Temperature- and atmosphere-driven phase evolution in sol–gel synthesized barium nickelate powders

Abstract

Controlling phase formation and stoichiometry in barium nickelate (BNO, BaNiO_3−x) is critical for the development of high-performance BNO-based catalysts, gas sensors, and piezoelectrics. The multiple oxidation states of nickel (Ni²⁺, Ni³⁺, and Ni⁴⁺) are thought to enable the broad range of BNO phases: however, the conditions that selectively access these phases and the stoichiometric ranges each phase accommodates remain unclear. This study presents a systematic calcination approach to map the relationship between calcination temperature, oxygen atmosphere, BNO phase, morphology, and stoichiometry. BNO sol–gels calcined from 800 to 1000 °C under controlled oxygen flow (0–3 L min⁻¹) reveal a phase-selective synthesis window: oxygen-rich calcination at 800 °C produces the piezoelectric P6₃mc phase, and higher temperatures promote formation of the metastable piezoelectric R32 phase and drive the phase transition to a previously unassigned BNO. In contrast, under oxygen-free conditions, calcination above 800 °C yields the centrosymmetric P6₃/mmc phase. Increasing the temperature and oxygen flow also drives the transition from a porous to a dendritic morphology, increases crystallite size, and tunes stoichiometry from BaNi_0.93O_2.76 to BaNi_0.91O_2.24. These findings offer a framework for controlling BNO phase formation, morphology, and stoichiometry, providing a broadly applicable strategy for synthesizing complex multivalent oxides and enabling the design of BNO-based catalysts, gas sensors, and piezoelectrics.