The novel arsenate Na4Co7−xAl2/3x(AsO4)6 (x = 1.37): crystal structure,charge‐distribution and bond‐valence‐sum investigations

The title compound, tetrasodium cobalt aluminium hexaarsenate, Na₄Co_7−xAl_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.