Raman and ab initio investigation of negative thermal expansion material TaVO5: Insights into phase stability and anharmonicity

Abstract

TaVO5 is a framework structured compound that exhibits negative thermal expansion (NTE) above room temperature, upto 1073 K. We report Raman spectroscopic investigation of TaVO5 as a function of temperature in the range 77-873 K, which confirms the reported reversible low temperature transition to monoclinic phase at 259 K. Structural stability of TaVO5 at high pressures investigated using in-situ Raman spectroscopy shows a reversible structural transition at around 0.2 GPa to a phase, which is probably the same monoclinic phase as the low temperature phase, indicating that this structural phase transition may be volume driven. From the pressure and temperature dependence of the Raman modes, some of the zone centre phonon modes, particularly, the librational modes, responsible for the NTE are identified and anharmonicity of the Raman modes is also estimated. We have found that explicit anharmonicity dominates over implicit anharmonicity and the low frequency modes have significant quartic anharmonicity. The vibrational properties in the ambient phase of TaVO5 are also investigated using ab initio calculations of phonon frequencies to understand the NTE behavior as well as to complement the Raman spectroscopic measurements. Further, the eigenvectors of specific phonon modes associated with phase transition and NTE behavior of the compound have been identified from these calculations.