Pressure-induced drastic collapse of a high oxygen coordination shell in quartz-like α-GeO2

With the combination of a single crystal diamond anvil cell and a polycapillary half-lens, the local structural evolution around germanium in tetrahedrally networked quartz-like alpha-GeO2 has been investigated using extended x-ray absorption fine structure spectroscopy of up to 14 GPa by multiple-scattering analysis method. While the first shell Ge-O bond distances show a slight contraction with increasing pressure, the third shell Ge-O bond distances are found to decrease dramatically. The sluggish lengthening of the first shell Ge-O bond distances, initiated by coordination increase from fourfold to sixfold, occurs in the 7-14 GPa range just when the third shell Ge-O bond distances fall in the region of the second shell Ge-Ge bond distances. Moreover, these features are accompanied by the closing of intertetrahedral Ge-O-Ge angles and the opening of two intratetrahedral O-Ge-O angles, whose topological configuration surprisingly exhibits a helical chirality along the c axis that is opposite to the double helices of the corner-linked GeO4 tetrahedra. These results suggest that the high-pressure phase transitions in quartz and quartz-like materials could be associated with a structural instability that is driven by the drastic collapse of the next-nearest-neighbour anion shell, which is consistent with the emergence of high-symmetry anion sublattice. Our findings provide crucial insights into the densification mechanisms of quartz-like oxides, which would have broad implications for our understanding of the metastability of various post-quartz crystalline phases and pressure-induced amorphization.