SYNTHESIS AND X-RAY IDENTIFICATION OF TITANIUM PHOSPHATE COMPOUNDS. M.L. Castro Reis¹, C. Castro Ribeiro², M.A. Barbosa², ¹IGM - Instituto Geologico e Mineiro, Rua da Amieira, 4465 S. Mamede de Infesta, Portugal, INEB- Instituto de Engenharia Biomedica, Pra,ca Coronel Pacheco, 1, 4000 Porto, Portugal, and ²FEUP- Faculdade de Engenharia da Universidade do Porto, Rua dos Bragas, 4099 Porfo Codex, Portugal.

In order to investigate the influence of Ti ions in the molecular structure of hydroxyapatite (HAp) dissolution studies were carried out using 0.9% sodium chloride solution to which different concentrations of Ti ions were added, ranging from 1 to 2000 ppm. The cristallinity of solids was studied by XRD techniques. For Ti concentrations in solution smaller than 350 ppm, hydroxyapatite undergoes an isomorphous substitution (Ca Ti) and the X-ray pattern revealed HAp single phase. By EDS it was possible to establish that this HAp is Ca-deficient (Ca/P, lower than 1.67) and it is transformed into an amorphous or a poorly crystallised phase for solutions containing Ti ions in concentrations of about 400 ppm. The "amorphous" phase can be a precursor of the formation of a titanium hydrogen phosphate compound, to which reference is made below. The lifetime of this precursor phase is sensitive to experimental conditions: with increasing time and temperature its cristallinity increases. When the concentration of Ti ions is higher than 400 ppm a crystalline phase is detected in early stages and its X-ray diffraction data seem to correspond to a mixture of the [[alpha]]-titanium hydrogen phosphate hydrate [[[alpha]]-Ti(HPO₄)2.H₂O] and [[gamma]]-titanium hydrogen phosphate hydrate [[[gamma]]-Ti(HPO₄)₂.2H₂O]. The X-ray diffraction data of the crystalline phase obtained were compared with data from the literature* for [[alpha]] form and [[gamma]] form. However, some reflections in positions non predicted, d=9.05, 6.4, 4.8Å, may correspond to a new phase or a mixed layered compound of the [[alpha]] and [[gamma]]-form. Usually, the layers are stacked along the c axis and the stacking matrix is formed by epitactic overgrowth. In this case a and b axis are exchangeable. The preliminary results of the mixed layered compound formed from both politypes ([[alpha]] and [[gamma]]) point towards a stacking matrix (l:l), with a layer spacing of approximately 19.2Å, which can be responsible for the reflections observed. On the other hand, the diffuse nature of the scattering pattern suggests that this material possesses a low level of crystallinity and/or a very small crystallite size.

*A. N. Christensen et al, Acta Chemica Scandinavica 44 (1990) 865-872.