A-type substitution in carbonated strontium fluor-, chlor- and hydroxylapatites

摘要

Carbonated strontium fluor-, hydroxyl- and chlorapatites (carbonated Sr-10(PO4)(6)X-2 = CSrApX, where X= OH, Cl and F) were synthesized in aqueous solution. The substitution mode of carbonate was determined from infrared (IR) stretching frequencies for carbonate and from the variation in unit-cell axial lengths as a function of carbonate content. The a-axis lengths of CSrApF and CSrApCl decrease, whereas the a-axis length of CSrApOH increases slightly with increasing carbonate substitution. The carbonate IR stretching region from similar to 1390 to 1590 cm(-1) contains two doublets for both CSrApOH and CSrApCl, indicating the presence of both A- and B-type carbonate. The carbonate spectral region for CSrApF is reminiscent of that for CCaApF, which contains one doublet for B-type carbonate with a small shoulder attributable to A-type carbonate. Activity-based K-sp values (assuming B-type substitution) were determined for all three series of carbonated Sr apatites and show that the solubilities of CSrApOH and CSrApCl increase at higher carbonate values, whereas those of CSrApF stay constant. The K-sp values for uncarbonated SrApOH, SrApCl and SrApF were determined by extrapolation to zero percent carbonate (10(-120), 10(-112) and 10(-113) for SrApOH, SrApCl and SrApF, respectively).