Ampoule Synthesis of Na-Doped Complex Bromide Cs₂AgBiBr₆ with Double Perovskite Structure

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.