Lattice and orbital fluctuations in TiPO₄

In the s = 1/2 antiferromagnetic spin chain material TiPO₄, the formation of a spin gap takes place in a two-step process with two characteristic temperatures, T_∗ = 111 K and T _SP = 74 K. We observe an unusual lattice dynamics over a large temperature regime as well as evidence for an orbital instability preceding the spin-Peierls transition. We relate different intrachain exchange interactions of the high temperature compared to the spin-Peierls phase to a modification of the orbital ordering pattern. In particular, our observation of a high-energy excitation of mixed electronic and lattice origin suggests an exotic dimerization process different from other spin-Peierls materials.