Synthesis and Characterization of a Monoclinic Crystalline Phase of Hydroxyapatite by Synchrotron X-ray Powder Diffraction and Piezoresponse Force Microscopy

In this work, we report the synthesis of a monoclinic hydroxyapatite [Ca-10(PO4)(6)(OH)(2)] (hereafter called HA) prepared by the sol-gel method assisted by ultrasound radiation at room temperature. The characterization of both the monoclinic and the hexagonal phases were performed by powder X-ray diffraction (PXRD) and using synchrotron radiation (SR). The measurement of the piezoelectricity was performed by piezoresponse force microscopy (PFM). The synthesis produced a mixture of monoclinic and hexagonal hydroxyapatite (HA). We also discuss the importance of stabilizing the monoclinic phase at room temperature with ultrasound irradiation. The existence of the monoclinic phase has important advantages in terms of showing piezoelectric properties for applications in the new medical rehabilitation therapies. Rietveld refinement of the PXRD data from SR indicated the monoclinic phase to be of about 81%. Finally, piezoelectric force microscopy was used to distinguish the phases of hydroxyapatite by measuring the average piezoelectric coefficient d(eff = 10.8 pm/V).