Woodhead, KE; (2013) High Pressure Polymorphism of Tantalum Nitrides and Oxynitrides. Doctoral thesis , UCL (University College London).

Abstract

The investigations presented here explore the nature of the Ta-O-N system at high pressures. The techniques that are commonly used for structural determination with the diamond anvil cell (DAC) are Raman spectroscopy and synchrotron-source X-ray diffraction (XRD); these methods are supported here by energy-dispersive X-ray absorption spectroscopy (EDXAS), a technique which is in its infancy when combined with in situ DAC studies. The studies presented here are among the first to exemplify the use of EDXAS in high-pressure structural determination, and are especially useful for nitride and oxynitride studies, where the greatest changes occur in the positions of the light elements that form the local environments around the metal atoms. The refractory ceramic, ε-TaN, has previously been shown through Raman and XRD studies not to undergo any phase transition up to 58 GPa, although some interesting features were observed in both cases. Here, in situ EDXAS probes the local structure around the tantalum atoms in this material, and the movement of the nitrogen atoms from their positions can be seen up to 58 GPa. The second investigation uses Raman, XRD and EDXAS experiments to explore phase transitions of TaON; the results presented here show a phase transition occurring from the baddelyite phase to the cotunnite phase between 20-35 GPa. The known baddeylite phase of TaON is also shown to be a highly incompressible material, possessing a bulk modulus value of 328 GPa. The third investigation presents a study of the complex structural transitions undergone by orthorhmobic Ta3N5-I at pressures up to 50 GPa. Both Raman and XRD experiments show a first order phase transition at about 11 GPa to the orthorhombic Ta3N5-II phase; amorphisation and a coexistence of phases are observed at pressures above 16 GPa in the XRD patterns, and hence the subtleties of the transitions are explored by EDXAS, such as an increase in coordination number and bond length. A novel high-pressure, high-temperature synthesis route to the new Ta3N5-II phase has also been shown as viable, by laser-heating an amorphous TaxNy precursor in the DAC.

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