High-pressure structural phase transitions in semiconducting niobium dioxide

Niobium dioxide was studied up to 47 GPa by angle-dispersive, x-ray powder diffraction. Semiconducting α-NbO₂ (space group I4₁/a, $Z=32)$ was found to transform to β-NbO₂ (space group I4₁, $Z=16)$ above 5 GPa. Both phases have distorted rutile-type structures and the transition involves a change in the long-range ordering of the metal-metal bonds. A further transition was observed above 8 GPa to a baddeleyite-related structure with $a=9.975(1),$ $b=9.963(1),$ c=9.933(1), β=104.023(4)^∘, and $Z=32$ at 12.8 GPa. The doubling of the unit cell along a, b, and c and the larger monoclinic angle with respect to a baddeleyite-type cell are due to the presence of metal-metal bonds. The d¹ cation dioxide, NbO₂, is thus shown to behave in a similar way to d⁰ cation dioxides, which follow a phase transition sequence involving the baddeleyite-type structure.