Iron in Hydroxyapatite: Interstitial or Substitution Sites?

Area of Science:

• Materials Science
• Solid State Chemistry
• Computational Materials Science

Background:

• Iron-doped hydroxyapatite (Fe-HAp) is a biocompatible magnetic material.
• A clear theoretical understanding of iron defect configurations in Fe-HAp is lacking.
• Existing literature lacks consensus on whether iron defects are interstitial or substitutional.

Purpose of the Study:

• To theoretically determine the geometry, electronic, magnetic, and thermodynamic properties of iron impurities in Fe-HAp.
• To identify the most probable defect configurations for iron in Fe-HAp.
• To elucidate the nature of iron inclusions in Fe-HAp.

Main Methods:

• Utilized modern first-principles methodologies, specifically hybrid density functional theory.
• Investigated a total of 26 distinct defect configurations, including substitutional (P and Ca sites) and interstitial defects.
• Estimated formation energies considering chemical potential boundaries for stable hydroxyapatite.

Main Results:

• Identified Fe3+ and Fe2+ substitutions at Ca(I) and Ca(II) sites as probable under calcium-poor conditions.
• Found Fe interstitials near hydroxyl channels to be favored in calcium-rich conditions.
• Determined that substitutional Fe on the P site is probable and adopts a low-spin state, unlike other configurations.
• Literature analysis of Fe K-XANES spectra indicates Fe-HAp typically contains multiple iron configurations.

Conclusions:

• The study provides a detailed theoretical description of iron defects in Fe-HAp.
• The nature and distribution of iron defects are highly dependent on the material's chemical environment (Ca-rich vs. Ca-poor).
• Fe-HAp materials commonly exhibit a mixture of iron defect types.