Study of the formation of the apatite-type phases La 9.33þx (SiO 4 ) 6 O 2þ3x/2 synthesized from a lanthanum oxycarbonate La 2 O 2 CO 3

International audiencea b s t r a c t Lanthanum silicated apatites with nominal composition La 9.33þx (SiO 4) 6 O 2þ3x/2 (À0.2 < x < 0.27) have been successfully synthesized by solid state reaction using a new reagent La 2 O 2 CO 3 and amorphous SiO 2 precursors. The formation mechanism of La 2 O 2 CO 3 reagent, which cannot be purchased, has been fol-lowed by in-situ temperature depend XRD of La 2 O 3 under CO 2 atmosphere. The stability of this reagent during the synthesis step allowed to limit the formation of secondary phase La 2 Si 2 O 7 and made the weighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 C. Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 C and then sintered at 1550 C. Traces of La 2 SiO 5 secondary phase present in synthesized powder disappeared after densification and pure oxyapatite materials were obtained for all the compositions. Electrical measurements confirmed that conductivity behaviors of the sintered pellets were dependent to the oxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 Â 10 À2 S cm À1 was exhibited at 800 C for the nominal composition La 9.60 (SiO 4) 6 O 2.405 with low activation energy around 0.79 eV. The ionic conductivity properties were comparable with that of the earlier obtained materials