Peculiarities of charge compensation in lithium-doped hydroxyapatite

Hydroxyapatite (HA) remains one of the most popular materials for various biomedical applications and its fields of application have been expanding. Lithium (Li⁺) is a promising candidate for modifying the biological behavior of HA. Li⁺ is present in trace amounts in the human body as an alkaline and bioelectric material. At the same time, the introduction of Li⁺ into the HA structure required charge balance compensation due to the difference in oxidation degree, and the scheme of this compensation is still an open question. In the present work, the results of the theoretical and experimental study of the Li⁺-doped HA synthesis are presented. According to X-ray diffraction data, Fourier transform infrared spectroscopy as well as the combination of electron paramagnetic resonance methods, the introduction of Li⁺ in the amount up to 0.05 mol% resulted in the preservation of the HA structure. Density functional theory calculations show that Li⁺ preferentially incorporates into the Ca (1) position with a small geometry perturbation. The less probable positioning in the Ca (2) position leads to a drastic perturbation of the anion channel.