Hydrothermal synthesis of superconductors Ba1−xKxBiO3 and double perovskites Ba1−xKxBi1−yNayO3

► We report a selective synthesis of Ba1−xKxBiO3 and Ba1−xKxBi1−yNayO3. ► The synthesis is under mild hydrothermal conditions. ► The influence factors for the crystal growths of the samples were discussed in detail. ► The purity and homogeneity of the crystals were detected by the ICP, SEM, EDX.