Raman Spectroscopy of γ-Si3N4 and γ-Ge3N4 nitride spinel phases formed at high pressure and high temperature:: Evidence for defect formation in nitride spinels

The discovery of dense spinel nitride phases, gamma-Si3N4 and gamma-Ge3N4, synthesized under high-pressure-high-temperature conditions has resulted in renewed interest in solid-state nitride materials chemistry. The new materials are high hardness ceramics, and they represent a new family of wide band gap semiconductors and optoelectronic materials. The compounds have been characterized by X-ray and electron diffraction and by Raman scattering carried out both on quenched samples and in situ at high pressure in the diamond anvil cell. Here, we assign the Raman-active modes of gamma-Si3N4 and gamma-Ge3N4 spinels, taking account of impurity phases that can be present within the samples. Laser-heating diamond anvil cell studies carried out at high pressures and at various temperatures lead to the identification of additional Raman features that we assign to the presence of defects, most likely N-3-vacancies, within the nitride spinels. The intensity of the "defect" peaks varies systematically with the temperature and pressure of synthesis, corresponding to changes in the N-2 activity in equilibrium between the solid nitrides and the surrounding N-2 fluid. The results point the way toward future measurement and control of the defect chemistry achieved within spinel nitrides prepared under high-pressure-high-temperature conditions.