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

Compounds of the general composition A2BIBIIIX6 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 Cs2Ag1−xNaxBiBr6 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 Cs2−xNaxAgBiBr6 in the temperature range of 300–650 °C. It is worth noting that the formation of Cs2Ag1−xNaxBiBr6 solid solutions by a solid-phase method is more favorable.