Effects of sintering temperature on structure of hydroxyapatite studied with Rietveld method

It is obvious that the sintering temperature of hydroxyapatite (HA) ceramics significantly affects their biological responses in vitro or in vivo. The HA ceramics sintered at different temperatures exhibit a wide variation of biological response, but the correlation of this variation with the parameters of HA crystalline structure is not fully investigated. In present study, the crystalline structure of HA powders sintered at different temperatures at 600, 800, 1000, 1200 degrees C, was characterized using X-ray diffraction (XRD), and then refined with Rietveld method. A series of structure parameters, such as, cell lattice parameters (a and c) and bond length, distances of special atoms on surface to nearest atom, a numerical index of distortion for PO(4) tetrahedron, as well as internal energy and density of HA cell, were calculated to characterize the crystalline structure of HA at atom level. The broadening effect of XRD reflections was also separated to calculate the micro-strain/crystalline size, respectively. These parameters of HA crystalline structure and cell demonstrated that with rising of the sintered temperature, the internal energy and micro-strain of HA cell decreased, but the crystalline size increased. The regularity of PO(4) tetrahedron and distance between Ca(1) and O(3) in HA cell showed the same tendency as the internal energy and micro-strain with sintering temperature. All of these parameters indicated that HA became more stable with the rising of sintering temperature.