Spin-Peierls transition in the S = 1 2 compound TiPO 4 featuring large intrachain coupling

We investigated the magnetic and structural properties of the quasi-one-dimensional 3${d}^{1}$ quantum chain system TiPO${}_{4}$ ($J~$ 965 K) by magnetic susceptibility, heat capacity, electron spin resonance, x-ray diffraction, and nuclear magnetic resonance (NMR) measurements, and by density functional theory (DFT) calculations. TiPO${}_{4}$ undergoes two magnetostructural phase transitions, one at 111 K and the other at 74 K. Below 74 K, NMR detects two different $^{31}\mathrm{P}$ signals and the magnetic susceptibility vanishes, while DFT calculations evidence a bond alternation of the Ti-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 and 74 K.