Crystal structure of the arsenate NaNb4O9AsO4. A framework structure with diffusion pathways

Single crystals of NaNb₄O₉AsO₄, were prepared, by solid state reaction at 875°C from a mixture of NaNO₃, Nb₂O₅ and NH₄H₂AsO₄. NaNb₄O₉AsO₄ crystallizes in the orthorhombic system, space group Pnma, Z=4, with a=9.9858(7) Å, b=10.3811(6) Å, c=10.4107(6) Å. The diffraction intensities were collected on a Siemens P4 diffractometer with MoKα radiation. The crystal structure was determined and refined from 3465 independent reflections. The three-dimensional framework consists of infinite chains (-NbO₆-NbO₆-AsO₄-)_n, parallel to the b axis and crosslinked via corner sharing by pairs of edge-sharing octahedra. Each pair links four chains. The Na ion partially occupies several independent close positions, with various occupancies, in the interconnected cavities delimited by the framework.