High pressure Raman spectroscopic study of phase transformation in TaO2F

Abstract

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO2F are carried out up to 19GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4GPa with rhombohedral (R-3c) structure above 4GPa, but the structure between 0.7GPa and 4GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8GPa and gets completed at 4.4GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO2F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO2F. Disorder-induced first order Raman modes at 176, 212, 381 and 485cm−1 soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91ppmK−1) is in fair agreement with the reported values (−1 to +1ppmK−1). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.