Combined X-ray and neutron diffraction Rietveld refinement in iron-substituted nano-hydroxyapatite

Simultaneous Rietveld refinements of X-ray and neutron powder diffraction patterns were applied to study the effect of Fe substitution on the crystal structure properties of the Ca_5-x Fe _x (PO ₄)₃OH system (0 ≤ x ≤ 0.3). From variations of the Ca(1) and Ca(2) site occupancies and modifications of interatomic distances with x, it is inferred that Fe substitutes at both crystallographic sites with a preference at the Ca(2) site. Such partiality is attributed to similar geometries of the sixfold coordinated Fe with the sevenfold coordinated Ca(2). The expected overall decrease of the lattice constants in the iron-substituted samples is followed by an increasing trend with x that is explained in terms of local lattice distortions. Hematite forms as a secondary phase starting at x = 0.1 up to 3.7 wt% for x = 0.3. Transmission electron microscopy reveals a nanosystem consisting of 15-65 nm rods and spheres, while hematite nanoparticles are distinguishable for x ≥ 0.1. A transition of the diamagnetic hydroxyapatite to paramagnetic Fe-hydroxyapatite was found from magnetic measurements, while the antiferromagnetic hematite develops hysteresis loops for x > 0.1.