Band-Gap-Engineered Ferroelectric Perovskite x KNbO3–(1 – x )BaFe0.5Nb0.5O3: Structural Phase Transition, Photophysics, DFT-Guided Charge-Transfer Mechanism and Superior Visible-Light Photocatalytic Degradation of Rhodamine B

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Abstract We synthesize and characterize the solid-solution series xKNbO3–(1 – x)BaFe0.5Nb0.5O3 (x = 1.0 to 0.0) through solid–state reaction. Rietveld refinement and Raman spectroscopy confirm a composition-driven Bmm2→Pm3¯m structural phase transition that completes at x ≤ 0.3. UV–vis Tauc analysis shows the widest band gap tuning (3.11 to 1.65 eV) reported for any single-phase KNO-based system. Progressive photoluminescence quenching confirms decreased electron–hole recombination as BFN content increases, while impedance spectroscopy indicates a 15-fold rise in dielectric permittivity εr, which broadens the space-charge depletion layer. Under a 250 W visible-light source, the x = 0.3 composition (KBFN3) achieves 98.1% photodecolorization (chromophore depletion) of rhodamine B in 50 min (kobs = 0.102 min–1), ∼99× the rate constant of a H2O2-only control lacking catalyst. This is among the highest rate constants reported for any modified KNbO3 system, and, to our knowledge, the only such report for which the catalytic contribution has been isolated from the H2O2/photolysis background via a quantified no-catalyst control, with at least 88% efficiency maintained over five cycles. This work is presented as a proof-of-concept demonstration under controlled laboratory conditions (fixed dye concentration, catalyst loading, and light source). DFT + U calculations link this performance to Fe 3d/O 2p hybridization, which narrows the gap through a valence-band upshift while keeping the conduction-band edge stable, making O2•- the main proposed reactive species. A Marcus outer-sphere electron-transfer analysis of the DFT + U total energies yields reorganization energies of λ(O2•-) = 0.963 eV and λ(•OHdirect) = 1.844 eV, giving activation barriers of 0.231 and 0.677 eV, respectively, and a kinetic rate ratio of ∼107 in favor of the superoxide pathway that remains robust to ±0.2 eV uncertainty in the Mulliken band-edge positions. Complementary climbing-image NEB calculations identify surface hydrogen migration (forward/reverse barriers of 0.536/0.487 eV) as the principal atomic-scale kinetic bottleneck. These findings indicate that dielectric and trap-mediated charge-separation mechanisms can replace ferroelectric polarization in narrow-gap perovskite photocatalysts.

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