High-pressure structural phase transitions in semiconducting niobium dioxide

Abstract

Niobium dioxide was studied up to 47 GPa by angle-dispersive, x-ray powder diffraction. Semiconducting α − NbO 2 (space group I 4 1 / a , Z = 32 ) was found to transform to β − NbO 2 (space group I 4 1 , 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 ) Å , 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 d 1 cation dioxide, NbO 2 , is thus shown to behave in a similar way to d 0 cation dioxides, which follow a phase transition sequence involving the baddeleyite-type structure.