First-principles study on strain-engineered photocatalytic performance in ferroelectric K(Ta_0.5Nb_0.5)O₃

Based on first-principles calculations, this study systematically investigated the evolution of structural stability, electronic structure, and bulk photoelectric properties of K(Ta_0.5Nb_0.5)O₃ (KTN) under biaxial strain ranging from -30% to 30%. The calculation results show that KTN maintains structural integrity throughout the strain range. Tensile strain (5-30%) enhances structural stability, with the system's binding energy decreasing from -7.3 eV to -7.9 eV, accompanied by a phase transition from tetragonal to monoclinic. Under -30% compressive strain, KTN stabilizes in the monoclinic phase. The projected crystal orbital occupation numbers indicate that the overall covalent bond strength of the system increases with the increase of strain, and the electron distribution of the O-Nb bond shows a non-monotonic change, reaching a peak of 0.085 at approximately -10% strain. Strain regulation significantly enhances ferroelectric polarization intensity and promotes the separation of photogenerated electron-hole pairs. Compressive strain continuously reduces the band gap, with the calculated minimum value being 0.838 eV at -30% strain (this value may be underestimated due to the limitations of the PBE functional). The carriers exhibit a "heavy hole-light electron" characteristic , which is conducive to hole migration to the surface. Compressive strain (-30% to -15%) and 5% tensile strain can induce a redshift of the absorption edge, expanding the visible light response range. Notably, monoclinic KTN under -30% compressive strain shows an optimized trend in polarization intensity, carrier mobility, and light absorption rate. Although achieving ±30% strain through standard epitaxial growth techniques is currently difficult (this technique is usually limited to ±4% or so), exploring such extreme conditions enables us to separate the correlation between electronic effects and lattice instability and clearly reveal the regularity of orbital activity. This study provides theoretical guidance for strain engineering in the design of KTN-based photocatalytic materials, but to accurately predict actual photocatalytic activity (such as hydrogen evolution efficiency), systematic research combining surface reaction energy barriers, carrier recombination kinetics, and interface band bending effects is still needed.