Enhanced dimerization of TiOCl under pressure: spin-Peierls - to -   Peierls transition
  We report high-pressure x-ray diffraction and magnetization measurements combined with ab-initio calculations to demonstrate that the high-pressure optical and transport transitions recently reported in TiOCl, correspond in fact to an enhanced Ti3+-Ti3+ dimerization existing already at room temperature. Our results confirm the formation of a metal-metal bond between Ti3+ ions along the b-axis of TiOCl, accompanied by a strong reduction of the electronic gap. The evolution of the dimerization with pressure suggests a crossover from the spin-Peierls to a conventional Peierls situation at high pressures. 
S. Blanco-Canosa, F. Rivadulla, A. Pineiro, V. Pardo, D. Baldomir, D.   I. Khomskii, M. M. Abd-Elmeguid, M. A. Lopez-Quintel...