Particle Size and Morphology Effects on the High-Pressure Behavior of NaNiF3 Fluoroperovskite

To investigate the influence of particle size and morphology on the high-pressure (HP) behaviour of fluoroperovskites, NaNiF3 samples exhibiting distinct microstructural characteristics were synthesised using precipitation-dehydration, solvothermal, and hydrothermal routes. Ambient-condition characterisation confirmed significant differences in particle crystallinity, size, morphology, and specific surface area between the samples. Their high-pressure structural evolution was investigated up to 17 GPa at room temperature using synchrotron X-ray diffraction (SXRD) in a diamond anvil cell (DAC) with sodium chloride (NaCl) as the pressure-transmitting medium (PTM). Although no perovskite-to-post-perovskite (Pv-pPv) transition was observed within the investigated pressure range, the samples exhibited distinct compression response, highlighting the influence of particle size and morphology on their response to pressure. This trend was further confirmed by multi-anvil (MA) experiments. While the nanosized samples remained in the Pv structure after treatment at 12.5 GPa and 500 °C, the microparticulate sample underwent a partial Pv-pPv transition under comparable pressure-temperature-time (P-T-t) conditions. These results indicate that microstructural parameters influence the transformation pathway and kinetics of ternary fluorides such as NaNiF3, with microparticulate materials transforming more readily than their nanosized counterparts.