A THERMOANALYTICAL STUDY OF SYNTHETIC CARBONATE-CONTAINING APATITES

Two series of carbonate-apatites, Ca-10(PO4)(6-x)(CO3)(x)(F,OH)(2), we re synthesized by precipitating them from aqueous solutions followed b y ripening at the precipitation temperature (20 or 80 degrees C). Init ial solutions contained Ca2+, Mg2+, NH4+, PO43-, F-, CO32- and NO3- io ns; in the second series Naf was added. The samples had low crystallin ity but, nevertheless. showed the apatite structure as judged from XRD and IR. Thermal degradation was followed by simultaneous TG/DTA and T G/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+-c ontaining 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 f irst two exothermic peaks at 250-300 degrees C could be assigned to th e release of H2O and H2O + NH3, respectively: the remaining three at 3 50-710 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 d egrees C had the hydroxy-fluorapatite structure. The TA and XRD data i ndicate that the crystal structure of the precipitated apatites is rel atively labile. After the release of volatiles, however, thermally ind uced rearrangements take place leading to a more stable and crystallin e phase.