Ab initio study of structural, electronic, and magnetic properties of pristine and Fe-doped Al₂Si₂O₅(OH)₄

Résumé

First-principles calculations based on density functional theory (DFT) were employed to investigate the structural, electronic, and magnetic properties of pristine and single Fe-doped Al₂Si₂O₅(OH)₄ (kaolinite). All calculations were performed within the GGA–PBE framework using a plane-wave pseudopotential approach. Convergence tests with respect to kinetic energy cut-off and k-point sampling confirmed that 40 Ry and a 5×5×5 Monkhorst–Pack grid ensure total energy convergence within 1 meV/atom for both systems. Structural optimization of pristine kaolinite reproduces the characteristic layered framework of SiO₄ tetrahedra and AlO₆ octahedra. Upon Fe substitution at an octahedral Al site, localized structural relaxation is observed, with elongated Fe–O bonds (≈1.95–2.12 Å), minor lattice expansion (<2%), and measurable octahedral distortion indices (~2–3%). The calculated binding energy (~0.68 eV) indicates that Fe incorporation is thermodynamically feasible under suitable conditions. Electronic structure analysis reveals that pristine Al₂Si₂O₅(OH)₄ is an indirect wide-band-gap insulator with a calculated GGA gap of 4.19 eV. Fe substitution induces spin polarization, reduces the band gap to 3.23 eV, and introduces Fe-3d states near the band edges. Spin splitting between majority and minority channels confirms magnetic ordering, yielding a local magnetic moment of ~3.8 μB per Fe atom, consistent with high-spin Fe³⁺ in octahedral coordination. Charge density and spin density analyses reveal pronounced Fe–O hybridization and localized magnetic moment formation. These findings demonstrate that Fe doping effectively tunes the structural stability, electronic band gap, and magnetic behavior of kaolinite, highlighting its potential for functional and catalytic applications.

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