Effect of processing temperature on the properties of Fe-Hydroxyapatite

Multi-substituted Hydroxyapatite (HAp), Ca$_{5}$(PO$_{4}$)$_{3}$OH, is the main mineral phase in physiological apatite. Fe is a minor substitution element in bone and enamel substituting Ca in the HAp structure. Crystal structure, magnetic and microstructure properties of Ca$_{\mathrm{5-x}}$Fe$_{\mathrm{x}}$(PO$_{4}$)$_{3}$OH depend on processing parameters. We present results from our research on the Ca$_{\mathrm{5-x}}$Fe$_{\mathrm{x}}$(PO$_{4}$)$_{3}$OH system ($x =$ 0.0, 0.05, 0.1, 0.2 and 0.3) prepared at 37$^{\circ}$ C, and 80$^{\circ}$ C. Hydroxyapatite single-phase was detected for $x$ \textless 0.1 in both sets of samples, while hematite and/or maghemite develops starting at $x =$ 0.1. Rietveld refinements of XRD and NPD patterns show that the $a$ and $c$ lattice constants decrease with increasing Fe concentration for both sets of samples. Pure HAp is diamagnetic but as $x$ increases, Fe-HAp transitions from paramagnetic to weak ferromagnetic behavior. TEM images show spherical particles in samples prepared at 37$^{\circ}$ C, and elongated particles in samples prepared at 80$^{\circ}$ C. XRF studies confirm the iron substitution and show that the Ca/P stoichiometric ratio of 1.67 decreases with increasing the Fe concentration. Further, the Fe/Ca$+$Fe atomic ratios of samples prepared at 37$^{\circ}$ C are greater than those prepared at 80$^{\circ}$ C.