Unveiling the dual bandgap structure and multiferroic coupling in KBiFe₂O₅: a combined experimental and first-principles investigation

Abstract

Multiferroic KBiFe₂O₅ (KBFO) has emerged as a promising candidate for high-temperature multiferroic and tandem photovoltaic applications, yet its fundamental electronic structure and thermal properties remain inadequately understood. Here, through integrated experimental characterization and hybrid DFT calculations, we establish an unconventional dual bandgap structure (E_g1 ≈ 1.69 eV, E_g2 ≈ 2.17 eV) in monoclinic KBFO, experimentally validated by UV-Vis spectroscopy and photoluminescence. This discovery resolves persistent discrepancies in reported optical absorption data and positions KBFO as a potential broadband photovoltaic absorber. We further elucidate the microscopic origins of magnetism (G-type antiferromagnetic ordering with weak ferromagnetic canting) and ferroelectricity (stereochemically active Bi³⁺ lone pairs), demonstrating intrinsic magnetoelectric coupling. Importantly, we report the first comprehensive thermal property characterization, quantifying standard molar entropy (297.59 J K⁻¹ mol⁻¹), enthalpy (45.438 kJ mol⁻¹), and pronounced magnetocaloric effects near the Curie temperature (T_c≈832 K). These findings provide a holistic theoretical framework for advancing KBFO in next-generation optoelectronic and solid-state refrigeration technologies.