Investigation of the hydroxyapatite obtained as hydrolysis product of alpha-tricalcium phosphate by transmission electron microscopy

Alpha tricalcium phosphate (alpha-TCP) is widely used as a reactant in many calcium phosphate cements, where the setting reaction occurs through its hydrolysis to a calcium deficient hydroxyapatite (CDHA), at room or body temperature. Transmission electron diffraction studies of cement single CDHA crystals produced at 37 degrees C have revealed the systematic presence of unexpected diffraction spots other than those assigned to the archetypal P6(3)/m apatite. Three possible causes can explain the existence of these extra spots: electron beam irradiation damage (inducing phase transformation to monoclinic apatite), presence of octacalcium phosphate (OCP) and presence of monoclinic apatite in the hydrolyzed product. Determination of the occurrence of any of these phases would have great implications in the properties of the hydrolyzed product. However, the identification of their corresponding electron diffraction patterns is not straightforward due to coincidental spots under basal zone axis orientation. In the present work, off-zone axis electron diffraction in the transmission electron microscope, TEM, has been used for phase identification, clearly indicating the presence of the monoclinic apatite phase. These findings could be a strong asset reinforcing the possibility of the monoclinic phase as a constituent of hard bone tissue.