Kanyuka Y. Isomorphic substitution of calcium in the synthetic hydroxyapatite by alkaline and rare-earth elements

Solid hydroxyapatite-based solutions in a diversity of compositions - Ca(5 - 2x)MIxMIIIx(PO4)3OH, where MIII = La, Pr, Nd, Sm, Gd, Tb, and Ho, MI = Li, Na and K - have been obtained using solid-phase synthesis at 1100 0C. In the system studied, the limits of substitution according to the Scheme 2Ca2+ - М+ + М3+ were determined using X-ray phase analysis. The solubility regions are changed in the range of x from 0,1 to 1,0. In the systems under study, as evidenced from NMR studies and IR- and EXAFS spectroscopy data, some portion of OH- groups leaves the structure of hydroxyapatite, the rest changes their orientation. An examination of experimental data and theoretical calculations for the systems studied and comparison with the literature data suggest that substitutions according to the Scheme 2Ca2+ - М+ + М3+ and Ca2+ + OH- - М3+ + O2- are in competition.