THE CRYSTAL STRUCTURE OF La3Pb0.1Ga1.6Se7 AND Pr3Pb0.1Ga1.6Se7 COMPOUNDS

Abstract

Samples of the stoichiometric composition La3Pb0.1Ga1.6Sе7 and Pr3Pb0.1Ga1.6Sе7, weighing 0.8 gram each, were prepared by fusing components of semiconductor purity in vacuumed quartz containers to a residual pressure of 10-2 Pa. The synthesis was carried out in a muffle furnace with software control of MP-30 technological processes. The maximum temperature of synthesis is 1100°C, homogenizing annealing lasted 240 hours at temperature of 500°C.
 The diffraction patterns for X-ray phase analysis were recorded at a DRON 4-13 diffractometer for 2Q range of 10-100° (CuKα radiation, scan step 0.02°, 20 s exposure in each point). Data processing and the determination of the crystal structure utilized WinCSD software package. Visualization of the crystal structure, stacking and coordination polyhedra was performed using the VESTA program.
 The crystal structure of the selenides La3Pb0.1Ga1.6Sе7 and Pr3Pb0.1Ga1.6Sе7 was studied by the X-ray powder diffraction method and its belonging to the hexagonal crystal system was established (space group P63). The refinement of atom coordinates and isotropic temperature displacement parameters in this model yielded satisfactory values of fit factors. Analysis of hkl reflex indices and their intensity indicates that the structural of La(Pr)3Pb0.1Ga1.6Sе7 compounds belong to the structural type La3CuSiS7 (space group P63; a = 1.028 nm, c = 0.575 nm)
 Crystal structure of selenides La3Pb0.1Ga1.6Se7 (a = 1.05595(4) nm, c = 0.63868(3) nm, RI = 0.0983, Rp = 0.1672) and Pr3Pb0.1Ga1.6Se7 (a = 1.03727(6) nm, c = 0.63772(5) nm, RI = 0.0868, Rp = 0.2286) was studied by X-ray powder method. It was established that the structure of the synthesized selenides belongs to the structural type La3CuSiS7 (SG P63; PS hP24). In their structure, R = La(Pr) atoms are localized in site 6c and, together with selenium atoms, form trigonal prisms with one additional atom [R 1Se13Se23Se3]. Atoms of statistical mixtures M1 (0.64 Ga + 0.09 Pb) and M2 (0.58 Ga + 0.11 Pb), concentrated in site 2a, form octahedra [M1 6Se2] and [M2 6Se2]. These octahedra are interconnected by faces and form columns in the direction of the c axis. The Ga (site 2b) atoms are surrounded by four selenium atoms. The formed tetrahedra are oriented in the direction of the c axis and are isolated from each other. Gallium-containing selenides La3Pb0.1Ga1.6Sе7 and Pr3Pb0.1Ga1.6Sе7 based on lanthanum and praseodymium are promising chalcogenides based on which materials for nonlinear optics and thermoelectricity can be created.
 Keywords: rare-earth metals; chalcogenide; crystal structure; X-ray powder diffraction.