A Thermoanalytical Study Of Synthetic Carbonate-Containing Apatites

Two series of carbonate-apatites, Ca-10(PO4)(6-x)(CO3)(x)(F,OH)(2), were synthesized by precipitating them from aqueous solutions followed by ripening at the precipitation temperature (20 or 80 degrees C). Initial solutions contained Ca2+, Mg2+, NH4+, PO43-, F-, CO32- and NO3- ions; in the second series Naf was added. The samples had low crystallinity but, nevertheless. showed the apatite structure as judged from XRD and IR. Thermal degradation was followed by simultaneous TG/DTA and TG/EGA (evolved gas analysis) methods and by ex situ studies.The NH4+-containing samples (A, 20 degrees C and B, 80 degrees C) and two Na+-containing samples (C and D, both at 80 degrees C) were subjected to a detailed study. On the basis of EGA studies of sample A by FTIR, the first two exothermic peaks at 250-300 degrees C could be assigned to the release of H2O and H2O + NH3, respectively: the remaining three at 350-710 degrees C were due to CO2 evolution and changes in the apatite structure. For samples synthesized at 80 degrees C, the DTA peaks were smaller than for sample A. The EGA peak due to NH3 was missing for Na+-containing samples (C and D). For all samples, the residue at 1000 degrees C had the hydroxy-fluorapatite structure. The TA and XRD data indicate that the crystal structure of the precipitated apatites is relatively labile. After the release of volatiles, however, thermally induced rearrangements take place leading to a more stable and crystalline phase.