X-ray absorption and EXAFS studies of Ge coordination and bonding in high-pressure nitrides: ?-Ge3N4 (phenacite) and ?-Ge3N4 (spinel)

Abstract

X-ray absorption spectra (XAS) have been obtained at the Ge K edge, and the extended X-ray absorption fine structure (EXAFS) oscillations have been analysed, for the low-pressure (β-) and high-pressure (γ-) phases of Ge3N4. β-Ge3N4 has the phenacite structure and it contains Ge atoms in tetrahedral coordination. Upon compression at ambient temperature, the structure undergoes progressive distortion resulting in pressure-induced amorphisation or metastable crystal-crystal phase transformations. The spinel-structured γ-Ge3N4 phase is stabilised under high pressure-high temperature conditions: it represents the first solid state compound containing Ge in octahedral coordination to nitrogen. The spectra in the XANES region are clearly correlated with changes in the local coordination geometry. Analysis of the EXAFS data gives bond lengths in excellent agreement with X-ray powder structure refinement. The XAS/EXAFS technique is a powerful method for studying the coordination and local structure in Ge-containing nitride and oxynitride materials synthesised at high pressure, including in situ studies in the diamond anvil cell. Here we report results of an in situ study of metastable compression of β-Ge3N4 to 30 GPa at ambient temperature, showing evidence for changes in the Ge coordination environment at 19–20 GPa associated with amorphisation or the newly-discovered β–δ transformation recently observed by Raman spectroscopy and X-ray diffraction. Heating samples to 500 °C at high pressure did not show evidence for transformation into the stable spinel-structured γ-Ge3N4 phase. The first order transition obviously requires high temperatures to initiate and complete the nucleation and growth process. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)