Stabilization of Carbonate Hydroxyapatite by Isomorphic Substitutions of Sodium for Calcium

Carbonate hydroxyapatite (CHA) is an analogue of the mineral component of bone tissue. Synthetic CHA is thermally unstable: it readily decomposes with carbon oxide evolution when sintered to ceramics. Its thermal stability has been studied as affected by partial isomorphic substitution of sodium for calcium intended to compensate a possible charge imbalance induced by CO₃^2- groups. Investigative tools were thermogravimetry and FTIR spectroscopy of the condensed vapor produced by heating CHA samples doped with 0.4 and 0.8wt % sodium. Sodium does not improve the thermal stability of CHA: weight loss on heating increases with increasing sodium level; evolution of carbon oxides occurs at lower temperatures and more intensively. Sodium enhances the generation of B-type defects (CO₃^2- -> PO₃^2- substitutions); these defects are thermodynamically less stable than AB-type defects (CO₃^2- -> PO₃^2-,OH^- substitutions), which are characteristic of sodium-free CHA.