Visualization of Pressure-Induced Phonon–Phonon Mode Coupling and Anharmonicity Analysis in <i>A</i> ₃ (VO ₄ ) ₂ ( <i>A</i> = Ni, Mg): A High-Pressure and High-Temperature Raman Spectroscopic Investigation

Phonon–phonon mode coupling induced by temperature and/or compositional changes has been observed in some materials; however, such behavior is rarely reported under pressure. Here, we report pressure-induced phonon–phonon mode coupling in two orthovanadates Ni3(VO4)2 and Mg3(VO4)2 using high-pressure Raman spectroscopy. In Ni3(VO4)2, pressure-induced phonon–phonon mode coupling is observed between a pair of phonon modes at bending and in the stretching regions. For Mg3(VO4)2, a similar coupling is observed only in the bending region. No structural phase transition is observed in either orthovanadate up to 23.7 GPa. Further, a high-temperature Raman spectroscopic investigation of Ni3(VO4)2 indicated large anharmonicity in the medium-frequency modes, with anomalous true anharmonic contributions for some of the phonon modes.