Synchrotron X‑ray Absorption Spectroscopy Reveals Local Structural Changes in Al‑Doped BiFeO₃ Powders

Background

Multiferroics exhibit simultaneous ferroelectric, ferromagnetic, and ferroelastic order, enabling electric–magnetic coupling that is promising for sensors, memory devices, and energy applications [1, 2]. The perovskite structure AB O₃ accommodates a larger A‑site cation and a smaller, high‑valence B‑site cation, typically resulting in antiferromagnetic or weak ferromagnetic behavior at low transition temperatures. Among single‑phase multiferroics, bismuth ferrite (BiFeO₃; BFO) is notable for its rhombohedrally distorted perovskite lattice (R3c), a Curie temperature of ≈1103 K, and a Néel temperature of ≈643 K, with a large spontaneous polarization (~90 µC cm⁻²) arising from Bi 6s² lone‑pair electrons and a complex G‑type antiferromagnetic spin structure [3–6].

However, BFO suffers from high leakage currents due to Bi and O vacancies, impurity phases (Bi₂Fe₄O₉, Bi₂₅FeO₃₉), and Fe valence fluctuations [7, 8]. Strategies such as strain engineering, divalent/rare‑earth ion doping, and co‑doping have been employed to suppress defects and enhance multiferroic properties [9–20]. Hydrothermal synthesis offers a low‑temperature, cost‑effective route that yields fine, well‑dispersed particles with reduced secondary phases [21–27].

Aluminum, a trivalent cation with a smaller ionic radius than Fe³⁺, has been doped at the B‑site to stabilize the perovskite framework and reduce Fe valence fluctuation, but its influence on the local electronic structure remains underexplored. XAFS, encompassing X‑ray absorption near‑edge structure (XANES) and extended X‑ray absorption fine structure (EXAFS), provides element‑specific insights into oxidation states, coordination environments, and bond distances, making it ideal for probing Al‑doped BFO [28–32].

In this study, we synthesize undoped and Al‑doped BFO powders (x = 0, 0.025, 0.05, 0.1) via hydrothermal processing and systematically examine their structural, electronic, and optical properties using XRD, Raman, XAFS, and UV‑Vis spectroscopy.

Methods

Precursor solutions were prepared from bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), aluminum nitrate hexahydrate (Al(NO₃)₃·6H₂O), and potassium hydroxide (KOH). 20 mL of each metal salt solution were mixed in an 80 mL stainless‑steel autoclave, followed by slow addition of KOH to reach 65–80 % volume. The mixture was stirred for 2–3 h at 80 °C to achieve a clear solution, then transferred to a Teflon‑lined autoclave and hydrothermally treated at 200 °C for 10 h under autogenous pressure. The resulting powders were cooled, washed repeatedly with acetone, deionized water, and ethanol until neutral pH, then dried at 70 °C for 6 h.

XRD patterns were recorded on a Mac Science M18XHF22‑SRA diffractometer. Raman spectra were collected using a Renishaw InVia Reflex microscope with an Ar⁺ laser. XAFS data at the Fe K‑edge and Bi L₃‑edge were acquired in transmission mode at the 1W2B beamline of the Beijing Synchrotron Radiation Facility (BSRF) with energy resolutions ΔE/E = 2 × 10⁻⁴ (Fe) and 1 × 10⁻⁴ (Bi). Powders were ground, mixed with BN, and pressed into pellets for measurement. ATHENA/IFEFFIT software processed the XAFS data, extracting χ(k) and performing Fourier transforms to obtain radial distribution functions. UV‑Vis spectra (300–800 nm) were measured on a UV 3900H spectrophotometer, and band gaps were calculated using Tauc plots for direct transitions (n = 2).

Results and Discussion

XRD and Raman – All samples crystallize in the rhombohedral R3c structure (JCPDS 20‑0169) with minor secondary phases (Bi₂Fe₄O₉, Bi₂₅FeO₃₉) appearing only at the highest Al content. The (101) peak shifts to higher 2θ values with increasing Al, confirming successful substitution of Al³⁺ (ionic radius 0.51 Å) for Fe³⁺ (0.65 Å). Raman spectra reveal six active modes (3 A₁ LO + 3 E TO), consistent with R3c symmetry. The A₁ 1(LO) mode shows a slight blue shift, while the A₁ 3(LO) mode broadens, indicating modified Bi–O and Fe–O covalency due to Al incorporation.

Fe K‑edge XANES – The absorption edge of Al‑doped samples shifts to lower energies, indicative of a mixed Fe²⁺/Fe³⁺ state that becomes more Fe²⁺‑rich with higher Al content. Pre‑edge intensity (A1) increases slightly, reflecting enhanced Fe 3d–O 2p hybridization and the contribution of Al 3d states. Post‑edge peaks (A2, A3) also intensify, signaling increased ligand‑to‑metal charge transfer and the emergence of Fe 4p character upon doping.

Fe K‑edge EXAFS – Fourier‑transformed data show a Fe–O peak at ~1.50 Å and a Fe–Fe/Al peak at ~3.53 Å. Both distances contract with Al addition, consistent with the smaller Al ionic radius and reduced lattice parameters observed in XRD. The Fe–O peak intensity rises, suggesting a slight rearrangement of the Fe–O network, while the Fe–Fe/Al peak intensity remains largely unchanged.

Bi L₃‑edge XANES – Edge energies remain near 13 429 eV, confirming Bi³⁺ oxidation across all compositions. Post‑edge peaks B1 and B2 grow with Al content, reflecting an increased 2p₃/₂ → 6d transition probability and a higher 6d energy level, likely due to altered Bi–O bonding environment.

Bi L₃‑edge EXAFS – The Bi–O coordination peak (~1.62 Å) shifts to longer distances upon Al doping, indicating an expanded Bi–O bond length. Slight intensity changes suggest a modest increase in local disorder around Bi atoms.

UV‑Vis Spectroscopy – All powders exhibit strong absorption in the visible range, with absorption edges shifting from 659 nm (x = 0) to 619 nm (x = 0.1). Band gap energies, derived from Tauc plots, increase from 1.83 eV to 1.91 eV as Al content rises, consistent with the blue shift and local structural perturbations. These optical properties point to potential application as visible‑light photocatalysts.

Conclusion

Hydrothermally synthesized BFA_xO powders (x = 0–0.1) retain the rhombohedral R3c perovskite framework while incorporating Al³⁺ at the B‑site. Al doping introduces minor secondary phases and contracts the lattice, but does not disrupt the overall symmetry. XAFS analysis confirms a mixed Fe²⁺/Fe³⁺ valence, enhanced Fe 3d–O 2p hybridization, and a shift of Bi–O bonds. Optical measurements reveal a visible‑light absorption band and a tunable band gap, positioning Al‑doped BFO as a promising photocatalytic material.

Abbreviations

AFM:

Antiferromagnetic

BFA_xO:

BiFe_1–xAl_xO₃

BFO:

BiFeO₃

EXAFS:

X‑ray absorption fine structure

FE:

Ferroelectric

FM:

Ferromagnetic

RT:

Room temperature

UV‑Vis:

Ultraviolet‑visible

XAFS:

X‑ray absorption fine structure

XANES:

X‑ray absorption near‑edge structure

XRD:

X‑ray diffraction