Diffusion of Copper Ions in the Lattice of Substituted Hydroxyapatite during Heat Treatment

The doping of hydroxyapatite with various substituent ions can give this material new and useful properties. Nonetheless, local distortions of structure after doping can change the properties of the material. In this work, the thermal stability of copper-substituted hydroxyapatite synthesized by the mechanochemical method was investigated. In situ diffraction analyses showed that copper ion diffusion during the heating of Cu-substituted hydroxyapatite promotes phase transformations in the substituted hydroxyapatite. The behavior of copper ions was studied in samples with ratios (Ca + Cu)/P = 1.75 and 1.67. It was found that in both cases, single-phase Cu-substituted hydroxyapatite with the general formula Ca_10-xCu_x(PO₄)_6-y(CO₃)_y(OH)_2-yO_y is formed by the mechanochemical synthesis. When heated at approximately 600-700 °C, the lattice loses copper cations, but at higher temperatures, CuO diffusion into the hydroxyl channel takes place. Cuprate-substituted hydroxyapatite with the general formula Ca₁₀(PO₄)₆(OH)_2-2x(CuO₂)_x forms in this context. At 1200 °C, the sample is single-phase at (Ca + Cu)/P = 1.75. Nonetheless, slow cooling of the material leads to the emergence of a CuO phase, as in the case of (Ca + Cu)/P = 1.67, where the material contains not only CuO but also Cu-substituted tricalcium phosphate. In the manufacture of ceramic products from Cu-substituted hydroxyapatite, these structural transformations must be taken into account, as they alter not only thermal but also biological properties of such materials.