Crystallographic studies of the role of Mg as a stabilizing impurity in β-Ca3(PO4)2. The crystal structure of pure β-Ca3(PO4)2 |
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| Authors: | B Dickens LW Schroeder WE Brown |
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| Institution: | Institute for Materials Research, National Bureau of Standards, Washington, DC 20234 USA |
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| Abstract: | β-Ca3(PO4)2 crystallizes in the rhombohedral space group R3c with unit cell parameters a = 10.439(1), c = 37.375(6) Å (hexagonal setting) and cell contents of 21 Ca3(PO4)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 Ba3(VO4)2, but has lower symmetry because of the widely different ionic sizes of Ca and Ba. Seven Ca3(PO4)2] units occupy a volume corresponding to eight Ba3(PO4)2] units. The requirement of the c glide in β-Ca3(PO4)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. |
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