K(MoO₂)₄O₃(AsO₄)

A new compound with a non-centrosymmetric structure, potassium tetra­kis­[dioxomolybdenum(IV)] arsenate trioxide, K(MoO₂)₄O₃(AsO₄), has been synthesized by a solid-state reaction. The [(MoO₂)₄O₃(AsO₄)]⁺ three-dimensional framework consists of single arsenate AsO₄ tetra­hedra, MoO₆ octa­hedra, MoO₅ bipyramids and bi­octa­hedral units of edge-sharing Mo₂O₁₀ octa­hedra. The [Mo₂O₈]_∞ octa­hedral chains running along the a-axis direction are connected through their corners to the AsO₄ tetra­hedra, MoO₆ octa­hedra and MoO₅ bipyramids, so as to form large tunnels propagating along the a axis in which the K⁺ cations are located. This structure is compared with compounds containing M₂O₁₀ (M = Mo, V, Fe) dimers and with those containing M₂O₈ (M = V) chains.

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.