Lattice Evolution and Proton Positioning in Hydrogen Molybdenum Bronzes under Pressure

Hydrogen molybdenum bronzes (H x MoO 3 ) are mixed-valence transition metal oxides whose structural and electronic properties are strongly influenced by their proton behavior. Despite their extensive study under ambient conditions, their structural evolution under compression remains largely unexplored. Here, we report the high-pressure structural behavior of three hydrogen molybdenum bronzes using synchrotron-based powder X-ray and neutron diffraction. Pressure induces structural phase transitions in all three compounds, yet each materials’ evolution is distinct, suggesting a coupling between hydrogen location/concentration and framework stress response. The bronze with the lowest reduction and proton incorporation level ( x ~ 0.36, Type I) exhibits a compression pathway similar to that of α -MoO 3 , whereas at higher hydrogenation levels ( x ~ 0.95, Type II; x ~ 1.68, Type III), distinct pressure-dependent changes in bulk modulus reveal altered elastic response during compression. Complementary in situ neutron diffraction measurements on D 0.36 MoO3 directly locate the hydrogen atom at an equatorial oxygen. Upon compression to 7.8 ± 1.5 GPa, the local hydrogen environment becomes increasingly constrained, exhibiting significant deviation of the O–D···O angle from 180° and shortening of the D···O distance by ca. 0.3 Å. These results establish the structural response of hydrogen molybdenum bronzes to high pressure and demonstrate how proton incorporation governs framework distortions and compressibility.