Crystallographic studies of cobalt arsenates, I. Crystal structure of Co3(AsO4)2

Co3(AsO4)2, an analogue of the mineral xanthiosite, crystallizes in the monoclinic space group P21/c with Z = 4. The unit cell parameters are a = 5.830(4) Å, b = 9.675(2) Å, and c = 10.34(2) Å with β = 93.42(5)°. The intensity of 2261 independent reflections were estimated either from integrated precession photographs or by direct measurement using a scintillation counter. The final value of the reliability index is 0.072 for a full matrix least-square refinement where all the atoms have anisotropic thermal parameters. The three independent cations are surrounded octahedrally by oxygen atoms with mean Co—O bond lengths of 2.12 Å and a range from 1.99 to 2.29 Å. One of the arsenate ions, AsO44−, is a nearly regular tetrahedron with a mean As—O bond length of 1.70 Å; while the second, with an angular irregularity primarily introduced by one oxygen atom, shows a mean As—O bond length of 1.69 Å. This structure consists of nearly hexagonally close packed layers of oxygen atoms normal to the b axis repeating every four layers.