Crystal structure of Na 4 Co 7− x Al 0.67 x (As 1− y P y O 4 ) 6 ( x  = 1.60; y = 0.116)

The title compound, tetrasodium hepta(cobalt/aluminium) hexa(arsenate/phosphate), Na 4 Co 5.40 Al 1.07 (As 0.883 P 0.116 O 4 ) 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 ( X O 4 ) 6 ( A : Na, K; M : Ni, Co; X : P, As) that is most similar to Na 4 Co 5.63 Al 0.91 (AsO 4 ) 6 . The Co 2+ ions in the title compound 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. 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.