NMR Study of the Spin-Peierls transition in TiPO$_4$*

We investigated the magnetic and structural properties of the quasi-one dimensional 3$d^1$-quantum chain system TiPO$_4$ ($J \sim$ 965 K) by NMR measurements. TiPO$_4$ undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different $^{31}$P signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti\ldots Ti distances within each chain. Thus, the 74~K phase transition is a spin-Peierls transition which evolves from an incommensurate phase existing between 111~K and 74~K.