THE CRYSTAL STRUCTURE OF Ce1.5Tb1.5Ga1.67S7

Abstract

The paper presents the results of the investigation of the crystal structure of Ce1.5Tb1.5Ga1.67S7 as a promising material, which is predicted to have interesting nonlinear optical properties. The synthesis of a sample of stoichiometric composition Ce1.5Tb1.5Ga1.67S7 with a total weight 0.8 g, was obtained by fusing simple substances in vacuumed quartz container to a residual pressure of 10 -2 Pa at a maximum synthesis temperature of 1100 °C. The crystal structure of Ce1.5Tb1.5Ga1.67S7 sulfide (a = 10.0316(4) Å, c = 6.0640(4) Å, V = 528.48(6) Å3, RI = 0.0498, Rp = 0.1553) was studied by the X-ray powder method. It was established that the structure of the synthesized sulfide belongs to the hexagonal symmetry (La3CuSiS7 structural type, SG P63; Pearson code hP24). The Ce1.5Tb1.5Ga1.67S7 is synthesized on the basis of La3Ga1.67S7 sulfide by replacement of lanthanum atoms in the site 6c by atoms of statistical mixture M (0.5 Ce + 0.5 Tb). The elemental composition of the Ce1.5Tb1.5Ga1.67S7 (Ce: 7.56 ± 1.34 %; Tb: 16.82 ± 3.18 %; Ga: 15.93 ± 1.44 %; S: 59.70 ± 3.07 %) was confirmed by EDAX analysis. In the structure of the investigated quaternary sulfide, atoms of the of statistical mixture M (0.5 Ce + 0.5 Tb) are localized in site 6c and, together with sulfur atoms, form trigonal prisms with one additional atom [M 3S13S21S3]. Trigonal prisms form “blocks” of 3[M 7S]. In these “blocks” trigonal prisms are connected to each other by ribs. The effective coordination number of atoms of the statistical mixture M (0.5 Ce + 0.5 Tb) is 6.84 due to the insignificant distortion index (calculated from bond lengths). The Ga1 (0.667) atoms, concentrated in site 2a, form octahedra [Ga1 6S1]. These octahedra are interconnected by faces and form columns in the direction of the c axis. The introduction of statistical mixture atoms into the structure of the La3Ga1.67S7 causes significant distortion of [Ga1 6S1] octahedra. In site 2b, Ga2 atoms are surrounded by four sulfur atoms [Ga2 3S21S3]. The insignificant distortion index (0.017 (calculated from the bond lengths)) of the tetrahedra indicates their high symmetry (the effective coordination number is 3.93).