Combination of X-ray Absorption Spectroscopy and Atomistic Simulations Reveals Features of Correlated Disorder in Quartz-like Germanium Oxide

Classical crystallography captures long-range symmetry in crystalline materials, but their local atomic configurations also play a key role in overall material order and its function. Emerging research shows that disorder in crystals can arise from electrostatic, magnetic, or orbital interactions, leading to short-range order that mismatches with global symmetry. Yet, the challenge lies in interpreting the type of disorder from locally sensitive spectroscopy at the subnanometre scale. In this paper, we establish correlated disorder in quartz-like germanium oxide (q-GeO2) by examining the extended local structure around Ge under ambient conditions as well as in low-temperature conditions. By integrating experimental X-ray absorption spectroscopy with reverse Monte Carlo/evolutionary algorithm and ab initio molecular dynamics simulations, we could pinpoint correlated disorder, which is different from the disorder due to thermal motion. We capture nearly a third of the Ge atoms in a triangular configuration (sp2-like), while the rest show a distorted tetrahedral configuration (sp3-like), accompanied by nonbridging oxygen atoms that influence network flexibility in q-GeO2. Ultimately, we propose a strategy to characterize nonthermal correlated disorder at cationic sites in metastable oxides, offering new insights into the complex interplay between local disorder and long-range periodic structure.