Sabov V. Interaction of Components and Properties of Phases in Tl(Ag)–Sb–P–Se Systems.

The scientific work is devoted to the establishment of the nature and regularities of physicochemical interaction in Tl(Ag) – Sb – P – Se systems and the effect of cationic substitution on the properties of single crystals Tl(Ag)Sb(In,Bi)P2Se6, by studying the phase equilibria on cross-sections based on Tl(Ag)SbP2Se6 quaternaries also polyhedration of quaternary systems and studying the properties of single crystal samples of quaternary compounds. The overview of the literature devoted to the interaction in double and ternary systems of thallium, argentum, antimony, phosphorus and selenium, the properties of the double and ternary compounds implemented in them, as well as the quaternary compounds Tl(Ag)Sb(In,Bi)P2Se6 is provided. The analysis of literature data shows that the information about phase equilibria in the Tl(Ag) – Sb – P – Se systems, and data on bulk single crystals growth of Tl(Ag)Sb(In,Bi)P2Se6 compounds don't present and regarding the properties of quaternary compounds are uncompleted. The starting components, general methods of synthesis and growth of single crystals, and a brief description of experimental methods: Differential thermal analysis (DTA), microstructural analysis (MSA), X-ray powder diffraction (XRD) for phase and structural analysis; scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS); X-ray photoelectron spectroscopy (XPS); Raman and UV-visible spectroscopy; density functional theory (DFT) calculations are described. The results of an experimental study of phase equilibria in the Tl – Sb – P – Se and Ag – Sb – P – Se systems are presented. The methods of growing single crystals of quaternary compounds and experimental results of studies of their properties (refinement of the crystal structure by the Rietveld method; confirmation of the morphology, quality and composition of single crystals by X-ray surface diffraction; scanning electron microscopy with energy dispersive X-ray spectroscopy; evaluation of the electronic structure combining ab initio calculations according to density functional theory with X-ray photoelectron spectroscopy data; determination of the bandgap from UV-visible optical data by Tauc’s plot) are presented. The regularities of the interaction of components in Tl(Ag)-Sb-P-Se systems and, using the singular polihedration method, additionally establishes the quasi-binarity/ternarity/tetrarity of a number of systems are summerized. The mechanisms of formation of solid solutions based on quaternary compounds are considered from the crystal-chemical formulae, the change in the nature of chemical bonding during cationic substitution is analyzed by comparing the sums of ionic and covalent radii with experimentally determined interatomic bond lengths and by difference parameters of the core levels electrons binding energies and the relationship between the change in the nature of the chemical bond and the width of the band gap is considered. The mechanisms of the formation of boundary solid solutions on the example of TlSbP2Se6 were studied from crystal-chemical formulae. Taking into account the experimental data on the wide of regions of solid solutions based on TlSbP2Se6, it was found that the cationic sublattice plays a decisive role in the stabilization of the solid solution, namely, the ratio of one- and three-charged cations (1:1) identical to the quaternary compound. The single crystals of AgSb(Bi)P2Se6 and TlSb(In)P2Se6 compounds were grown using the Bridgman method. The results of the single crystals study by scanning electron microscopy with energy dispersive X-ray spectroscopy showed their homogeneity and composition close to stoichiometry. According to the results of spectral studies, an increase in the width of the band gap in the AgBiP2Se6  AgSbP2Se6  TlInP2Se6  TlSbP2Se6, which, given the similarity of the studied compounds, indicates an increase in the ionic component of the chemical bond. Also, the difference parameter P revealed an increase in the covalent component of the bonds in the anionic sublattice [P2Se6]4- in the same series, which, due to the trans-influence in the Me-Se-P bond system, leads to an increase in the ionicity of Me-Se bonds. This pattern can probably be used for a qualitative assessment of the change in the band gap in similar hexahalcohypodiphosphates. Confirmation of this and the possibility of creating a more accurate correlation model will require a set of spectral studies on a wider range of compounds. Keywords: single crystal; phase analysis; crystal structure; solid solutions; electronic structure; ab initio (first principal) calculation/computation; X-ray photoelectron spectroscopy (XPS); optical properties; differential thermal analysis (DTA); phase diagram; scanning electron microscopy (SEM); energy dispersive X-ray spectroscopy (EDS); X-ray diffraction; phase equilibria.