Revisiting the structures and phase transitions of PrNiO₃ nickelate using symmetry-mode analysis

We have investigated the complete sequence of temperature-induced structural phase transitions in PrNiO₃ using high-angular-resolution synchrotron X-ray diffraction and symmetry-adapted distortion-mode analysis. By employing a high-purity sample synthesized under high pressure (3.5 GPa at 1073 K), we resolve the monoclinic (P2₁/n) → orthorhombic (Pbnm) → rhombohedral (R3c) evolution over an extended temperature range (10-900 K). Beyond confirming the role of oxygen stretching modes in the insulator-metal transition at ∼130 K, we show that these phonon modes also govern the high-temperature metallic-metallic transition at ∼750 K. This work provides a comprehensive mode-resolved picture of both insulator and metallic regimes in PrNiO₃ and highlights the central role of phonon-mediated processes in driving charge disproportionation and lattice symmetry breaking across all the crystalline phases.