Synthesis, structure and Na+ migration pathways of new Wylleite-type Na1.25Co2.187Al1.125(AsO4)3

A new sodium cobalt (II) aluminum arsenate Na1.25Co2.187Al1.125(AsO4)3 has been synthesized by solid state reaction method and its structure has been determined by single crystal X-ray diffraction at room temperature. The title material crystallizes in the monoclinic system, space group P21/c with the unit cell parameters: a = 6.532(2) Å, b = 12.492(2) Å, c = 11.060(2) Å, β = 99.44 (2)°, V = 890.3(3) Å3 and Z = 4. The peculiarity of this structure is the Al3+ and Co2+ distribution over four crystallographic independent sites. Charge distribution (CHARDI) calculations and bond-valence sum (BVS) analysis were used to support the proposed structural model. The crystal structure of the title compound features a 3D anionic framework built of corner-edge sharing (Al3+/Co2+)O6 and AsO4 polyhedra, with interconnecting channels where the Na+ cations are located. The title compound is a new member of Wylleite family with an additional new Na+ site denoted X(3). The modelling of pathways transport of Na+ in the anionic framework shows that only the sodium Na1 can diffuse through an infinite 1D pathway along [100] direction. The empirical activation energy deduced from the Bond-Valence Site Energy (BVSE) model is about 5.45 eV.