Smitiukh O. Phase equilibria and crystal structure of intermediate phases in the R2X3 – R'2X3 – PbX (DIVX2) systems (R, R' – Y, La, Ce, Pr, Tb, Dy, Ho, Er; DIV – Si, Ge , Sn; X – S, Se) at 770 K

The thesis is devoted to the study of phase equilibria at 770 K in quasi-ternary systems R2X3 – R'2X3 – PbX (DIVX2) (R, R' – Y, La, Pr, Er; DIV – Si, Ge , Sn; X – S, Se) and the crystal structure of new quaternary phases that form in these systems. Isothermal sections of the phase diagrams of 28 quasi-ternary systems of the R2X3 – R´2X3 – PbX (DIVX2) type (R, R´– Y, La, Pr, Er; DIV – Si, Ge, Sn; X – S, Se) at 770 K were investigated by X-ray phase analysis. The new quaternary phases Er2.34R0.66Ge1.28S7 (R – La, Ce, Pr) crystallize in the Dy3Ge1,25S7 structure type (SG P63). The 6c site of Dy atoms is occupied by the statistical mixture (Er + R), with the predominant fraction of Er atoms. The quaternary phases Y(Dy, Er)1.5La(Ce, Pr)1,5SixSe7 (x = 1.66; 1.67; and 1.75) crystallize in the Pr3Si1.25Sе7 structure type (PG Р63). Quantum chemical calculations of the equilibrium spatial structure of charge-stoichiometric molecular models of the mixed selenosilicate R3Si1.75Se7 (R – 1.5 Y + 1.5 La) were performed to ascertain the charge state of silicon. The results of quantum chemical calculations indicate that tetrahedrally coordinated Si (IV) atoms form relatively short, mainly covalent bonds, whereas the bonds in the octahedra centered on Si (II) atoms are longer and have substantially ionic character. The existence of solid solution ranges R4xLa4-xGe3S12 (R – Y, Tb, Dy, Ho, Er; x = 0–0.75) based on La4Ge3S12 was found in the R2S3 – La2S3 – GeS2 systems. The substitution of La atoms in the La4Ge3S12 structure decreases in the series Y → Tb → Dy → Ho → Er. The crystal structure of several quaternary phases R4xLa4-xGe3S12 (R – Y, Tb, Dy, Ho, Er; x = 2–2.64, у = 1.36–2.0) was studied by the single crystal method. These structures contain two types of the statistical mixtures of rare earth metals which are described by different ratios of REM at various sites. The content of La atoms is predominant at the 6a site, and of R atoms (R – Y, Tb, Dy, Ho, Er) at the 18b site.