Manganese behavior in hydroxyapatite crystals revealed by X-ray difference Fourier maps

Abstract The use of magnetic nanoparticles in association with scaffolds is considered an important way to transform typical passive scaffolds into active scaffolds. Manganese can develop magnetic properties in hydroxyapatites, as can iron, copper, cobalt and samarium, but lacks the high toxicity of these latter compounds. Because the magnetic properties exhibited by transition metal-containing hydroxyapatite are entirely dependent on site occupation and on the formation of magnetic oxides under heating, it is extremely important to understand how manganese behaves when inserted into hydroxyapatite. In this paper, we demonstrated by using X-ray difference Fourier maps that the insertion of Mn into the hydroxyapatite structure induces several perturbations in its hexagonal channels, with a preference for occupying Ca(2) sites, particularly when the hydroxyapatite lattice is poorly ordered and has CO32− inserted at PO43− sites. When CO32− is released at high temperature and the structure is better ordered, Ca(1) sites become more susceptible to occupation by Mn atoms, while the occupation of PO43− sites by MnO43− is reduced. However, with increasing time and calcination temperature, Mn atoms tend to be released from the hydroxyapatite structure in the form of Mn3O4 nanoparticles through the hydroxyl channels, occupying Ca(2) sites in the final stage of the segregation pathway. These findings are fundamental to the development of new strategies to synthesize active hydroxyapatite-based scaffolds under remote control by a magnetic field.