Phase Transitions and Resistivity Anomaly of Layered MoO₃ at High Pressure

Abstract

MoO3 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 MoO3 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 MoO3-I (Pnma) undergoes a series of structural phase transitions, initially to MoO3-II (P21/m) at 11 GPa and then to MoO3-III (Pmma) at 60 GPa. The former transition occurs displacively within nearly a same lattice of aI = 2 cII, bI = bII, and cI = aII, whereas the latter transition occurs reconstructively with ∼3–5% volume collapse at the transition. Interestingly, MoO3-II is absent in hydrostatic helium pressure medium, underscoring the presence of helium resisting shear deformation of this layered structure. MoO3-I directly transforms to MoO3-III at ∼26 GPa. These structural phase transitions also accompany a strong modification of electronic structure from insulating MoO3-I to poor metallic MoO3-III. The refined crystal structure of MoO3-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 MoO3-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.