Structural and Magnetic Phase Transformations of Hydroxyapatite-Magnetite Composites under Inert and Ambient Sintering Atmospheres

The present work reports the impact of sintering conditions on the phase stability in hydroxyapatite (HA)−magnetite (Fe 3 O 4 ) bulk composites, which were densified using either pressureless sintering in air or by rapid densification via hot pressing in inert atmosphere. In particular, the phase abundances, structural and magnetic properties of the (1– x )HA- x Fe 3 O 4 ( x = 5, 10, 20, and 40 wt %) composites were quantified by corroborating results obtained from Rietveld refinement of the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Mössbauer spectroscopy. Post heat treatment phase analysis revealed a major retention of Fe 3 O 4 in argon atmosphere, while it was partially/completely oxidized to hematite (α-Fe 2 O 3 ) in air. Mössbauer results suggest the high-temperature diffusion of Fe 3+ into hydroxyapatite lattice, leading to the formation of Fe-doped HA. A preferential occupancy of Fe 3+ at the Ca(1) and Ca(2) sites under hot-pressing and conventional sintering conditions, respectively, was observed. The lattice expansion in HA from Rietveld analysis correlated well with the amounts of Fe-doped HA determined from the Mössbauer spectra. Furthermore, hydroxyapatite in the monoliths and composites was delineated to exist in the monoclinic ( P 2 1 / b ) structure as against the widely reported hexagonal ( P 6 3 / m ) crystal lattice. The compositional similarity of iron doping in hydroxyapatite to that of tooth enamel and bone presents HA-Fe 3 O 4 composites as potential orthopedic and dental implant materials.