The aluminoarsenate Na_1.67K_1.33Al₃(AsO₄)₄

The title compound, sodium potassium trialuminium tetra­kis­(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 octa­hedra (site symmetries .2. and 2/m..) that are connected to isolated AsO₄ tetra­hedra (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 inter­layer 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.