Some Structural Properties of the Mixed Lead–Magnesium Hydroxyapatites

Resumo

Lead–magnesium hydroxyapatite solid solutions Pb_(10–x)Mg_x(PO₄)₆(OH)₂ have been prepared via a hydrothermal process. They were characterized by X-ray powder diffraction, Transmission Electron Microscopy (TEM), chemical and IR spectroscopic analyses. The results of the structural refinement indicated that the limits of lead-magnesium solid solutions (x ≤ 1.5), a regular decrease of the lattice constant a and a preferential magnesium distribution in site S(I). Through the progressive replacement of Pb²⁺ (r = 0.133 nm) by the smaller cation Mg²⁺ (r = 0.072 nm), all interatomic distances decrease in accordance with the decrease of the cell parameters. According to what could be expected from the coordinance of the metallic sites S(I) (hexacoordination) and S(II) (heptacoordination), the small magnesium cation preferentially occupies the four sites S(I). The results of the TEM analysis confirm the presence of magnesium in the starting solution and reveals the decrease in the average size of crystals. The IR spectra show the presence of the absorption bands characteristic for the apatite structure.