PHASE-TRANSITION AND V4+-V4+ PAIRING IN VO2(B)

The study of low-temperature X-ray diffraction and magnetic properties of VO2(B) has revealed a structural phase transition and formation of V4+ ion pairs. The phase transition occurs over a temperature range from 300 to 180 K; however, a complete structural change is not attained. The low-temperature phase has the same monoclinic symmetry as the high-temperature phase. The X-ray Rietveld refinements revealed a significant decrease in the V(2)-V(2) distance from 0.2867 to 0.2670 nm, indicating V4+ - V4+ pairing for half of the V sites in the low-temperature phase. The pairing has been confirmed by the observation of a spin-singlet state in the 51V NMR measurements and corresponds to the decrease in magnetic susceptibility. The behavior of phase transition and pairing in VO2(B) are analogous to those in V6O13, which has a related structure. © 1993 Academic Press, Inc.