Giant Lattice Expansion through Structural Frustration Release in a Dense Oxide

Large lattice responses in dense inorganic oxides are typically driven by structural phase transitions or redox processes that alter the crystal symmetry or composition. Here, we identify a distinct mechanism for such responses: giant lattice expansion through symmetry-preserving cation redistribution in a metastable, structurally frustrated, dense oxide. High-pressure-quenched Ba4Ru3O12 undergoes an irreversible volumetric expansion of 4.4% between 450 and 650 K while retaining R3̅m symmetry and oxygen stoichiometry. High-resolution synchrotron diffraction reveals cooperative redistribution of Ru within face-sharing RuO6 trimers, directly linking the intratrimer cation configuration to the lattice volume. Thermogravimetric, transport, and magnetic measurements exclude decomposition, redox processes, and electronic or magnetic phase transitions. First-principles calculations show that compression stabilizes a low-volume cation configuration, which is retained after recovery to ambient pressure and relaxes upon heating through intratrimer cation exchange. Together, these results establish symmetry-preserving cation redistribution as a mechanism for giant lattice responses in dense oxides and identify metastable frustrated configurations as a route to large structural responses without symmetry-breaking or compositional change.