Abstract
Monoclinic hydroxyapatite was prepared by a wet method and followed by heating in air at 1473K for 1 hour, and its phase transition was investigated by a high temperature X-ray powder diffractometry and a differential scanning calorimetry. The prepared hydroxyapatite was 98% monoclinic. The Rietveld refinement results revealed that the low temperature phase had monoclinic symmetry, space group P21/b and the high temperature phase had hexagonal symmetry, space group P63/m. In monoclinic hydroxyapatite, two different sizes of oxygen triangles existed. The rotation of phosphate tetrahedra at about 6° and the change of the hydroxyl order to disorder arrangement occurred due to the phase transition. The phase transition took place reversibly at 480.5(1)K in heating process and 477.5(1)K in cooling process. The enthalpy value was found to be 130(1) J mol-1. Further, the transition was observed to resolve into two steps that were attributed to the rotation of phosphate tetrahedra and the order to disorder arrangement of hydroxyls.
