Anomalous Lattice Evolution-Mediated Electrical Properties in Transparent KNN-Based Lead-Free Ferroelectric Films

The epitaxial (K_0.49Na_0.49Li_0.02)(Ta_0.2Nb_0.8)O₃ with 2 wt % MnO₂ addition (KNNLT-M) film on the transparent La_0.03Ba_0.97SnO₃-coated LaAlO₃ (001) substrate is chosen to investigate how the lattice evolution, as well as the electrical properties, optical bandgap energy, and thermal stability, changes with the growth oxygen pressure [P(O₂)]. Compared to the other perovskite oxide films, for example, (La,Ca)MnO₃, PbTiO_3, and BaTiO₃, an anomalous lattice evolution with an increased (decreased) out-of-plane (in-plane) lattice constant was observed in KNNLT-M films as P(O₂) increases. Such anomalous lattice evolution can improve the electric properties of KNNLT-M films; for example, the ferroelectricity is significantly optimized and the dielectric constant is enhanced from 451 to 1248 at 1 kHz. The X-ray photoelectron spectra results have demonstrated that high P(O₂) can make more K cations to enter the perovskite lattice and the Mn²⁺/Mn³⁺ existing in KNNLT can effectively suppress the leakage behavior, thus promoting the electrical nature of KNNLT-M films. The optical measurements show that the KNNLT-M film heterostructures are highly transparent with a maximum transmittance of ∼80%, and both direct and indirect bandgap energies increase with increasing P(O₂). Meanwhile, all these KNNLT-M films exhibit good thermal stability with stable ferroelectricity up to the high temperature of at least 125 °C. These results demonstrate that the control of the lattice structure and electrical properties by P(O₂) is one of the important prerequisites for the application of KNN-based films.