Solid state solubility of copper oxides in hydroxyapatite

Highlights: • Copper ions enter into the apatite trigonal channels up to 58% of stoichiometry. • Copper content in hydroxyapatite increases strongly with the annealing temperature. • Copper content grows with the water vapor partial pressure reduction. • Copper content value peaks at an intermediate partial pressure of oxygen. - Abstract: Samples containing copper oxide doped hydroxyapatite with the composition Ca₁₀(PO₄)₆(Cu_xOH_1-x-δ)₂, x = 0.054 – 0.582, in the mixture with CuO/Cu₂O were prepared by a solid-state high-temperature treatment at varying annealing temperatures and at different partial water vapor and oxygen pressures. The crystal structures of the apatite compounds were refined using powder X-ray diffraction patterns and the content of copper ions x in the apatite was determined. Copper ions enter exclusively into the apatite trigonal channels formally substituting protons of OH-groups and the hexagonal cell parameters grow approximately linearly with x, the channel volume mostly expanding while the remaining volume of the crystal lattice changing only slightly. The equilibrium copper content in the apatite increases drastically, by almost a factor of 10 with the annealing temperature rising from 800° to 1200°C. The reduction of the water partial pressure leads to a further increase of x, while the dependence of x on the oxygen partial pressure exhibits a maximum. The observed relations are consistent with the proposed chemical reactions implying the copper introduction is followed by the release of a considerable quantity of gaseous products – water and oxygen. The analysis of interatomic distances suggests that the maximum content of copper ions in the channel cannot exceed 2/3.