Incorporation of 3d-metal ions in the hexagonal channels of the Sr5(PO4)3OH apatite

Strontium phosphate apatites with compositions Sr₅(PO₄)₃Zn_0.15O_0.3(OH)_0.7, Sr₅(PO₄)₃Ni_0.2O_0.4(OH)_0.6, and Sr₅(PO₄)₃Co_0.2O_0.5(OH)_0.4 were synthesized by solid state reaction at 1400 °C in air. The samples were characterized by powder X-ray diffraction, EDX analysis, magnetic measurements and IR spectroscopy. The crystal structures were refined by the Rietveld method in the space group P6₃/m with lattice constants a = 9.7499(1), 9.7722(1), 9.7507(1) Å and c = 7.3066(1), 7.2962(1), 7.2988(1) Å, respectively. The 3d-metal atoms were found randomly distributed in the hexagonal channels formally substituting hydrogen in the initial hydroxyapatite. Zn and Ni atoms were twofold coordinated by oxygen atoms such that the linear O–M–O groups formed in the channel separated by the OH groups. Co atom was shifted from the channel center giving the O–Co–O fragment distorted from a linear geometry probably due to the additional coordination by the oxygen atoms of the phosphate groups.