A HIGH-PRESSURE RAMAN-STUDY OF TEO2 TO 30 GPA AND PRESSURE-INDUCED PHASE-CHANGES

The effect of pressure on the Raman modes in TeO2 (paratellurite) has been investigated to 30 GPa, using the diamond cell and argon as pressure medium. The pressure dependence of the Raman modes indicates four pressure-induced phase transitions near 1 GPa, 4.5 GPa, 11 GPa and 22 GPa. Of these the first is the well studied second-order transition from D4(4) symmetry to D2(4) symmetry, driven by a soft acoustic shear mode instability. The remarkable similarity in the Raman spectra of phases I to IV suggest that only subtle changes in the structure are involved in these phase transitions. The totally different Raman spectral features of phase V indicate major structural changes at the 22 GPa transition. It is suggested that this high pressure-phase is similar to PbCl2-type, from high pressure crystal chemical considerations. The need for a high pressure X-ray diffraction study on TeO2 is emphasized, to unravel the structure of the various high pressure phases in the system.