The Raman spectra of tetragonal TbPO4 (zircon-type I41/amd structure, D4h19 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 Pc≈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.

The high-pressure Raman spectra of ZnF 2 have been measured up to 6.8 GPa and provided unambiguous evidence of a phase transition from its ambient-pressure rutile ͑D 4h 14 ͒ to the CaCl 2 -type orthorhombic ͑D 2h 12 ͒ structure at a pressure P c Ϸ 4.5 GPa. Both macro-and micro-Raman spectra have been independently obtained at high pressures leading to the same conclusion about the high-pressure phase and the P c . The transition appears to be closely associated with the B 1g ͑A g ͒ Raman-active soft mode of the rutile ͑CaCl 2 ͒ structure, implying that ZnF 2 undergoes a second-order ferroelastic phase transition.

No abstract

No abstract

No abstract