Anomalous structural disorder and distortion in metal-to-insulator-transition Ti₂O₃

Mott proposed that impurity bands in corundum-symmetry Ti₂O₃ at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti₂O₃ using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti₂O₃ powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti₂O₃ with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRD reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c-axis for T > 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3d a_1g and e_g^π bands, resulting in a collapse of the band gap for T > 450 K.