Crystal structure of Na4Co7−xAl0.67x(As1−yPyO4)6(x = 1.60;y= 0.116)

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Abstract
The title compound, tetrasodium 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 formulaA4M7(XO4)6(A: Na, K;M: Ni, Co;X: P, As) that is most similar to Na4Co5.63Al0.91(AsO4)6. The Co2+ions in the title compound 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. With regard to the P and As atoms, one site (site symmetrym) 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.

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