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 alpha-NbO2 (space group I4(1) /a, Z = 32) was f ound to transform. to beta-NbO2 (space group I4(1), Z = 16) above 5 GPa. Bo th phases have distorted rutile-type structures and the transition involves a change in the long-range ordering of the metal-metal bonds. A further tr ansition was observed above 8 GPa to a baddeleyite-related structure with a = 9.975(1), b = 9.963(1), c = 9.933(1) Angstrom, beta = 104.023(4)degrees, and Z = 32 at 12.8 GPa. The doubling of die unit cell along a, b, and c an d the larger monoclinic angle with respect to a baddeleyite-type cell are d ue to the presence of metal-metal bonds. The d(1) cation dioxide, NbO2, 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. [S01 63-1829(99)13021-2].