Synthesis of Unusually High Valent Perovskite Oxide from the Highly Oxidized Coprecipitation Precursor

Perovskite oxides containing cations with unusually high-valent states such as Fe⁴⁺, Ni³⁺, and Cu³⁺ have attracted significant attention, often requiring a strong oxidizing atmosphere. This study addresses the challenges associated with the conventional precursor preparation for the high-pressure synthesis of BiNi_1-xFe_xO₃, a material exhibiting negative thermal expansion (NTE), specifically the need for mixed oxidants and the emission of NO_x. We successfully established a modified method for preparing a highly oxidized, amorphous precursor containing Bi⁵⁺ and Ni³⁺ ions, ensuring high elemental dispersion by employing a reverse coprecipitation method with simultaneous oxidation using hypochlorite ions, enabling the synthesis of the target phase without the addition of external oxidizing agents. When subjected to high-pressure and high-temperature treatment, the BiNi_1-xFe_xO₃ phase crystallized directly from the amorphous precursor at a relatively low temperature (750 °C) and in a short time of less than 1 min, unlike the crystalline precursor prepared by conventional methods. Furthermore, leveraging the advantage of direct crystallization from the amorphous phase, we demonstrated that reducing the heating duration allows for the fabrication of fine particles, decreasing the size from 15 to 5 μm. These fine particles exhibited NTE over a wider temperature range without any degradation of the volume shrinkage magnitude. This synthetic method provides a safe, pollution-free, and potentially scalable route for BiNi_1-xFe_xO₃ fine particles and is expected to be applicable to the synthesis of other oxides containing anomalously high-valent ions.