Spin-Peierls transition in TiOCl

Temperature-dependent x-ray diffraction of the low-dimensional spin-$1∕2$ quantum magnet TiOCl shows that the phase transition at ${T}_{c2}=90\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ corresponds to a lowering of the lattice symmetry. Below ${T}_{c1}=66\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure one-dimensional chains of atoms. The first-order character of the transition at ${T}_{c1}$ is explained by the competition between the structurally deformed state below ${T}_{c2}$ and the spin-Peierls state below ${T}_{c1}$.