LAUSR

The application of pressure reveals a rich phase diagram for the quantum $S=1/2$ spin chain material TiOCl. We performed x-ray diffraction on single-crystal samples in a diamond-anvil cell down to $T=4$ K and pressures up to 14.5 GPa. Remarkably, the magnetic interaction scale increases dramatically with increasing pressure, as indicated by the high onset temperature of the spin-Peierls phase. The spin-Peierls phase was probed at $\ensuremath{\sim}6$ GPa up to 215 K but possibly extends in temperature to above $T=300$ K, indicating the possibility of a quantum singlet state at room temperature. Near the critical pressure for the transition to the more metallic phase, coexisting phases are exemplified by incommensurate order in two directions. Further comparisons are made with the phase diagrams of related spin-Peierls systems that display metallicity and superconductivity under pressure.