Site Preference of Rare Earth Elements in Hydroxyapatite [Ca10(PO4)6(OH)2]

Crystals of rare earth element-(REE)-bearing hydroxyapatites [La–OHAp, Nd–OHAp, Sm–OHAp, Dy–OHAp; Ca10-6x2yNayREE6x+y(P1-xSixO4)6(OH)2, with x=0.036, y=0.045; space group P21/b] have been grown from H2O- and Na-rich phosphate melts and their structures refined in space group P63/m at room temperature with single crystal X-ray intensities to R=0.019–0.025. Hydrogen bond valences are in good agreement with a hydrogen-bonded model for the c-axis columns of OHions. The crystal/melt partition coefficients for REEs are 3.1, 3.3, 3.0, and 2.4 and REE site occupancy ratios (REE–Ca2/REE–Ca1) are 11, 2.0, 1.6, and 3.3, respectively. The uptake of REEs peaks at Nd, as in REE-substituted fluorapatite (FAp). The REE site occupancy ratio in REE–OHAp and REE–FAp is proportional to the change in unit-cell volume, pointing to some control by spatial accommodation of substituents in the apatite structure. Both the site occupancy ratio and uptake are influenced also by the substitution mechanism, which varies with the volatile anion component.