Structural study and stability of hydroxyapatite and beta-tricalcium phosphate: two important bioceramics

The stability of hydroxyapatite (OHAP) and beta-tricalcium phosphate (beta-TCP) ceramics depends on the temperature of preparation and the partial pressure of water in the reaction atmosphere. In bioceramics used in the study of biomaterials, these two phosphates often coexist in distinct proportions; therefore, structural characterization of the individual phases is hindered. The structure of these two individual calcium phosphates have been studied using X-ray diffraction, and mainly by high-resolution electron microscopy. Whereas the X-ray diffraction profile of OHAP has been refined in the space group P6(3)/m, the HREM images oriented along the [001] direction at the edges of the crystal show a threefold symmetry. The X-ray diffraction pattern of beta-TCP has been refined in the space group R3c and the HREM image oriented along [0001] zone axis shows regular contrast. Both these calcium phosphate ceramics are unstable to the electron beam for a prolonged exposure time, and show areas of decomposition in HREM images of thin regions of the crystal. The regions of decomposition in the beta-TCP crystal show the presence of CaO as one of the major phases.