Phase Transitions and Resistivity Anomaly of Layered MoO₃ at High Pressure

MoO 3 has large electronic and mechanical anisotropy arising from diverse chemical bonding in its layered structure. Here, we report high-pressure structural and electronic transitions in MoO 3 to 100 GPa, using confocal micro-Raman spectroscopy, synchrotron X-ray diffraction, and electric conductivity measurements at both ambient and low temperatures. The results indicate that MoO 3 -I ( Pnma ) undergoes a series of structural phase transitions, initially to MoO 3 -II ( P 2 1 / m ) at 11 GPa and then to MoO 3 -III ( Pmma ) at 60 GPa. The former transition occurs displacively within nearly a same lattice of a I = 2 c II, b I = b II, and c I = a II, whereas the latter transition occurs reconstructively with ∼3–5% volume collapse at the transition. Interestingly, MoO 3 -II is absent in hydrostatic helium pressure medium, underscoring the presence of helium resisting shear deformation of this layered structure. MoO 3 -I directly transforms to MoO 3 -III at ∼26 GPa. These structural phase transitions also accompany a strong modification of electronic structure from insulating MoO 3 -I to poor metallic MoO 3 -III. The refined crystal structure of MoO 3 -III indicates that Mo atoms occupy two different sites at the centers of octahedra and slightly distorted square planar resulting in the O 2p–Mo 4d hybridization. This unique arrangement of Mo atoms in MoO 3 -III gives rise to an interesting resistive anomaly with a T –1/4 dependence of log(ρ) between 256 and 67 K, showing a variable range hopping mechanism of a Mott insulator.