A POWDER NEUTRON-DIFFRACTION STUDY OF SEMICONDUCTING AND METALLIC NIOBIUM DIOXIDE

Rietveld analysis of neutron powder diffraction patterns from niobium dioxide at 295 and 1300 K have been performed. At 1300 K NbO2 adopts a rutile-type structure (P4(2)/mnm, a = 4.8463 angstrom, c = 3.0315 angstrom, x = 0.2924), whereas the room temperature form can be described as a superstructure with a subcell of the rutile type (I4(1)/a, a = 13.7020 angstrom, c = 5.9850 angstrom). Both the c axis and the oxygen position parameter in the rutile phase are anomalously small compared with other rutile dioxides. This can be rationalized in terms of M-M bonding. Both the niobium and the oxygen thermal displacement are large in the rutile phase. (C) 1994 Academic Press, Inc.