Incommensurate structure of the spin-Peierls compound TiOCl in zero and finite magnetic fields

We report on a detailed single crystal x-ray diffraction study of the unconventional spin-Peierls compound TiOCl. The intermediate phase of TiOCl is characterized by an incommensurate modulation which is virtually identical to that recently found in the homologue compound TiOBr. The first order phase transition between the spin-Peierls ground state and the incommensurate phase reveals the same kind of thermal hysteresis in both its crystal structure and magnetic susceptibility. A weak, but significant magnetic field effect is found for this phase transition with a field induced shift of the transition temperature of $\Delta T_{c1}=-0.13\mathrm{K}$ for an applied field of $B=10\mathrm{T}$ along the chain direction. The field induced changes of the incommensurate crystal structure are compatible with a scenario of competing intrachain and interchain interactions.

Figure 1

Temperature dependence of the intensity of the (0, 1.5, 0) superlattice reflection of TiOCl in the vicinity of the first phase transition around $T_{c1}=67\mathrm{K}$. Shown are measurements in a zero field (open circles) and in an external field of $B=10\mathrm{T}$ (full circles), respectively. The inset shows intensities of both commensurate and incommensurate reflections over the whole temperature range.

Figure 2

Evolution of the (0, 1.5, 0) superlattice reflection of TiOCl for various temperatures on passing through the first phase transition from the dimerized spin-Peierls ground state into the intermediate phase in zero field. The peak splitting indicates an incommensurate modulation of the intermediate phase.

Figure 3

Temperature dependence of the incommensurate components $ϵ$ (a) and $\delta$ (b) of the $(\pm \delta ,2-ϵ,0)$ reflection of TiOCl in the intermediate phase in zero field (open circles) and in an external field of $B=10\mathrm{T}$ (full circles). The insets show the incommensurate positions $\delta$ and $ϵ$ in more detail around $T_{c1}$.