Crystal Structure of Ca3(AsO4)2

Abstract

Ca3(AsO4)2 crystallizes in the rhombohedral space group R3c with unit cell parameters a = 14.05(1) Å, α = 45.05(2)°, and ZR = 7. The equivalent hexagonal axes are a = 10.77(1) and c = 37.81(3) Å with ZH = 21. The structure was refined, using 769 symmetry independent reflections and a full matrix least squares program from a trial structure obtained from Patterson functions, to a final R value of 0.063. The structure consists of interconnected chains of CaOn polyhedra and AsO4 tetrahedra paralleling the c axis. The six chains surrounding the three-fold axes are each much like those in Ba3(PO4)2 with 3Ca followed by two AsO4 groups per half cell. The chain on the three-fold axis has only one AsO4 per half cell with one and half cations between adjacent arsenic atoms. The mean As—O bond lengths are 1.69, 1.65, and 1.69 Å for the three independent tetrahedra. The cation sites show coordination to 6, 7, or 8 oxygen atoms in contrast to the 10 or 6 + 6 found in Ba3(PO4)2. These structural differences arise primarily from the shorter cation – oxygen atom bond lengths in this compound. This structure is the same as that of Ca3(VO4)2 and although closely related to that of Whitlockite differs from it because of the need to accommodate hydrogen atoms in the latter compound.