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

The epitaxial (KNaLi)(TaNb)O with 2 wt % MnO addition (KNNLT-M) film on the transparent LaBaSnO-coated LaAlO (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 [(O)]. Compared to the other perovskite oxide films, for example, (La,Ca)MnO, PbTiO and BaTiO, an anomalous lattice evolution with an increased (decreased) out-of-plane (in-plane) lattice constant was observed in KNNLT-M films as (O) 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 (O) can make more K cations to enter the perovskite lattice and the Mn/Mn 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 (O). 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 (O) is one of the important prerequisites for the application of KNN-based films.