Structural stability and anharmonicity of phonon modes of metastable Zn4V2O9: In-situ Raman spectroscopic investigation

•The metastable compound Zn4V2O9 is synthesized by solid state reaction method.•A series of pressure induced structural phase transitions are identified by Raman spectroscopy.•First high pressure phase transition is reversible and the second phase transition is irreversible.•It decomposes to stable phase Zn3V2O8 (orthorhombic Cmca) after cooling from 1073 K.•Contribution of four phonon decay process is dominant for most of the Raman modes. [Display omitted] The structural stability of metastable Zn4V2O9 has been investigated using in-situ Raman spectroscopy at high pressures up to 25 GPa at ambient temperature and high temperature, upto 1073 K at ambient pressure. Pressure dependent Raman studies show pressure induced structural phase transitions around 5 GPa and 14 GPa, followed by an onset of structural disorder above17 GPa. The transition observed at 5 GPa is reversible while the transition to disordered phase is irreversible. The changes in Raman spectra above 14 GPa point out towards an increase in coordination of vanadium atoms. The disordered high pressure phase could be retrieved upon release of pressure. Temperature dependent Raman studies indicated many anomalous phonon modes but none of them show softening behaviour with pressure. The anharmonicities of Raman modes have been estimated for all the observed Raman modes in the ambient monoclinic phase and interestingly most of the modes show positive explicit anharmonic contribution.