JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 6(1995) 85--89 Structural analyses of carbonate-containing apatite samples related to mineralized tissues F. J. G. CUISINIER., P. STEUER, J.-C. VOEGEL INSERM, CJF 92-04, Centre de Recherches Odontologiques 1, Place de I'H6pital 67000 Strasbourg, France F. APFELBAUM, I. MAYER Department of Inorganic and Analytical Chemistry, Hebrew University, Jerusalem 91904, Israel Crystallographic and structural aspects of two carbonate-containing (one with and another without Zn ions) and a pure hydroxyapatite sample were investigated by high resolution electron microscopy. For five different zone axes and for the three samples a good correspondence was always found between simulated lattice images and observed micrographies. Numerous structural defects were detected in the carbonate-containing apatites, whereas none were found in the similarly prepared pure hydroxyapatite. The incorporation of Zn ions in the crystal lattice of the carbonate-containing apatite reduces the number of defects. 1. Introduction Calcium phosphates forming inorganic phases of mammal calcified tissues are structurally described by hydroxyapatite (HA) [1]. X-ray diffraction and in- frared spectroscopical studies showed that the mineralized calcified tissues could be related to non- stoichiometric carbonate-containing hydroxyapatites [2, 31. Non-stoichiometry is principally due to ionic losses like calcium deficiencies and/or to ionic substi- tutions. The most abundant foreign ions are the carbonate ions which either replace OH- (type A carbonato-apatite) or PO]- groups (type B carbona- to-apatite) [4, 5]. Ionic losses and substitutions, occur- ring simultaneously in biological crystals, induce com- plex structures at the unit-cell level [6, 7]. Synthetic carbonated apatites (c.HAS) are classically used as structural models for the study of the growth and dissolution processes of biological crystals [8, 9]. However, the atomic structure of c.HAS is not com- pletely known because neither pure type A carbon- ated-apatite nor pure type B carbonated-apatite can yet be synthetized. Synthetic apatites are currently used for bone re- storation and for metallic implant coating in ortho- paedics and dentistry. However, in these clinical fields carbonated apatites are only seldom employed. Car- bonated apatites possess properties which at first sight could give them interesting clinical potentialities. Among these properties one may in particular notice that: (a) the presence of carbonate ions within the apatite lattice induces structural disorders as observed for gallium containing carbonate apatites [10] and that the crystallographic defects may subsequently favour biological apatite nucleation; (b) the presence of carbonate ions increases the solu- 0957-4530 © 1995 Chapman & Hall bility product of the samples and for the same pH value, more calcium and phosphate ions are avail- able for the further calcification, or biological mineralization; (c) as a side effect, the increased solubility of the carbonated apatite may also favour the formation of stronger metal-biological apatite bonds. Synthetic hydroxyapatite, carbonated apatite and zinc-containing carbonated apatites were studied by high resolution transmission electron microscopy (HRTEM). HRTEM is a powerful tool for the study of crystallized materials with a Scherzer resolution close to 0.15 nm. Unfortunately HRTEM is not so well adapted for the study of non-stoichiometric materials because of the small sizes of the analysed areas and because of the difficulty in detecting chemical vari- ations (except for weak phase objects) [11]. Used for the analyses of calcified tissues, HRTEM allows a precise description of biological crystals with a resolu- tion close to 0.2 nm [12, 13], as well as the character- ization of structural defects [13]. The aim of this study was to investigate carefully the structure and morpho- logy of these synthetic apatites and to specify the structural modifications induced by the presence of foreign ions (carbonate and zinc) in the crystal lattice. Particular attention has been paid to the nature and density of structural defects. 2. Materials and methods 2.1. Sample preparation and characteiization Apatite samples were prepared by a precipitation method, precisely described elsewhere [14]. Briefly, the synthesis was performed by the dropwise addition of a phosphate solution (Na2HPO4) over 2 h to a stirred Ca solution in the form of Ca(NO3)z at 87 °C. 85