THE DEFECT STRUCTURE AND IONIC TRANSPORT PROPERTIES OF CALCIUM APATITE

Résumé
L'intérêt des processus de transport des apatites de calcium est dû au fait que la phase microcristalline minérale des os et des dents est l'hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. Les résultats obtenus par diffusion, relaxation diélectrique et conductivité montrent : a) la nature unidimensionnelle des chemins possibles pour le transport des ions hydroxyles parallèlement à l'axe c cristallographique et b) l'existence de deux sous-réseaux calcium s'interpénétrant dans la structure avec des énergies différentes d'énergie de formation et de migration des lacunes.

Abstract
The transport properties of the calcium apatites are of interest since the microcrystalline inorganic phase of bones and teeth is hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂. This material plays a structural role in hard tissue but is also involved in the dynamic exchange of its constituents with the blood supply and its in vivo environment. In order to provide data for the quantitative evaluation of possible biological processes involving ion exchange and impurity uptake, theoretical and experimental studies of the defect structure and ionic transport processes have been undertaken. The results of diffusion, dielectric relaxation and conductivity measurements will be presented. These indicate that two features are of particular importance in bulk transport processes : a) The one dimensional nature of the paths available for the motion of hydroxyl ions parallel to the crystalline c-axis, and b) The existence of two interpenetrating calcium sub-lattices within the structure with different energies of vacancy formation and frequency of vacancy jump to adjacent sites. The application of these data to the in vivo problem indicates that the bulk diffusion of calcium and phosphate ions at body temperatures is improbable, emphasizing the importance of the surface properties to the biological role of apatite. It is also indicated that fluoride ions, incorporated into hydroxyapatite substitutionally for the hydroxyl ions, are not capable of blocking the migration of hydroxyl ions along the one dimensional c-axis paths. Consequently the resistance to dental caries associated with fluoridation does not arise from the decreased enamel solubility resulting from the blocking of hydroxyl ion migration.