| Summary: | Rare-earth labeling in biological apatite could provide critical information for the pathologic transition (osteoclastic) and physiologic regeneration (osteogenesis) of bone and teeth because of their characteristic site-sensitive fluorescence in different coordinative conditions of various tissues in many biological processes. However, the rare-earth labeling method for biological apatites, i.e., carbonated-hydroxyapatite, has been rarely found in the literature. In this paper, we report a Pourbaix-diagram guided mineralizing strategy to controllable carbonation and doping of rare-earth ions in the hydroxyapatite (HA) lattice. The carbonation process of hydroxyapatite was achieved by controllable mineralization in hydrothermal condition with K<sub>2</sub>CO<sub>3</sub> as the carbonate source, which results into the pure B-type carbonated hydroxyapatite (CHA) with tunable carbonate substitution degree. All of the as-synthesized materials crystalized into P63/m (No. 176) space group with the lattice parameter of a decreases and c increases with the increasing of carbonate content in the reactants. Structural refinement results revealed that the substitution of planar CO<sub>3</sub><sup>2−</sup> is superimposed on one of the faces of PO<sub>4</sub><sup>3−</sup> tetrahedral sub-units with a rotation angle of 30° in reference to c-axis. All of the hydrothermally synthesized CHA nanocrystals show hexagonal rod-like morphology with the length of 70–110 nm and diameter of 21–35 nm, and the decreasing length/diameter ratio from 3.61 to 2.96 from low to high carbonated level of the samples. Five rare-earth cations, of Pr<sup>3+</sup>, Sm<sup>3+</sup>, Eu<sup>3+</sup>, Tb<sup>3+</sup>, and Ho<sup>3+</sup>, were used as possible probe ions that can be doped into either HA or CHA lattice. The site-preference of Tb<sup>3+</sup> doping is the same in the crystallographic site of HA and CHA according to characteristic emission peaks of <sup>5</sup>D<sub>4</sub>–<sup>7</sup>F<i><sub>j</sub></i> (<i>j</i> = 3–6) transitions in their photoluminescent spectroscopy. Our work provides a controllable carbonation method for rare-earth labeling hydroxyapatite nanomaterials with potential biologically active implant powders for bone repair and tissue regeneration.
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