Na(3)Co(2)(AsO(4))(As(2)O(7)): a new sodium cobalt arsenate

In the title compound, tris­odium dicobalt arsenate diarsenate, Na(3)Co(2)AsO(4)As(2)O(7), the two Co atoms, one of the two As and three of the seven O atoms lie on special positions, with site symmetries 2 and m for the Co, m for the As, and 2 and twice m for the O atoms. The two Na atoms are disordered over two general and special positions [occupancies 0.72 (3):0.28 (3) and 0.940 (6):0.060 (6), respectively]. The main structural feature is the association of the CoO(6) octa­hedra in the ab plane, forming Co(4)O(20) units, which are corner- and edge-connected via AsO(4) and As(2)O(7) arsenate groups, giving rise to a complex polyhedral connectivity with small tunnels, such as those running along the b- and c-axis directions, in which the Na(+) ions reside. The structural model is validated by both bond-valence-sum and charge-distribution methods, and the distortion of the coordination polyhedra is analyzed by means of the effective coordination number.