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REGULARITIES OF PHYSICO-CHEMICAL INTERACTION IN THE QUASIBINARY SYSTEMS BASED ON THE TERNARY HALIDES Rb3(Cs3)Sb2(Bi2)Br9(I9) AND K2(Rb2,Cs2,Tl2)TeBr6(I6): EXPLANATION WITHIN THE FRAMEWORK OF THE BOND VALENCE MODEL

The work is the first attempt to employ the bond valence model (BVM) in the explanation and interpretation of the phase diagrams observed for the quasibinary systems based on the ternary halides Rb3(Cs3)Sb2(Bi2)Br9(I9) (A3B2X9) and K2(Rb2,Cs2,Tl2)TeBr6(I6) (A2BX6) belonging to the structural family of perovskite. The most characteristic features of the phase diagrams of such systems are (a) a monotonic increase in the melting and crystallization temperatures when the smaller A atom is replaced by a larger chemical analogue; (b) a gradual decrease in the melting and crystallization temperatures when the bromine atoms are replaced by iodine atoms, reaching the temperature minimum at a molar ratio of ~1 : 1, and then an increase of the temperatures to the pure iodine component of the system. With the isostructurality of the starting components, in case (a) a continuous series of solid solutions (CSSS) of substitution is formed, without a temperature minimum; if the starting components belong to different structural types, then a peritectic interaction with the wide concentration limits of solid solutions is observed. Under the same conditions concerning the isostructurality/non-isostructurality of starting components, in case (b), correspondingly, either a CSSS of substitution with a temperature minimum is formed or eutectic interaction with the wide concentration limits of solid solutions is observed. 
Within the framework of the BVM, in case (a) a monotonic increase of the melting and crystallization temperatures when replacing a smaller atom A with a larger analogue can be interpreted as a consequence of reducing the deficit of the bond valence sum (BVS) for cations A, resulting in the stabilization of the structure as a whole. In case (b), partial replacement of one halogen ion by a halogen ions of another sort leads either to a deficit (if a larger anion is replaced by a smaller one) or to an excess (if a smaller anion is replaced by a larger one) of the BVS values for the cations A and for the anion that replaced the main halogen sort. An increase in the excess or deficit of the BVS values destabilizes the structure of ternary halides, reducing the melting and crystallization temperatures; the maximum destabilization is achieved at a ~1 : 1 molar ratio of the components, which is expressed in reaching the characteristic temperature minimum of the respective systems. 
The approach described in the work can expectedly be employed in explanation and interpretation of another phase diagrams based on the structures featuring the closest packing formed by alkaline metals and halogens. 
Keywords: ternary halides; crystal structures; phase diagrams; bond valence model.