Crystal and band energy structure of Nd1.5Y(Tb)1.5Ga1.67S7 sulfide

Two sulfide phases of composition Nd1.5Y(Tb)1.5Ga1.67S7 were obtained from high-purity simple substances in evacuated quartz ampoules at a maximum synthesis temperature of 1100 °C. The synthesized alloys were homogenized by annealing at a temperature of 500 °C for one month. The crystal structure of the sulfides Nd1.5Y1.5Ga1.67S7 (a = 9.720(1) Å, c = 6.103(1) Å, RI = 0.0804, Rp = 0.2806) and Nd1.5Tb1.5Ga1.67S7 (a = 9.8866(7) Å, c = 6.0877(6) Å, RI = 0.0800, Rp = 0.2536) was studied by powder X-ray diffraction. Their structure belongs to the structural type La3CuSiS7 (space group P63; Pearson symbol hP24). In these structures, the rare-earth (R) atoms [Nd and Y(Tb)] are statistically distributed in the site 6c (x y z) and together with the S atoms form trigonal prisms with one additional atom [R S6+1]. The Ga1 site 2a is defective with an atomic packing factor APF = 66.7 %, and Ga2 site 2b with full atomic occupancy. The Ga1 and Ga2 atoms have octahedral and tetrahedral environments, respectively. Sulfur in the crystal lattice has three atomic positions: S1, S2 (site 6c) and S3 (site 2b). The primitive hexagonal unit cell contains two formula units Nd1.5Y(Tb)1.5Ga1.67S7. Trigonal prisms [R S6+1] are connected by edges and form "blocks" (three prisms each). Octahedrons [Ga1 S6] have common faces and form "columns" in the direction of the main axis. Trigonal prisms with octahedra form common faces. Tetrahedra [Ga2 S4] are isolated from each other. Using density functional theory within the framework of the Kohn-Shem formalism, the band-energy structure of synthesized sulfides was studied.