To attain a deep understanding of ferroelectric and piezoelectric characteristics of K0.5Na0.5NbO3 as a promising lead-free compound, the ferroelectric and piezoelectric responses of its epitaxially grown films with three primary orientations of [001], [110], and [111] were investigated with an emphasis on the influence of crystallographic orientation. The films were prepared by sol-gel processing using Nb-doped SrTiO3 single-crystalline substrates with various cutting directions. A peak remnant polarization value (Pr) of 17.3 μC/cm2 was obtained along the [110] direction due to the coincidence between the spontaneous polarization and the film orientation, which is significantly higher than 10.5 μC/cm2 in [111]-oriented and 10.1 μC/cm2 in [001]-oriented ones. However, a better piezoelectric response was achieved in the [001]-oriented films with an average local effective piezoelectric coefficient (d33) of 50.5 pm/V, as compared with 45.1 pm/V and 39.7 pm/V in [110]- and [111]-oriented films, respectively.

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Epitaxial Pb(Zr,Ti)O3 (PZT) films were deposited on Nb-doped SrTiO3 (Nb:STO) monocrystalline wafers by a sol-gel method. It was observed that phase structure of prepared films depends on the orientation of Nb:STO substrates. Interestingly, tetragonal structure was only found for [001]-oriented PZT film on Nb:STO(100) substrate, whereas Nb:STO(111) substrate resulted in a [111]-oriented rhombohedral PZT film, regardless of Zr∕Ti ratios ranging from 40∕60 to 60∕40. The influence of substrate orientation on phase structure of epitaxial PZT films was discussed on basis of the lattice matching. Ferroelectric and piezoelectric properties of PZT films on different Nb:STO substrates were also studied.

Lead-free piezoelectric xLiNbO3–(1−x)(K0.52Na0.58)NbO3 (LKNN) (x=0, 0.04, 0.06, and 0.08) thin films on Pt/Ti/SiO2/Si(100) substrates were prepared by a sol-gel method. The effects of the Li substitution content on the crystal structure, microstructure, and electrical properties of LKNN films were investigated. The Li substitution leads to a remarkable improvement in the piezoelectric properties of the films. The LKNN film with an optimized Li substitution of x=0.06 showed a high local effective piezoelectric coefficient d33∗ of 192 pm/V, which is comparable to the piezoelectric performance of Pb(Zr,Ti)O3 films. An enhanced ferroelectricity was also obtained in the x=0.06 LKNN film, with a remnant polarization of 9.7 μC/cm2 and a coercive field of 36.5 kV/cm. Furthermore, an in-depth analysis of the leakage current characteristics of the LKNN films was also conducted in this study.

[100]-textured Nb-doped Pb(Zr0.52Ti0.48)O3 (PNZT) films with different thicknesses from 80 to 600 nm were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by a sol−gel process. It was found that the local effective longitudinal piezoelectric coefficient, d 33, initially increased with film thickness and reached a peak (∼220 pm/V) for an intermediate thickness (∼350 nm), but then decreased with further increasing thickness. XRD and Raman analyses revealed that, even for an identical Zr/Ti ratio of 52/48, which is near the morphotropic phase boundary of bulk PZT, a pseudophase transition from a tetragonal structure to a rhombohedral structure was induced in [100]-textured PNZT films because of changes in stress with film thickness. This finding revealed a special approach to enhance the piezoelectric properties of PZT-based thin films by combining compositional optimization and the substrate constraint effect.