The crystal structure of magnesium arsenate,Mg8.5As3O16

Single crystals of magnesium arsenate have been grown from a PbOAs₂O₅ eutectic by flux crystallization of a ceramic preparation having an initial composition of 6MgO·As₂O₅. The crystals are rhombohedral, space group $\text{R}\text{3}\text{m}$, with hexagonal unit cell parameters a = 6.0278(6) and c = 27.600(3) Å. A three-dimensional structural analysis using automatic diffractometer data has been completed and refined by full-matrix least-squares procedures to a residual R = 0.049 (R_w = 0.059) with a data-parameter ratio of 36 in the final anisotropic refinement. The structure analysis indicated that magnesium arsenate has the composition Mg_8.5As₃O₁₆ and is isostructural with the previously reported Co₈As₃O₁₆. The structure is based upon a cubic close-packed oxygen array with charge balance achieved in magnesium arsenate through partial occupancy of a unique magnesium site occurring in an arsenic-substituted MgO-type layer.