The double perovskite halides A₂B^IB^IIIX₆ provide flexibility for various formulation adjustments and are of less toxicity in comparison with well-discussed complex lead halide derivatives. Such type of structure can be formed by replacing two Pb²⁺ ions in the cubic lattice with a pair of non-toxic heterovalent (monovalent and trivalent) metal cations, such as silver and indium. The aim of this work is to briefly characterize the phase equilibria in the ternary system CsBr-AgBr-InBr₃ and investigate the thermodynamic availability of synthesis of Cs₂AgInBr₆ double perovskite phase by solid-state sintering or melt crystallization. The results demonstrate the unfeasibility of the Cs₂AgInBr₆ phase but high stability of the corresponding binary bromides perspective for optoelectronics.

Compounds of the general composition A₂B^IB^IIIX₆ with a double perovskite (elpasolite) structure are currently considered as an alternative to lead halide perovskites APbX₃ in electronics and photovoltaics due to their greater compositional flexibility and low toxicity. One such alternative is the recently synthesized double perovskite Cs₂AgNaBiBr₆ and a number of various substituted compounds. The close values of the radii of silver and sodium cations make tuning the optoelectronic properties of the double perovskite via the substitution of Ag⁺ by Na⁺ promising if the formation of the substitution solid solution Cs₂Ag_1−xNa_xBiBr₆ takes place. We explored different possible routes for the synthesis of this class of materials, including solid-phase or melt crystallization ampoule syntheses. Varying heating temperature and duration and using standard cooling processing or a quenching-like process, we demonstrate the instability of Cs₂NaBiBr₆ and Na-substituted compounds Cs_2−xNa_xAgBiBr₆ in the temperature range of 300–650 °C. It is worth noting that the formation of Cs₂Ag_1−xNa_xBiBr₆ solid solutions by a solid-phase method is more favorable.

New bromide compounds A₂B^IB^IIIBr₆ with a double perovskite structure provide variety and flexibility of optoelectronic properties, and some of them are of poor toxicity in comparison with such popular lead halides. The promising compound with a double perovskite structure was proposed recently for the ternary system of CsBr-CuBr-InBr₃. Analysis of phase equilibria in the CsBr-CuBr-InBr₃ ternary system showed stability of the quasi-binary section of CsCu₂Br₃–Cs₃In₂Br₉. Formation of the estimated phase Cs₂CuInBr₆ by melt crystallization or solid-state sintering was not observed, most likely, as a result of higher thermodynamic stability of binary bromides CsCu₂Br₃ and Cs₃In₂Br₉. The existence of three quasi-binary sections was observed, while no ternary bromide compounds were found.