DETERMINATION OF THE ANTIMONY SUBSTITUTION SITE IN CALCIUM FLUORAPATITE FROM POWDER X-RAY-DIFFRACTION DATA

Comparison of powder X-ray diffraction data from a 2.2 wt% antimony-substituted calcium fluorapatite [0.185 Sb atoms per unit cell containing Ca10F2(PO4)6] with data from an undoped sample shows electron density changes corresponding to antimony substitution at the Ca(2) (mirror) site. Many properties of this 2.2 wt% Sb sample indicate that it corresponds to commercial halophosphate phosphors of lower Sb concentration. Differing properties are shown by a 3.1 wt% Sb sample for which no diffraction evidence is found for substitution at the Ca(2) site, but for which electron density difference maps do suggest substitution at the Wyckoff 2(c) and 2(d) sites of P63/m, between Ca(1) (threefold) positions. Both Rietveld refinements and Fourier inversion of the differences between observed intensities were used to reach these conclusions.