HP-MoO₂: A High-Pressure Polymorph of Molybdenum Dioxide

High-pressure molybdenum dioxide (HP-MoO₂) was synthesized using a multianvil press at 18 GPa and 1073 K, as motivated by previous first-principles calculations. The crystal structure was determined by single-crystal X-ray diffraction. The new polymorph crystallizes isotypically to HP-WO₂ in the orthorhombic crystal system in space group Pnma and was found to be diamagnetic. Theoretical investigations using structure optimization at density-functional theory (DFT) level indicate a transition pressure of 5 GPa at 0 K and identify the new compound as slightly metastable at ambient pressure with respect to the thermodynamically stable monoclinic MoO₂ (α-MoO₂; ΔE_m = 2.2 kJ·mol^-1).