Local structural investigation of non-crystalline materials at high pressure: the case of GeO₂ glass

Local structures play a crucial role in the structural polyamorphism and novel electronic properties of amorphous materials, but their accurate measurement at high pressure remains a formidable challenge. In this article, we use the local structure of network-forming GeO2 glass as an example, to present our recent approaches and advances in high-energy x-ray diffraction, high-pressure x-ray absorption fine structure, and ab initio first-principles density functional theory calculations and simulations. Although GeO2 glass is one of the best studied materials in the field of high pressure research due to its importance in glass theory and geophysical significance, there are still some long-standing puzzles, such as the existence of appreciable distinct fivefold Ge-[5] coordination at low pressure and the sixfold-plus Ge[6+] coordination at ultrahigh pressure. Our work sheds light on the origin of pressure-induced polyamorphism of GeO2 glass, and the Ge-[5] polyhedral units may be the dominant species in the densification mechanism of network-forming glasses from tetrahedral to octahedral amorphous structures.