High pressure X-ray study of the lattice dimerization in the S$=$1/2 chain quantum magnet TiOCl.

TiOCl, composed of S$=$1/2 chains of Ti ions, is a good realization of a spin-Peierls system. Considerable effort has been put into doping the material with the goal of achieving metallization. Interestingly, high pressure resistivity shows a dramatic decrease of its insulating behavior, which may coincide with the emergence of charge density wave order. Therefore, high pressure of the structure with x-rays are important in further exploring the phase diagram. Upon cooling under normal pressure conditions, TiOCl exhibits a transition to an incommensurate nearly dimerized state at Tc2 $=$ 92 K and to a commensurate dimerized state at Tc1 $=$ 66 K. Here we map its complex temperature vs. pressure phase diagram with x-rays on a single crystal in a high pressure diamond anvil down to 6 K in temperature and to pressures up to approximately 14.5 GPa.