Phase change of monoclinic hydroxyapatite prepared by wet method

摘要

Monoclinic hydroxyapatite was prepared by a wet method and followed by heating in air at 1473 K for 1 h, and its phase transition was investigated by high temperature X-ray powder diffractometry and differential scanning calorimetry. The prepared hydroxyapatite is 98percent monoclinic. The Rietveld refinement results revealed that the low temperature phase has monoclinic symmetry, space group P2_1/b and the high temperature phase has hexagonal symmetry, space group P6_3/m. In monoclinic hydroxyapatite, two different sizes of oxygen triangles exist. The rotation of phosphate tetrahedra at about 6 deg 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 with the enthalpy value of 130(1) Jmol~(-1). Further, a sintered monoclinic hydroxyapatite was prepared. Discontinuity of dielectric curves was observed at 483 K The discontinuity of dielectric curves was concluded to the change of hydroxyl order to disorder arrangement.