# Symmetry disquisition on the TiOX phase diagram

## TL;DR

This study investigates the phase transitions and symmetries of TiOX (TiOBr and TiOCl) using inelastic light scattering, revealing the role of symmetry differences and interchain interactions in the formation of various phases.

## Contribution

It provides new insights into the symmetry changes and transition characteristics of TiOX, highlighting the importance of magnetic interchain interactions and clarifying the phononic nature of high energy scattering.

## Key findings

- First-order transition to spin-Peierls phase explained by symmetry differences.
- Pressure dependence indicates magnetic interchain interactions influence phase formation.
- High energy scattering is phononic, not electronic.

## Abstract

The sequence of phase transitions and the symmetry of in particular the low temperature incommensurate and spin-Peierls phases of the quasi one-dimensional inorganic spin-Peierls system TiOX (TiOBr and TiOCl) have been studied using inelastic light scattering experiments. The anomalous first-order character of the transition to the spin-Peierls phase is found to be a consequence of the different symmetries of the incommensurate and spin-Peierls (P$2_{1}/m$) phases. The pressure dependence of the lowest transition temperature strongly suggests that magnetic interchain interactions play an important role in the formation of the spin-Peierls and the incommensurate phases. Finally, a comparison of Raman data on VOCl to the TiOX spectra shows that the high energy scattering observed previously has a phononic origin.