High pressure Raman spectroscopic study of phase transformation in TaO2F |
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Affiliation: | 1. Laboratory of Fine Chemistry and Industry, Henan University, Kaifeng 475004, PR China;2. National Engineering Center of Environmental Photocatalysis, Fuzhou University, Fuzhou 350002, PR China;1. Key Laboratory of Functional Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, 241002, PR China;2. Department of Food Science and Technology, Yeungnam University, Gyeongsan, Gyeongbuk, 712749, South Korea;1. Department of Physics, Dhanalakshmi College of Engineering, Tambaram, Chennai 601 301, Tamil Nadu, India;2. Research Centre for Solar Energy, Department of Physics, Koneru Lakshmaiah Education Foundation, Green Fields, Guntur District, Vaddeswaram, Andhra Pradesh 522502, India;3. Research Centre for Renewable Energy, Forty-five Enterprises, Chennai, Tamilnadu 600 094, India |
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Abstract: | High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO2F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. 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 4.4 GPa 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 4.4 GPa 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 485 cm−1 soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K−1) is in fair agreement with the reported values (−1 to +1 ppm K−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. |
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Keywords: | Raman spectroscopy High pressure Negative thermal expansion |
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