Abstract
The title compound, tetrasodium cobalt aluminium hexaarsenate, Na4Co7-x Al-2/3x (AsO4)(6) (x = 1.37), is isostructural with K4Ni7(AsO4)(6); however, in its crystal structure, some of the Co2+ ions are substituted by Al3+ 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 Co2+ 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.