Preparation and characterization of magnesium/carbonate co-substituted hydroxyapatites

Abstract

A new synthesis/processing method has been devised to produce magnesium/carbonate co-substituted hydroxyapatite ceramics that do not decompose to tricalcium phosphate (TCP) on sintering. Using this method, a series of magnesium/carbonate co-substituted hydroxyapatite (Mg/CO₃–HA) compositions, containing between 0 and 0.35 wt % Mg and approximately 0.9 wt % CO₃ were prepared. Sintering the Mg/CO₃–HA compositions in a CO₂/H₂O atmosphere yields a single crystalline phase that appears to be identical to stoichiometric HA. In contrast, when the compositions were prepared in the absence of carbonate and were sintered in air, the phase composition was a biphasic mixture of HA and TCP e.g. for 0.25 wt % Mg substitution the phase composition was approximately 60%HA/40% TCP. Clearly, both the synthesis route and the processing (i.e. sintering) route are of importance in the production of a single-phase Mg/CO₃–HA ceramic. Fourier transform infrared (FTIR) spectroscopy has indicated that the Mg/CO₃–HA ceramics still contained carbonate groups after sintering at 1200 °C. Chemical analysis by X-ray fluorescence spectroscopy (XRF) and C–H–N analysis has shown that the cation/anion molar ratio (i.e. [Ca+Mg]/[P+C/2]) of the different compositions were 1.68(±0.01), which is equivalent to the Ca/P molar ratio of stoichiometric HA. Although the magnesium/carbonate co-substitution had a positive effect in preventing phase decomposition during sintering, it appeared to have a negative effect on the densification of the MgCO₃–HA ceramics, compared to stoichiometric HA.