The key role of the A-site composition of oxy-hydroxyapatites in high-temperature solid–gas exchange reactions

The manufacture of carbonated hydroxyapatite-based bioceramics with control of the composition and microstructure remains challenging and reveals our lack of knowledge regarding the thermal behavior of such materials, particularly at high temperatures under reactive atmospheres. This work lays a foundation for addressing this issue by investigating the solid–gas exchange reactions occurring between oxy-hydroxyapatites (O x HA) and a CO 2 -rich atmosphere during thermal treatment. Accordingly, O x HA reference powders with different oxygen contents (0 ≤ x ≤ 0.79) were produced, extensively characterized and heat-treated under a CO 2 -rich atmosphere at 950 °C for 5 h. The results of physicochemical, thermal and microstructural analyses showed that the A-site composition of O x HA controls the exchange reactions: a high initial OH content induced concomitant A-site dehydration and carbonation; conversely, a high OH vacancy content induced A-site hydration as a first step. Furthermore, the specific surface area significantly influenced the solid–gas exchange reactions by controlling their kinetic.