S = 1 2 chains and spin-Peierls transition in TiOCl

We study TiOCI as an example of an S= ½ layered Mott insulator. From our analysis of susceptibility data, combined with local-density-approximation (LDA) and LDA+ U band-structure calculations, we conclude that orbital ordering produces quasi-one-dimensional spin chains and that TiOCI is an example of Heisenberg chains that undergo a spin-Peierls transition. The energy scale is an order of magnitude larger than that of previously known examples. The effects of nonmagnetic Sc impurities are explained using a model of broken finite chains.