Critical Properties of the Unconventional Spin-Peierls System TiOBr

TiOBr is one of only three inorganic compounds which have been shown to undergo a spin-Peierls (SP) transition, developing a dimerized singlet ground state at low temperatures. However, unlike conventional SP systems, TiOBr exhibits not one, but two successive phase transitions: a continuous transition to an incommensurate SP state at T$_{c2}$$\sim$48K, followed by a discontinuous transition to a commensurate SP state at T$_{c1}$$\sim$27K. We have performed x-ray diffraction measurements on single crystal TiOBr, and carried out a detailed analysis of the critical exponent $\beta$ near T$_{c2}$. Our results yield a transition temperature of T$_{c2}$=47.85(5)K and an exponent of $\beta$=0.30(3). This value is consistent with conventional 3D Ising-like behavior ($\beta$=0.326), and closely compares to the exponent reported for CuGeO$_{3}$ ($\beta$=0.36(3)) [1], the canonical inorganic SP system. In contrast with measurements performed on isostructural TiOCl [2], we observe no evidence of commensurate dimerization fluctuations in either the incommensurate SP phase (T$_{c1}$$<$T$<$T$_{c2}$) or the so-called pseudogap phase (T$_{c2}$$<$T$<$T*$\sim$100-150K). Furthermore, the incommensurate scattering between T$_{c1}$ and T$_{c2}$ appears to be shifted in Q-space relative to the commensurate scattering below T$_{c1}$. [1] M.D. Lumsden et al., PRL {\bf 76}, 4919 (1996). [2] J.P. Clancy et al., PRB {\bf 75}, 100401(R) (2007).