Electrical properties of K0.5Na0.5NbO3 thin films grown on Nb:SrTiO3 single-crystalline substrates with different crystallographic orientations

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.