Pressure-induced phase transitions in TmVO₄ investigated by Raman spectroscopy

The structural behavior of TmVO₄ at pressures up to ∼ 55 GPa was investigated by Raman spectroscopy. The changes in Raman spectra suggest the existence of three phase transitions upon compression. The first phase transition appeared at ∼ 7.7 GPa, which was an irreversible phase transition from the ambient-pressure zircon phase to the scheelite phase, confirming previous X-ray measurements. Subsequently, the second reversible phase transition from the scheelite phase to the fergusonite phase occurred at ∼ 23 GPa. Additional changes in the Raman spectra were observed at ∼ 37 GPa, validating the third phase transition. Based on a comparison to related rare earth orthovanadates, we assumed that the post-fergusonite of TmVO₄ has an orthorhombic structure described by space group Cmca. The wavenumbers of the Raman modes and their pressure coefficients for all four phases of TmVO₄ are reported. Our study provides the vibrational difference in various polymorphs of TmVO₄, which will refine our understanding of the structural behavior of rare-earth orthovanadates.