Carbonate substitution in lead hydroxyapatite Pb5(PO4)3OH

Synthetic carbonate lead hydroxyapatite Pb-5(PO4,CO3)(3)(OH,CO3) was precipitated from aqueous solution and characterized. The maximum content of CO32- ion in lead apatites does not exceed 2.25 wt%. For precipitation from aqueous solutions this is even lower and controlled by the solubility of cerussite PbCO3. Carbonate substitution occurs simultaneously in two structural positions: at OH- sites (A-type substitution) and at PO43- sites (B-type substitution). This is the most pronounced in FTIR (Fourier Transform Infrared Spectroscopy) spectra at 865 cm(-1) and within the range of 1300-1500 cm(-1). The substitution results in slight increase of the unit cell parameter a from 9.874 to 9.904 A. The presence of CO32- in two structural positions results in two stages of the release of CO2 upon heating: at 300-350 degrees C and at 400 degrees C. The presence of carbonates has little effect on thermal decomposition of lead hydroxyapatite which starts at about 450 degrees C resulting in the formation of lead pyrophosphate. (C) 2017 Elsevier B.V. All rights reserved.