Fe5O5[B6O10(OH)3] x nH2O: wave-layered iron borate and frustrated antiferromagnetism.

The first layered iron borate, Fe(5)O(5)[B(6)O(10)(OH)(3)] x nH(2)O, has been prepared by the boric acid flux method. Its structure, determined by single crystal X-ray diffraction, contains a double FeO(6)-octahedral layer and an unusual [B(6)O(13)] chain. The rigid and cambered [B(6)O(13)] chains bend the octahedral layers, resulting in a wave-like and sandwiched structure. Crystallographic study indicates the structural modulation is mainly from the [B(6)O(13)] chains because of the insertion of water molecules in between. Nevertheless, FeO(6) layers in the average structure, which are well separated by borate chains, is still a reasonable model to understand the two-dimensional magnetism. The strong antiferromagnetic interactions and the complex Fe(3+)-net suggest a possible geometrically magnetic frustration, which may be the reason for the second-order temperature-induced magnetic transition at approximately 125 K. The condensed Fe(3+) layers and the relatively low redox potential at about 1.25 V versus Li(+)/Li show its potentials as an anodic material.