Axially dependent dielectric relaxation response of natural hydroxyapatite single crystals

A thermally stimulated depolarization current (TSDC) study of single crystals of natural fluorine-rich hydroxyapatite, polarized with the electric field (Ep) applied parallel and perpendicular to the c axis, has revealed relaxation modes with axially dependent dielectric behavior. The (re)orientation of the OH− ions in the columns and the hydroxyls of the structural H2O molecules give rise to two prominent thermocurrent bands in the temperature range of ∼200–340 K. Several weak current bands, below ∼200 K, are related to the relaxation of impurity–Ca2+ vacancy (I–V) dipoles. The I–V mechanisms are particularly effective in the case where Ep⊥c axis and are considered to take place via cation jumps on planes perpendicular to the [0001] crystal axis, at regions near “distorted” Ca(II) triangles (i.e., triangles which include impurity ions and Ca2+ vacancies). The band attributed to rotational relaxation of the OH− ions in the columns shows particular strength when Ep⊥c axis, and an extensive distribution of relaxation times. The distribution is due to the multiplicity of physicochemical interactions between the OH− ions and their ionic and molecular neighbors (hydrogen bonds with adjacent F−, Cl−, and O2− ions, varying steric hindrances, etc.). The changes induced in the TSDC spectra after annealing processes at selected high temperatures are interpreted based on the gradual loss of structural H2O molecules located in oxygen or calcium vacancies and the resulting local changes in the crystal lattice.