Fe- and Mn-substituted hydroxyapatite: Distinguishing single- and co-substitution effects

This study investigates Fe- and Mn-substituted hydroxyapatite (HA) as a platform for engineering HA/oxide hybrid materials with potential in electrochemical biosensing. HA containing Mn and/or Fe was synthesized by aqueous precipitation and characterized by X-ray fluorescence, X-ray diffraction with Rietveld refinement, Fourier-transform infrared spectroscopy, and transmission electron microscopy/energy-dispersive X-ray spectroscopy to assess dopant incorporation, structural distortion, and phase evolution after calcination at 1000 °C. Both substituents induced distinct crystal-chemical effects before calcination: Mn caused unit-cell contraction, whereas Fe promoted unit-cell expansion and a stronger loss of crystallinity. After calcination, pure HA remained largely stable, while the substituted samples showed lower thermal stability and formed composition-dependent oxide phases: Mn 3 O 4 in Mn-containing HA, Fe 3 O 4 and Fe 2 O 3 in Fe-containing HA, and predominantly Fe 3 O 4 in the co-substituted system. Analysis indicated that a fraction of the dopants remained associated with the apatite phase after calcination, and suggested the presence of Mn within the segregated ferritic phase. These results show that Fe/Mn substitution is an effective strategy for tuning the structural response and thermal decomposition pathway of HA, thereby enabling the design of functional HA/oxide hybrid materials.