High-Pressure Phase Transition of Micro- and Nanoscale HoVO4 and High-Pressure Phase Diagram of REVO4 with RE Ionic Radius

In situ Raman spectra of HoVO4 micro- and nanocrystals were obtained at high pressures up to 25.4 and 18.0 GPa at room temperature, respectively. The appearance of new peaks in the Raman spectra and the discontinuities of the Raman-mode shift provided powerful evidence for an irreversible zircon-to-scheelite structure transformation for HoVO4 microcrystals at 7.2 GPa and for HoVO4 nanocrystals at 8.7 GPa. The lattice contraction caused by the size effect was thought to be responsible for the different phase-transition pressures. Also, the higher stability of HoVO4 nanocrystals compared with the microcrystals was also confirmed using the Raman frequencies and pressure coefficients. The results of the phase transition of HoVO4 were compared with previously reported rare-earth orthovanadates, and the phase diagram of REVO4 with RE ionic radius at different pressures was presented.