Crystal Structure of Ca₃(AsO₄)₂

Abstract

Ca 3 (AsO 4 ) 2 crystallizes in the rhombohedral space group R3c with unit cell parameters a = 14.05(1) Å, α = 45.05(2)°, and Z R = 7. The equivalent hexagonal axes are a = 10.77(1) and c = 37.81(3) Å with Z H = 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 CaO n polyhedra and AsO 4 tetrahedra paralleling the c axis. The six chains surrounding the three-fold axes are each much like those in Ba 3 (PO 4 ) 2 with 3Ca followed by two AsO 4 groups per half cell. The chain on the three-fold axis has only one AsO 4 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 Ba 3 (PO 4 ) 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 Ca 3 (VO 4 ) 2 and although closely related to that of Whitlockite differs from it because of the need to accommodate hydrogen atoms in the latter compound.