NONCENTROSYMMETRIC CRYSTAL STRUCTURE OF SELENIDES M3Si1.75Se7 (M = 0.5Y+0.5La(Pr))

Abstract

The article studies the problems of the crystal structure of tetrar chalcogenides M3Si1.75Se7 (M = 0.5Y + 0.5La (Pr)). The compounds were obtained by the method of solid-phase reactions by the interaction of elementary components at a temperature of 1323 K. The diffraction patterns of powder samples were taken at room temperature on DRON-4-13. The problem of these structures is not only the localization of a mixture of atoms of rare earth elements (REE), but also the appearance of silicon atoms in two charge states +2 and +4, which has not been previously studied in scientific papers. This approach allows a deeper study of the crystal structure of the obtained phases for the relationship between the charge of the atom and its coordination environment. According to many studies, the localization of atoms carrying the opposite charge relative to the central coordination atom is determined by the position of the atom in the periodic system and its charge. In most studies, there is a variant of the description of the structure, when the composition includes ions with a certain constant conditional charge + or -. In this paper, we propose a solution of the structure in which Silicon with charge 2+ is localized in a site 2a, and Silicon with charge +4 is localized in a site 2b. In addition, this leads to a different coordination environment of these atoms (C.N. = 6 and 4, respectively) The structure was analyzed by the formation of cation-cation pairs between silicon atoms, as the most electronegative atom. A comparative evaluation with known studies that reflect similar issues and found that such pairs in this structure are impossible due to the significant interatomic distances of Si-Si, which leads to the destruction of such a geometric configuration. Silicon atoms in a tetrahedral environment with selenium atoms have relatively short interatomic Si-Se distances, which indicates their predominantly covalent nature. In an octahedral environment, silicon atoms are much larger, indicating an increase in the ionicity of the bond.