Crystallographic properties of a series of synthetic hydroxyapatites

The crystallographic properties of hydroxyapatites synthesized at temperatures of 200 degrees C, 500 degrees C, 900 degrees C and 1250 degrees C, designated HAP200, HAP500, HAP900 and HAP1250, respectively, were studied using X-ray powder diffraction. Values of a-axis length for the hydroxyapatites ranged from 9.406 A to 9.416 A, which were smaller than the corresponding value for stoichimetric hydroxyapatite, a = 9.418 A. Generally, the a-axis length decreased with the synthesis temperature, except for HAP500 which showed the smallest value. Crystallinity of the hydroxyapatites, measured by X-ray diffraction peak broadening as full width at the half-maximum value (FWHM), increased with the synthesis temperature, although HAP500 showed a decrease. These results suggest that the synthetic hydroxyapatites contained some carbonate ions, probably at the A-site of the apatite structure, and that the structure was depleted of carbonate ions at about 500 degrees C.