Commensurate Fluctuations in the Pseudogap and Incommensurate Spin-Peierls Phases of TiOCl

We have performed x-ray scattering measurements on the unconventional spin-Peierls system TiOCl and the closely related doped compound Ti$_{(1-x)}$Sc$_{x}$OCl (x = 0.01, 0.03). In pure TiOCl these measurements reveal the presence of commensurate dimerization peaks within both the incommensurate spin-Peierls phase (T$_{c1}<$ T $<$ T$_{c2})$ and the so-called pseudogap phase (T$_{c2}<$ T $<$ T*). This commensurate scattering is slightly shifted in Q-space relative to the commensurate long-range ordered state below T$_{c1}$, and has a fairly narrow width in Q, suggesting correlation lengths greater than 100 angstroms. Below T* $\sim $ 130K, the integrated intensity of the scattering over the commensurate and incommensurate positions grows continuously as a function of decreasing temperature. In addition, measurements performed on the doped compound show that the substitution of non-magnetic Sc$^{3+}$ ions (S = 0) onto Ti$^{3+}$ (S = 1/2) sites appears to suppress commensurate fluctuations and prevent the development of a commensurate long-range ordered state.