Crystal structure of Na4Co7-x Al0.67x (As1-y P y O4)6 (x = 1.60; y = 0.116)

The title compound, tetra-sodium hepta-(cobalt/aluminium) hexa-(arsenate/phosphate), Na4Co5.40Al1.07(As0.883P0.116O4)6, was prepared by a solid-state reaction. It is a new member of the family of isostructural compounds with the general formula A 4 M 7(XO4)6 (A: Na, K; M: Ni, Co; X: P, As) that is most similar to Na4Co5.63Al0.91(AsO4)6. The Co(2+) ions in the title compound are substituted by Al(3+) in a fully occupied octa-hedral site (site symmetry 2/m) and a partially occupied tetra-hedral site (site symmetry 2). A third octa-hedral site is fully occupied by Co(2+) ions only. With regard to the P and As atoms, one site (site symmetry m) is simultaneously occupied by As and P, whereas in the second site there is only arsenic. The alkali cations are, as in the isostructural compounds, distributed over half-occupied crystallographic sites, with a positional disorder of one of them. The proposed structural model is based both on a careful investigation of the crystal data, as well as validation by means of bond-valence-sum (BVS) and charge-distribution (CHARDI) calculations. The correlation between the X-ray refinement and the validation results is discussed.