| Autor: |
Kalogeras, Ioannis M., Vassilikou-Dova, Aglaia, Katerinopoulou, Anna |
| Předmět: |
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| Zdroj: |
Journal of Applied Physics; 7/1/2002, Vol. 92 Issue 1, p406, 9p, 1 Diagram, 4 Charts, 7 Graphs |
| Abstrakt: |
A thermally stimulated depolarization current (TSDC) study of single crystals of natural fluorine-rich hydroxyapatite, polarized with the electric field (E[sub p]) applied parallel and perpendicular to the c axis, has revealed relaxation modes with axially dependent dielectric behavior. The (re)orientation of the OH[sup -] ions in the columns and the hydroxyls of the structural H[sub 2]O 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-Ca[sup 2+] vacancy (I-V) dipoles. The I-V mechanisms are particularly effective in the case where E[sub p]⊥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 Ca[sup 2+] vacancies). The band attributed to rotational relaxation of the OH[sup -] ions in the columns shows particular strength when E[sub p]⊥c axis, and an extensive distribution of relaxation times. The distribution is due to the multiplicity of physicochemical interactions between the OH[sup -] ions and their ionic and molecular neighbors (hydrogen bonds with adjacent F[sup -], Cl[sup -], and O[sup 2-] 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 H[sub 2]O molecules located in oxygen or calcium vacancies and the resulting local changes in the crystal lattice. [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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