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.