the magnetizations σ(T), taken in the ZFC and FC modes (see Figure 6b). The sample has a remanent magnetization σ_R at T = 4.2 K (see inset in Figure 6b). The temperature of the magnetic phase transition is Tc = 4.7 K. The temperature dependence of the inverse magnetic susceptibility (Figure 6c) is typical for magnets with a ferrimagnetic type of ordering [52].

1/χ = H/σ

At high temperatures, the magnetic susceptibility χ obeys the Curie-Weiss law. The dependence 1/χ (T) is linear:

$$\chi \left(T\right)=\frac{C}{T-\Theta } ,$$

where C and Θ are constants. With decreasing temperature, the inverse magnetic susceptibility 1/χ is decreased steeply, tending to zero, as T→Tc. Linear extrapolation of the experimental dependence 1/χ (T), in the temperature range 100 < T < 273 K, gives for EuErCuSe₃ the following Curie-Weiss constant and paramagnetic Curie temperature values: C = 0.031 K cm³/g; Θ = –1 K, respectively.

Presumably, below the temperature of the magnetic phase transition, the magnetic moments of rare-earth ions Eu²⁺ (7.94 μB) and Er³⁺ (9.59 μB) [23,56] form two magnetic sublattices with a mutual anticollinear orientation and uncompensated total magnetic moment. A more accurate picture of the magnetic sublattices behavior can be obtained by neutron diffraction studies.

Temperatures, enthalpies of melting of EuLnCuSe₃ (Nd, Sm, Gd, Er), were determined by finding of samples in evacuated quartz ampoules, as well as in a graphite crucible (Er) (Figure 7, Figure 8 and Figure 9,

Table 4. Temperature (K) and phase transition enthalpy (kJ mol¹) values in the EuErCuSe₃ compound.

NETZSCH Jupiter STA 449 F3
Tα↔β	ΔHα↔β	Tβ↔γ	ΔHβ↔γ	Tγ↔δ	ΔHγ↔δ
1561	31.4	1579	4.4	1600	10.1
SETARAM SETSYS Evolution
1561	30.7	1579	4.6	1600	10.7

and

Table 5. Temperatures and enthalpies of melting of EuLnCuSe₃ compounds.

Compound	Melting Type	Tmelt, K	ΔHmelt, kJ/mol	Compound	Melting Type	Tmelt, K	ΔHmelt, kJ/mol
EuLaCuSe3	Solid phase decay	1202	2.6	EuSmCuSe3	Incongruent	1449	18.8
EuCeCuSe3		1256	5.5	EuGdCuSe3		1588	17.9
EuNdCuSe3		1296	8.2	EuHoCuSe3		1645	-

). The thermal stability of the compounds increases markedly, depending on rLn³⁺ (Figure 9). EuNdCuSe₃ decomposes by a solid-phase reaction (Figure 7a). The state of the sample after DSC has not changed. The thermal stability of EuSmCuSe₃ increases by more than 150 K, which already leads to the incongruent melting of the compound (Figure 7b); after DSC, the sample is partially melted. The investigated samples did not show phase transitions when they were in the solid state. The EuGdCuSe₃ compound has two thermal effects of phase transformations at 1494 and 1530 K (Figure 7c). The EuErCuSe₃ compound has three endothermic effects on the temperature range 1561–1608 K (Figure 7d,e). Samples of phases heated to 1550 K (Gd) to 1613 K (Er) remained in the polycrystalline state, and no signs of the appearance of a liquid phase were found. Thermal effects are reproduced in heating–cooling cycles. For EuErCuSe₃, the values of temperature and enthalpy of the heat effect obtained at different installations practically coincide, as shown in Table 4. As for the shape of the peaks, there are linear sections characteristic of phase transformations that correspond to invariant phase equilibria [55,57]. After cooling, the samples have the same crystal structure as before heat treatment. For the sample of EuErCuSe₃, ST KZrCuS₃, SG Cmcm, a = 4.057 (3), b = 13.357 (9), and c = 10.463 (7) Å were determined. All the data obtained indicate that the thermal effects are most likely caused by polymorphic transitions in the compounds. High-temperature modifications were not fixed by quenching. Greek letters indicate the observed modifications of compounds with increasing temperature. Thus, the α-EuErCuSe₃ modification exists from standard conditions to 1561 K, β-EuErCuSe₃—from 1561 to 1579 K; γ-EuErCuSe₃—from 1579 to 1600 K; and δ-EuErCuSe₃—from 1600 K to melting.

EuGdCuSe₃ and EuErCuSe₃ melt incongruently. Broadened peaks of endothermic effects of melting of the samples were recorded at 1588 K (Gd) and 1664 K (Er) (Figure 7c–e). After cooling, the samples had an oval shape, which indicates a partial transition of the samples into the melt. The character of phase melting was studied in detail, during DSC-TG studies of a sample of EuErCuSe₃. The initial EuErCuSe₃ sample becomes multiphase after cooling. According to XRD data, the following phases were found: 65 mol. % EuErCuSe₃, 2 mol. % EuSe, 19 mol. % Eu_1−x−yEr_xCu_ySe, 8 mol. % Cu_2−xSe, and 6 mol. % Er₂Se₃. The results of studying a sample cut after DSC are in complete agreement with the SEM data (Figure 8).

The sample microstructure has a continuous EuErCuSe₃ phase field, in which isolated regions of several phases are located. The phase composition of the multiphase regions is approximately similar, and one of them is shown in Figure 8. In the EuErCuSe₃ phase field, oval EuSe grains are located, and, at the perimeter of EuSe grains, the Eu_1−xEr_xCu_ySe solid solution is located. The solid solution contains erbium, but copper is fixed at the trace level. A eutectic is formed between the Eu_1−xEr_xCu_ySe solid solution and Cu_2−xSe. Separate Er₂Se₃ crystals are in contact with the eutectic. The crystallization peak of the Eu_1−xEr_xCu_ySe + Cu_2−xSe eutectic was recorded during the cooling at 1091 K. The formation of primary EuSe crystals allows one to draw up the following incongruent EuErCuSe₃ melting scheme (1).

EuErCuSe₃ → Eu_1−xEr_xCu_ySe + liquid

(1)

In the temperature range of the endothermic effect at 1664–1687 K, several processes occur simultaneously: the incongruent EuErCuSe₃ melting, partial thermal dissociation of the compound, and melting of primary crystals of solid solution of the EuSe phase.

From the general peak of the processes ΔH = 41.3 J/g, an inconsistent melting peak of EuErCuSe₃ with ΔH = 29 ± 4 J/g, 18.0 ± 2.5 kJ/mol is roughly isolated.

Of particular interest is the comparison of the results, obtained from the thermal analysis of EuLnCuSe₃ compounds (Table 5) with the thermal properties of related sulfide and selenide compounds (Figure 9). The thermal stability of EuLnCuSe₃ increases significantly, depending on rLn³⁺. The enthalpies of incongruent melting of sulfide and selenide quaternary compounds are comparable.

The values of melting points and enthalpies of incongruent decomposition of ALnCuX₃ (A = Eu, Sr, Ba), (X = S, Se), found in the literature, are presented in Figure 9 [23,58,59,60].

4. Conclusions

In the series of EuLnCuSe₃ (Ln = Nd, Sm, Gd, Er), regularities are observed that correlate with the electronic structure of rare earth elements and the value of the ionic radius rLn³⁺. Compounds for the rare earth elements of the first and second tetrads (Nd, Sm, Gd) have SG Pnma ST Cu₂EuS₃, for the element of the fourth tetrad (Er), SG Cmcm ST KZrCuS₃ was determined. Depending on the value of the ionic radius rLn³⁺, the parameters of e.u. and compounds naturally decrease, a trend of increasing microhardness appears, and the thermal stability of compounds significantly increases. The crystal structure of EuErCuSe3, established on the isolated single crystal, is crystallochemically similar to the structure of EuErCuS₃. Like sulfide compounds, EuErCuSe₃ exhibits a magnetic phase transition Tc = 4.7 K. Below Tc, the compound is ferromagnetic; above Tc, it exhibits paramagnetic properties. For EuLnCuSe₃, depending on the value of the ionic radius rLn³⁺, the characteristics change naturally: the unit cell parameters naturally decrease, the optical bandgap increases 1.58 eV (Nd), 1.58 eV (Sm), 1.72 eV (Gd), 1.79 eV (Er), the microhardness of the 205 (Nd), 210 (Sm), 225 (Gd) 235 ± 4 HV (Er) phases increases, and the thermal stability of the phases increases significantly. EuNdCuSe₃ decomposes, as a result solid-phase reaction, the rest of the compounds (Sm, Gd, Er) melt incongruently to form a solid solution of EuSe and a liquid phase. EuGdCuSe₃ has two polymorphic transitions, while EuErCuSe₃ has three. Thermal effects of polymorphic transitions are recorded during both heating and cooling. High-temperature modifications of joints are not fixed by quenching.

Further directions in the study of EuLnCuSe₃ should be highlighted. First of all, it is necessary to develop synthesis methods that ensure the preparation of samples without impurity phases. Establish the structures of high-temperature modifications. During the annealing of the samples of compounds at 1270 K, the formation of a selenium phase was observed, which did not cause the appearance of additional impurity phases in the samples. It is possible that the EuLnCuSe_3−x phases, with vacancies in the structural positions of selenium, will turn out to be more stable. Particles of the EuLnCuSe_3−x phases have a certain plasticity and are sintered during annealing. The creation of functional ceramics of compounds for studying the spectral characteristics of phases, their certification as new thermoelectrics, and possible components of underwater solar converters is urgent.