Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at T_{c2} = 90 K corresponds to a lowering of the lattice symmetry. Below T_{c1} = 66 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 1D 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}.
@article{arxiv.cond-mat/0503203,
title = {Spin-Peierls transition in TiOCl},
author = {M. Shaz and S. van Smaalen and L. Palatinus and M. Hoinkis and M. Klemm and S. Horn and R. Claessen},
journal= {arXiv preprint arXiv:cond-mat/0503203},
year = {2009}
}