STRUCTURAL MODIFICATIONS OF HYDROXYAPATITE INDUCED BY LEAD SUBSTITUTION FOR CALCIUM

Continuous series of solid solutions of lead and calcium hydroxyapatites have been prepared by solid-state reaction as well as from aqueous solutions and characterized by X-ray diffraction and IR spectroscopic analyses. While the lattice constant a of the solid solutions varies linearly with composition, the variation of the lattice constant c does not follow Vegard's law but shows a discontinuity at about 50% lead atom content. A similar discontinuity has been observed in the variation of the IR frequencies with composition. Although lead incorporation seems to destabilize the apatite structure, the solid solutions prepared from aqueous media at a lead content around 50% exhibit a high thermal stability. The variation of c, thermal stability, as well as the frequencies and intensities of the IR bands of lead-calcium hydroxyapatite are interpreted on the basis of the inhomogeneous lead distribution in the two non-equivalent cation sites of the hydroxyapatite structure.