First single-crystal synthesis and low-temperature structural determination of the quasi-2D quantum spin compound (CuCl)LaNb2O7.

We provide a full description of the first single-crystal synthesis of the low-dimensional quantum spin compound (CuCl)LaNb(2)O(7) through the low-temperature topotactic ion exchange route. Very fast diffusion of ion-exchanged CuCl and Cs ions is observed. In addition, thorough structure determination at very low temperature is outlined following use of single-crystal X-ray diffraction and powder neutron diffraction, taking advantage of the better sensitivity of the latter method for describing O and Cl atoms. State-of-the-art calculations (Maximum Entropy Method) are used for visualising the fine structural features of this unusual pseudo-tetragonal superstructure. Our study unambiguously throws light on the long-standing controversy concerning the crystal structure of this compound, and allows for a relevant description of its magnetic properties. Incidentally, it is demonstrated that the low-temperature structural model tentatively proposed very recently by Tsirlin et al. (Phys. Rev. B, 2010, 82, 054107), according to powder synchrotron X-ray diffraction, is correct.