LiCo₂As₃O₁₀: une nouvelle structure à tunnels inter­connectés

The title compound, lithium dicobalt(II) triarsenate, LiCo₂As₃O₁₀, was synthesized by a solid-state reaction. The As atoms and four out of seven O atoms lie on special positions, all with site symmetry m. The Li atoms are disordered over two independent special (site symmetry -1) and general positions with occupancies of 0.54 (7) and 0.23 (4), respectively. The structure model is supported by bond-valence-sum (BVS) and charge-distribution (CHARDI) methods. The structure can be described as a three-dimensional framework constructed from bi-octahedral Co₂O₁₀ dimers edge-connected to As₃O₁₀ groups. It delimits two sets of tunnels, running parallel to the a and b axes, the latter being the larger. The Li⁺ ions are located within the inter­sections of the tunnels. The possible motion of the alkali cations has been investigated by means of the BVS model. This simulation shows that the Li⁺ motion appears to be easier mainly along the b-axis direction and that this material may possess inter­esting conduction properties.

β-Xenophyllite-type Na₄Li_0.62Co_5.67Al_0.71(AsO₄)₆

The title compound, tetrasodium lithium cobalt aluminium hexa­(orthoarsenate), was synthesized by a solid state reaction route. In the crystal structure, Co²⁺ ions are partially substituted by Al³⁺ in an octa­hedral environment [M1 with site symmetry 2/m; occupancy ratio Co:Al = 0.286 (10):0.714 (10)]. The charge compensation is ensured by Li⁺ cations sharing a tetra­hedral site with Co²⁺ ions [M2 with site symmetry 2; occupancy ratio Co:Li = 0.690 (5):0.310 (5)]. The anionic unit is formed by two octa­hedra and three tetra­hedra linked only by corners. The CoM1M2As₂O₁₉ units associate to an open three-dimensional framework containing tunnels propagating along the a-axis direction. One Na⁺ cation is located in the periphery of the tunnels while the other two are situated in the centres: all Na⁺ cations exhibit half-occupancy. The structure of the studied material is compared with those of various related minerals reported in the literature.

Na₃Co₂(As_0.52P_0.48)O₄(As_0.95P_0.05)₂O₇

The title compound, trisodium dicobalt(II) (arsenate/phosphate) (diarsenate/diphosphate), was prepared by a solid-state reaction. It is isostructural with Na₃Co₂AsO₄As₂O₇. The framework shows the presence of CoX2₂O₁₂ (X2 is statistically disordered with As_0.95P_0.05) units formed by sharing corners between Co1O₆ octa­hedra and X2₂O₇ groups. These units form layers perpendicular to [010]. Co2O₆ octa­hedra and X1O₄ (X1 = As_0.54P_0.46) tetra­hedra form Co2X1O₈ chains parallel to [001]. Cohesion between layers and chains is ensured by the X2₂O₇ groups, giving rise to a three-dimensional framework with broad tunnels, running along the a- and c-axis directions, in which the Na⁺ ions reside. The two Co²⁺ cations, the X1 site and three of the seven O atoms lie on special positions, with site symmetries 2 and m for the Co, m for the X1, and 2 and m (× 2) for the O sites. One of two Na atoms is disordered over three special positions [occupancy ratios 0.877 (10):0.110 (13):0.066 (9)] and the other is in a general position with full occupancy. A comparison between structures such as K₂CdP₂O₇, α-NaTiP₂O₇ and K₂MoO₂P₂O₇ is made. The proposed structural model is supported by charge-distribution (CHARDI) analysis and bond-valence-sum (BVS) calculations. The distortion of the coordination polyhedra is analyzed by means of the effective coordination number.