Soignard, Emmanuel; (2003) High pressure - high temperature synthesis and studies of nitride materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis develops the topics of synthesis and study of nitride materials under extreme conditions of high pressure and high temperature. The thesis presents two main research projects. The investigations were only possible after developing and building the Raman spectroscopy and laser heating system presented in details in the second chapter of this thesis. We designed and built this equipment specifically for diamond anvil cell research and micro-Raman spectroscopy studies on very weakly scattering samples. The first investigations develop the study of group IV nitrides, silicon and germanium nitrides. The experiments determined the compressibility of the high pressure phase y-Si3N4 and the synthesis of the first ternary phases in the spinel system Si3N4 - Ge3N4. In particular, the phase GeSi2N4 was found to be a normal spinel with silicon atoms in octahedral sites and germanium atoms in octahedral sites. The Raman spectroscopy study of the spinel nitride phases following laser heating at high pressure indicated that they commonly possess N vacancies. Experiments were performed on the low pressure phases alpha and beta- germanium nitride at low temperature. Upon metastable compression, beta-Ge3N4 transformed, into a new 5-Ge3N4 phase at 22 GPa, followed by amorphisation above 35 GPa. The second set of investigations presents a study of transition metal nitrides at high pressure. Here we show the results of the first bulk modulus measurements for two phases of molybdenum nitride, TiNx, Cr2N and TaN. These materials are highly incompressible with bulk modulus values exceeding 300 GPa. We also performed for the first time a Raman spectroscopy study, including measurements at high pressure for selected transition metal nitrides. We observed that 6-MoN and 8-TaN, which are both hexagonal, have very similar Raman spectra, although the structures are different. Finally, we measured the Raman spectrum of B1-structured nitrides, TiNx, VNx and Y-M02N and observed their phonon density of states. No phase transition was observed upon room pressure compression for any of the transition metal nitrides studied.

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