The elucidation of the structures of crystalline and disordered solids (glasses) based on the packing of the constituent atoms is a very powerful concept. An understanding of the underlying topical ordering can be used to forecast a variety of properties. Most information on the local structure was obtained from x-ray diffraction. However, at very high pressure (> 60 GPa), there are significant discrepancies on the coordination number (CN) extracted from the diffraction patterns between different measurements, even for the prototypical SiO₂ and GeO₂ glasses. The difficulty arises from the fact that diffraction does not provide direct information on the change in the electronic structure associated with a structural transformation. In this presentation, it will be shown that element specific experimental probes, such as X-ray Raman (XRS) and X-ray emission (XES) spectroscopy, combined with First-Principles simulation of the glass structures and quantitative electronic structure calculations of the spectra can provide a consistent and reliable description of the local structure of oxide glasses. The results show, different from a recent experimental study on SiO₂, the Si-O CN > 6 is only reached beyond 140 GPa [1]. On the other hand, the octahedral coordination (Ge-O CN~6) in GeO₂ persisted up to 100 GPa, albeit with significant distortion of the environment [2]. We further show a recently proposed relationship between topological ordering and the oxygen volume fraction [3], which relies completely on the choice of oxygen size parameter, is too simplistic [4].

[1] S. Pettigirard, et.al., Geochem. Persp. Lett. (2019) 9, 32-37.

[2] G. Spiekermann, et.al., Phys. Rev. X. (2019) 9, 011025.

[3] A. Zeidler, P.S. Salmon and L.B. Skinner, Proc. Natl. Acad. Sci. U.S.A. (2014), 111, 10045−

10048.

[4] X. Du and J.S. Tse, J. Phys. Chem. B, (2017) 10726-10732.