The aluminoarsenate Na1.67K1.33Al3(AsO4)4

The title compound, sodium potassium trialuminium tetra-kis-(orthoarsenate), was prepared by solid-state reactions. The anionic framework consists of corrugated layers parallel to (010) and is made up of corner-sharing AlO6 octa-hedra (site symmetries .2. and 2/m..) that are connected to isolated AsO4 tetra-hedra (site symmetries .2. and m..) through edge- and corner-sharing. The alkali cations are occupationally disordered. The two K(+) cations [site symmetries .2. and m..; occupancies 0.314 (7) and 0.035 (12)] are situated in the inter-layer space, whereas the smaller Na(+) cations [both with site symmetry m..; occupancies = 0.725 (14) and 0.112 (14)] are located in the cavities of the anionic framework. The K(+) cations are surrounded by six and seven O atoms, the Na(+) cations by seven and nine O atoms. The resulting coordination polyhedra of the two types of cations are highly distorted.