La ions in precipitated hydroxyapatites

Hydroxyapatites were synthesized by precipitation from an aqueous solution with La³⁺ (0-0.75%) and with carbonate (0-6.1%) at controlled pH 7.0. Uptake of La³⁺ was 90-95% complete. Relatively low Ca/P (1.54-1.63) ratios were attributed to nonstoichiometry. Carbonate in samples was identified by IR spectroscopy as B-type carbonate. Lattice parameters of the hexagonal apatite structure were not affected by the La³⁺ content. Noncarbonated samples heated to 800°C transform partially to β- Ca₃(PO₄)₂. Thermogravimetric analysis showed release of 0.4 mol adsorbed and 1 mol crystalline water up to 400°C and decomposition of carbonate up to 900°C in the samples. Luminescence data obtained for Gd-containing hydroxyapatites prove that Gd³⁺ ions are not incorporated in the precipitated hydroxyapatite. These findings suggest that, in the La- containing samples, La³⁺ is surface adsorbed and not incorporated in hydroxyapatite.