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 pre ssure of water in the reaction atmosphere. In bioceramics used in the study of biomaterials, these two phosphates often coexist in distinct proportion s; therefore, structural characterization of the individual phases is hinde red. The structure of these two individual calcium phosphates have been stu died using Xray diffraction, and mainly by high-resolution electron microsc opy. 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 p attern of beta-TCP has been refined in the space soup R3c and the HREM imag e oriented along [0001] zone axis shows regular contrast. Both these calciu m phosphate ceramics are unstable to the electron beam for a prolonged expo sure time, and show areas of decomposition in HREM images of thin regions o f the crystal. The regions of decomposition in the beta-TCP crystal show th e presence of CaO as one of the major phases. (C) 2000 John Wiley & Sans, I nc.