The influence of structural disorder and phonon on metal-to-insulator transition of VO .

We used temperature-dependent x-ray absorption fine structure (XAFS) measurements to examine the local structural properties around vanadium atoms at the V K edge from VO films. A direct comparison of the simultaneously-measured resistance and XAFS regarding the VO films showed that the thermally-driven structural transition occurred prior to the resistance transition during a heating, while  this change simultaneously occured during a cooling. Extended-XAFS (EXAFS) analysis revealed significant increases of the Debye-Waller factors of the V-O and V-V pairs in the {111} direction of the R-phase VO that are due to the phonons of the V-V arrays along the same direction in a metallic phase. The existance of a substantial amount of structural disorder on the V-V pairs along the c-axis in both M and R phases indicates the structural instability of V-V arrays in the axis. The anomalous structural disorder that was observed on all atomic sites at the structural phase transition prevents the migration of the V 3d electrons, resulting in a Mott insulator in the M-phase VO.