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

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) Å3. 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 EuGdCuSe3, two (Tα↔β = 1494 K, ΔHα↔β = 14.8 kJ/mol, Tβ↔γ = 1530 K, ΔHβ↔γ = 4.8 kJ/mol) and for EuErCuSe3 three polymorphic transitions (Tα↔β = 1561 K, ΔHα↔β = 30.3 kJ/mol, Tβ↔γ = 1579 K, ΔHβ↔γ = 4.4 kJ/mol, and Tγ↔δ = 1600 K, ΔHγ↔δ = 10.1 kJ/mol). The compounds melt incongruently at the temperature of 1588 K, ΔHmelt = 17.9 ± 1.8 kJ/mol and 1664 K, ΔHmelt = 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.