Compression effect on structure of the Li-stabilized high-temperature phase of Mn₃(VO₄)₂ with composition Li_0.2Mn_2.9(VO₄)₂ - Raman spectroscopic and X-ray diffraction investigations

Highlights: • Raman spectroscopic study of tetragonal Li_0.2Mn_2.9(VO₄)₂ suggests a first order structural transition above 13 GPa. • Synchrotron X-ray diffraction at high pressures suggests anisotropic compression of the unit cell. • Equation of state of the ambient tetragonal phase is obtained. • High pressure phase is structurally disordered and is different from the known thermodynamically stable orthorhombic structure of Mn₃(VO₄)₂. -- Abstract: Tetragonal Mn₃(VO₄)₂ is a metastable high temperature phase of Mn₃(VO₄)₂ showing unusual coordination of Mn atoms. A nominal Li-substitution helps to stabilize the tetragonal modification. We report room temperature compression of Li-stabilized high-temperature tetragonal phase of Mn₃(VO₄)₂ with composition Li_0.2Mn_2.9(VO₄)₂ studied by in-situ Raman spectroscopy and synchrotron X-ray diffraction up to pressures of 20 and 26.5 GPa respectively. Raman spectroscopy suggests onset of a structural phase transition at around 10 GPa which completes above 13 GPa. X-ray diffraction also suggested a first order structural transition above 10 GPa, with a coexistence range of two phases up to 13 GPa. High pressure phase is found to be different from the known thermodynamically stable orthorhombic structure of Mn₃(VO₄)₂. The equation of state data of the ambient tetragonal phase has been obtained. Anisotropic compression is observed with the c-axis being more compressible than a-axis. Using experimental bulk modulus, the mode Grüneisen parameters for the observed Raman frequencies for the ambient phase have been obtained. On pressure release, the high pressure phase could be retrieved.