RECIPROCAL Cs3Sb2Br9+Cs2TeI6  Cs3Sb2I9+Cs2TeBr6 SYSTEM: PHASE EQUILIBRIA, ELECTRONIC STRUCTURE AND OPTICAL PROPERTIES OF INTERMEDIATES COMPOUNDS

Abstract

Cs- and Rb-containing halide double perovskites of A2B1B2X6 type and vacant-ordered perovskites of A2B□X6 type (where □ vacancy) are potential compounds for photovoltaic materials.Phase equilibria in the reciprocal Cs3Sb2Br9–Cs2TeI6 « Cs3Sb2I9–Cs2TeBr6 system were studied by methods of physicochemical analysis (DTA, X-ray, MSA) with the involvement of mathematical modeling.The Cs3Sb2Br9, Cs3Sb2I9, Cs2TeBr6, Cs2TeI6 ternary compounds melts congruently at 864 K, 1019 K, 806 K, 876 K, respectively. The compound Cs3Sb2Br9 has a polymorphic transformation at 791 K. Limited solid solutions are formed in the Cs3Sb2Br9–Cs2TeI6 « Cs3Sb2I9–Cs2TeBr6 system: a based on Cs2TeBr6–хIх, b and b’ based on the low- and high-temperature modification of Cs3Sb2Br9, respectively, and g based on Cs3Sb2I9. Due to the formation an unlimited solid solution in the Cs2TeB6–Cs2TeI6 system (can be considered as one component Cs2TeBr6–хIх) the investigated system may be presented as the Cs3Sb2Br9–Cs3Sb2I9–Cs2TeBr6–хIх quasiternary system (characterized be eutectic invariant process L«a+b’+g at 737 K, 26 mol.% Cs3Sb2Br9, 32 mol.% Cs3Sb2I9, 19 mol.% Cs2TeB6, mol.% Cs2TeI6).Consider the structure of octahedral [SbX6]3– complex ions within ligand field theory (LFT) and the molecular orbital (MO) method are detected. The structure of octahedral [SbX6]3– complex ions are characterized by sp3d2 hybridization. Ab initio quantum-mechanical calculations of the electronic structure by DFT method were showed that all investigated compounds (except Cs3Sb2Br9) belongs to indirect type semiconductors with band gaps Eg = 2.24 eV (Cs2TeBr6), Eg = 1.43 eV (Cs2TeI6), Eg = 2.18 eV (direct type semiconductor Cs3Sb2Br9), Eg = 1.87 eV (Cs3Sb2I9) and Eg = 1.35 eV (Cs2TeBr3I3).Optical studies of the fluorescence spectra of perovskite compounds of the Cs3Sb2X9 type indicate the possibility of their use as promising functional photovoltaic materials for solar cell elements.