Thermally-induced structural modification of dental enamel apatite: Decomposition and transformation of carbonate groups

Dental enamel is mainly composed of non-stoichiometric hydroxyapatite with A- and B-type carbonate groups in OH and phosphate sites, respectively. Structural and chemical modifications of dental enamel apatite were studied using FTIR and XRD techniques after heat treatment in air for 1h from 300 to 1193 K. Both IR and XRD results show a high degree of crystallinity of apatite that is enhanced with increasing temperature. The loss of B-type and A-type carbonate was studied; the amount of B-type carbonate and the total carbonate content decrease on heating while the amount of A-type carbonate first decreases up to 573 K and then increases from 573 to 973 K. Almost 50% of the carbonate ions were released from dental enamel with the formation of beta-tricalcium phosphate phase (beta-TCP) after heat treatment at 973 K for I It. The incorporation of CO, and cyanate species in dental enamel was observed in the temperature range of 273-973 K and 673-1073 K, respectively. The content of CO2 in dental enamel increases from 473 K to a maximum near 773 K and decreases thereafter. The mechanism of the decomposition and transformation of carbonate groups at different sites in enamel apatite structure is discussed.