Abstract
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 (X-2 is statistically disordered with As P-0.95(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 = As0.54P0.46) tetrahedra form Co2X1O8 chains parallel to [001]. Cohesion between layers and chains is ensured by the X22O7 groups, giving rise to a threedimensional framework with broad tunnels, running along the a-and c-axis directions, in which the Na+ ions reside. The two Co(2+)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 (x2) 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, alpha-NaTiP2O7 and K2MoO2P2O7 is made. The proposed structural model is supported by charge-distribution (CHARDI) analysis and bond-valencesum(BVS) calculations. The distortion of the coordination polyhedra is analyzed by means of the effective coordination number.