Observation of phase transition with lattice distortion in the low-dimensional quantum spin system TiOBr by synchrotron X-ray diffraction -evidence of spin-Peierls transition?-

Here, we report on the direct observation of lattice distortion in the new quasi-one-dimensional spin system TiOBr by X-ray diffraction experiment using synchrotoron radiation. By comparing the crystal structure between TiOBr and TiOCl, the difference in b-axis length was considered to play a key role in determining the physical properties of both systems. Using a single crystal sample of TiOBr, superlattice reflections were observed at (0, 1.5, 0) and (0, 2.5, 0). The temperature dependence of the superlattice reflections implied the first-order transition at T_c1 = 27 K. We discussed the phase transition and analyzed the data using a simple dimerization model.