Crystal structure of lanthanum-silicate co-substituted apatite obtained by the mechanochemical synthesis

This paper presents a structural study of lanthanum-silicate co-substituted apatites synthesized by the mechanochemical method. The compositions of the reaction mixtures were taken such that, theoretically, compounds containing equal concentrations of lanthanum and silicon could be formed. As a high degree of crystallinity is required to carry out the refinement of the crystal structure, the synthesized nanosized co-substituted apatites were annealed at 1200 °C. It was found that, in all annealed samples, the concentration of lanthanum is slightly higher than that of silicon. In the structure of the co-substituted apatites, lanthanum occupies both crystallographic positions of calcium with site 2 preference. It was found that the simultaneous replacement of calcium by lanthanum and phosphate by silicate results in an increase in the thermal stability of the compounds in comparison with unsubstituted hydroxyapatite.