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

Abstract

AbstractCarbonated strontium fluor-, hydroxyl- and chlorapatites (carbonated Sr10(PO4)6X2 = 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 ∼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 Ksp 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 Ksp 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).