The novel arsenate Na4Co(7-x)Al(2/3x)(AsO4)6 (x = 1.37): crystal structure, charge-distribution and bond-valence-sum investigations.

The title compound, tetrasodium cobalt aluminium hexaarsenate, Na(4)Co(7-x)Al(2/3x)(AsO(4))(6) (x = 1.37), is isostructural with K(4)Ni(7)(AsO(4))(6); however, in its crystal structure, some of the Co(2+) ions are substituted by Al(3+) in a fully occupied octahedral site (site symmetry 2/m) and a partially occupied tetrahedral site (site symmetry 2). A third octahedral site is fully occupied by Co(2+) ions only. One of the two independent tetrahedral As atoms and two of its attached O atoms reside on a mirror plane, as do two of the three independent Na(+) cations, all of which are present at half-occupancy. The proposed structural model based on a careful investigation of the crystal data is supported by charge-distribution (CHARDI) analysis and bond-valence-sum (BVS) calculations. The correlation between the X-ray refinement and the validation results is discussed.