Formation of a sodium bicarbonate cluster in the structure of sodium-substituted hydroxyapatite

Ceramic sodium-substituted carbonated hydroxyapatite has been synthesized using the method of the solid-phase reaction in the temperature range of 640–820$^\circ$C in water vapor. It has been established that substitutions of Ca$^{2+}$ ions in the cation and anion subsystems with Na$^+$ ions and the PO$^{3-}_4$ and OH$^-$ groups with СО$^{2-}_3$ ions lead to a considerable acceleration of the shrinkage and synthesis of dense ceramics at substantially lower temperatures than in the case of unsubstituted hydroxyapatite. Sintering in water vapor leads to densification of carbonate groups in channel positions, which induces the appearance of orderings of A2 and B2 types (bands with wave numbers 867 and 865 cm$^{-1}$ in IR spectra, respectively) as well as the protonation of carbonate groups both in A and B sites and the formation of sodium bicarbonate clusters (856 and 859 cm$^{-1}$) in addition to carbonate ordering of A1 and B1 types (879 and 872 cm$^{-1}$).