New arsenates (V) NaKAl2O[AsO4]2 and Na2KAl3[AsO4]4

The title compounds have been synthesized by a solid state reaction route using a salt flux. Their crystal structures were determined from single crystal X-ray data. NaKAl2O[AsO4]2 crystallizes with the orthorhombic K2Fe2O[AsO4]2-type, Pnma, a = 8.2368(6) Å, b = 5.5228(3) Å, c = 17.0160(13) Å and Z = 4, whereas Na2KAl3[AsO4]4 crystallizes with the orthorhombic K3Fe3[AsO4]4-type, Cmce, a = 10.5049(9), b = 20.482(2), c = 6.3574(6) Å and Z = 4. The NaKAl2O[AsO4]2 structure is built up of [Al2As2O9]2− layers perpendicular to the c-axis which are separated by A+ alkali layers. The [Al2As2O9]2− layers consist of ribbons of edge-sharing AlO6 octahedra, running along the a direction and which are connected through AsO4 tetrahedra by sharing corners. The Na2KAl3[AsO4]4 structure contains [Al3As4O16]3− layers perpendicular to the b-axis separated by A+ alkali layers. The [Al3As4O16]3− layer consists of a layer of corner-sharing AlO6 octahedra which are also connected to the AsO4 tetrahedra by sharing corners.