Thermal impurity reactions and structural changes in slightly carbonated hydroxyapatite

Abstract

Lattice and surface impurity reactions and structural changes induced by them in slightly carbonated hydroxyapatite (SCHA) treated at 25–1100ºC were comprehensively studied. The SCHA was processed by a conventional wet synthesis at a high possible temperature (96ºC) using ammonium containing parent reagents. IR-spectroscopy, XRD, TG-DTA technique and mass spectrometric thermal analysis (MSTA) were employed for characterization of the samples. ${\text{NH}}_{4}{}^{+ }$ with ${\text{H}}_{{\text{3}}} {\text{O}}^{{\text{ + }}}$ in cationic- and ${\text{CO}}_{3}{}^{2 - }$ (A- and B-positions) with ${\text{HPO}}_{4}{}^{2 - }$ in anionic sites, and H₂O, ${\text{CO}}_{3}{}^{2 - }$(${\text{HCO}}_{3}{}^{-}$) ${\text{NO}}_{3}{}^{- }$, N _x H _y on the surface of particles were found and considered as impurity groups. Complicated changes in lattice constants of the SCHA stepwise annealed in air (for 2 h) were revealed; the changes were associated with reactions of the impurity groups. Filling the hexed sites with hydroxyl ions above 500ºC was shown to happen partly due to lattice reactions but was mainly owing to hydrolysis of the SCHA by water molecules in air. Decomposition of ${\text{CO}}_{3}{}^{2 - }$ groups proceeded through both thermal destruction and reactions with some of the impurity ions. The decarbonation in A-sites occurred at much lower temperatures (450–600ºC) than in B-sites (700–950ºC) and was first revealed to happen in two stages: due to an impurity reaction around 500ºC, and then through thermal destruction at 570ºC. A redistribution of ${\text{CO}}_{3}{}^{2 - }$ ions, decreasing in amount on the whole, was observed upon annealing above 500ºC. To avoid possible erroneous conclusions from TG-data, a sensitive method was shown to be required for monitoring gaseous decomposition products (such as the MSTA in this study), in case several impurity groups were present in a SCHA.