Raman study of tetragonal TbPO4 and observation of a first-order phase transition at high pressure

The Raman spectra of tetragonal TbPO4 (zircon-type I4(1)/amd structure, D-4h(19) space group) have been measured at ambient conditions and under variable pressure up to 15.5 GPa inside a diamond anvil cell (DAC). Assignment of the Raman active modes of the tetragonal phase has been carried out based on polarized measurements from a single oriented crystal of TbPO4 at ambient conditions. The abrupt Raman mode discontinuities and the appearance of numerous new Raman peaks at a pressure P-c approximate to 9.5 GPa have provided strong evidence for a first-order phase transition to a lower crystal symmetry, most likely monoclinic. The high-pressure structure appears to be more compact compared to the tetragonal one, and is retained upon bringing the crystal to ambient pressure.