Monoclinic-Hexagonal Phase Transition in Hydroxyapatite Studied by X-ray Diffraction and Differential Scanning Calorimeter Techniques

H. Suda, M. Yashima, M. Kakihana, and M. Yoshimura
[J. Phy. Chem., Vol.99, pp.6752-6754, 1995]

The transition between the monoclinic (P2₁/b) and hexagonal (P6₃/m) phases of hydroxyapatite, Ca₅(PO₄)₃OH, was investigated both by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) techniques. Peak intensities of XRD of the low-temperature monoclinic phase were almost unchanged up to 473 K but dropped abruptly to zero above 483 K. Endo- and Exothermic DSC peaks and a specific heat anomaly associated with the transition were observed. The transition enthalpy and entropy were estimated to be 630± 25 J/mol and 1.30± 0.05 J/molƕ K, respectively.

Fig. 1. Specific heat capacity curves of hydroxyapatite, compared with the previously reported values.