Na 3 Co 2 (As 0.52 P 0.48 )O 4 (As 0.95 P 0.05 ) 2 O 7

The title compound, trisodium dicobalt(II) (arsenate/phosphate) (diarsenate/diphosphate), was prepared by a solid-state reaction. It is isostructural with Na 3 Co 2 AsO 4 As 2 O 7 . The framework shows the presence of Co X 2 2 O 12 ( X 2 is statistically disordered with As 0.95 P 0.05 ) units formed by sharing corners between Co1O 6 octahedra and X 2 2 O 7 groups. These units form layers perpendicular to [010]. Co2O 6 octahedra and X 1O 4 ( X 1 = As 0.54 P 0.46 ) tetrahedra form Co2 X 1O 8 chains parallel to [001]. Cohesion between layers and chains is ensured by the X 2 2 O 7 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 Co 2+ cations, the X 1 site and three of the seven O atoms lie on special positions, with site symmetries 2 and m for the Co, m for the X 1, 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 K 2 CdP 2 O 7 , α-NaTiP 2 O 7 and K 2 MoO 2 P 2 O 7 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.