Molten salt synthesis of metastable perovskite cobaltites with high-temperature metal to insulator transitions

While perovskite cobaltites ( Re CoO 3 , Re : rare-earth) exhibit multiple spin states and complex electromagnetic phase diagram, their intrinsic metastability at heavy rare-earth composition ( Re H CoO 3 ) impedes their synthesis. Herein, we effectively synthesized Re H CoO 3 ( Re H = Ho, Er, and Tm), without the necessity to use the previous high pressures, by descending their reaction temperatures within molten-salts. Owing to the intermediate valence of Co 3 + , their metastable perovskites cannot be stabilized by simply elevating oxygen pressures, in contrast to the synthesis of high-transitional valence metastable perovskites such as Re NiO 3 . Alternatively, the positive formation free energy of the Re H CoO 3 was reduced by introducing KCl that dissolve the precursors at lower temperatures to prevent varying the valence of Co without scarifying the reaction kinetics. The effective synthesis of metastable Re CoO 3 towards heavy rare-earth composition further extends the range for regulating both the electronic and magnetic phases of Re H CoO 3 , paving the way to their potential electromagnetic applications.