Mechanism of Substitution in Carbonated Apatites

Single-phase AB-type carbonate apatites were prepared by sintering appropriate mixtures of CaHPO₄ and CaCO₃ at 870°C in a CO₂ atmosphere with a partial water vapor pressure of 5 mm Hg. Chemical and physical analyses indicate that at a constant CO₃²⁻/OH⁻ ratio in the hydroxyl sublattice, carbonate substitutes for phosphate on a 1:1 mole basis. For every three PO₄³⁻ ions substituted, two vacancies in the Ca²⁺ sublattice and one in the OH⁻ sublattice are created. The same substitution mechanism seems to apply in pure B-type carbonate apatite.