Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca 3(PO 4) 2. The crystal structure of pure β-Ca 3(PO 4) 2
β-Ca 3(PO 4) 2 crystallizes in the rhombohedral space group R3 c with unit cell parameters a = 10.439(1), c = 37.375(6) Å (hexagonal setting) and cell contents of 21 [Ca 3(PO 4) 2]. The structure was refined to Rw = 0.026, R = 0.030 using 1143 X-ray intensities collected from a single crystal by counter methods. Corrections were made for absorption, secondary extinction, and anomalous dispersion. The structure is related to that of Ba 3(VO 4) 2, but has lower symmetry because of the widely different ionic sizes of Ca and Ba. Seven [Ca 3(PO 4) 2] units occupy a volume corresponding to eight [Ba 3(PO 4) 2] units. The requirement of the c glide in β-Ca 3(PO 4) 2 has been shown in the least squares refinements to be attained by disorder of one cation over two sites. This disorder has a far-reaching effect on the structure.