K_0.78Na_0.22MoO₂AsO₄

The title compound, potassium sodium dioxidomolybden­um(VI) arsenate, K_0.78Na_0.22MoO₂AsO₄, was synthesized by a solid-state reaction route. The structure is built up from corner-sharing MoO₆ octa­hedra and AsO₄ tetra­hedra, creating infinite [MoAsO₈]_∞ chains running along the b-axis direction. As, Mo and all but one O atom are on special positions (4c) with m symmetry and K (occupancy 0.78) is on a position (4a) of -1 in the tunnels. The possible motion of the alkali cations has been investigated by means of the bond-valance sum (BVS) model. The simulation shows that the Na⁺ motion appears to be easier mainly along the b-axis direction. Structural relationships between the different compounds of the AMoO₂AsO₄ (A = Ag, Li, Na, K, Rb) series and MXO₈ (M = V; X = P, As) chains are discussed.

Na₇Al₃(As₂O₇)₄

The title compound, hepta­sodium trialuminium tetrakis(diarsenate), has been isolated as single crystals from a solid-state reaction. Its structure, which is isotypic with that of the Na₇Fe₃(X₂O₇)₄ (X = As, P) family of compounds, consists of AlO₆ octa­hedra sharing their vertices with As₂O₇ groups, forming a three-dimensional [Al₃(As₂O₇)₄]_∞ framework incorporating channels occupied by the sodium ions. One of the aluminium ions lies on a crystallographic twofold axis. The sodium ions are situated over ten positions (one with site symmetry 2), all but one of which are partially occupied.