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

The epitaxial (K 0.49 Na 0.49 Li 0.02 )(Ta 0.2 Nb 0.8 )O 3 with 2 wt % MnO 2 addition (KNNLT-M) film on the transparent La 0.03 Ba 0.97 SnO 3 -coated LaAlO 3 (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 (O 2 )]. Compared to the other perovskite oxide films, for example, (La,Ca)MnO 3 , PbTiO 3, and BaTiO 3 , an anomalous lattice evolution with an increased (decreased) out-of-plane (in-plane) lattice constant was observed in KNNLT-M films as P (O 2 ) 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 (O 2 ) can make more K cations to enter the perovskite lattice and the Mn 2+ /Mn 3+ 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 (O 2 ). 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 (O 2 ) is one of the important prerequisites for the application of KNN-based films.