Ionic conduction mechanism in Ca-deficient hydroxyapatite whiskers

Hydroxyapatite (HA) whiskers were hydrothermally synthesized from amorphous calcium phosphate and their ionic conduction properties were investigated with the goal of developing biomedical electrets made of HA whiskers. The obtained whiskers were assumed to be Ca-deficient HA with a composition of Ca9.18[HPO4]0.82[PO4]5.18[OH]1.18·nH2O. Based on the results of complex impedance measurements, a multiple conduction mechanism was indicated by the distinctive dependence of ionic conductivity on temperature within a range of 250–600 °C; the conductivity once increased to 5.0 × 10−9 S/cm at 500 °C and then decreased to 1.7 × 10−9 S/cm as the temperature increased to 600 °C. Since HPO42− ions are reportedly supposed to convert into P2O74− in approximately the same temperature range where the particular change appeared, the increase in conductivity up to 500 °C is presumably the result of an increase in the number of mobile protons generated from HPO42−. The subsequent decrease in conductivity seemed to be caused by a decrease in the number of protons brought about by the conversion of HPO42− into P2O74− with the elimination of water.