Ferroelectric and piezoelectric properties of La-modified lead-free (Bi0.5Na0.5)TiO3–(Bi0.5K0.5)TiO3–SrTiO3 thin films

Lead-free piezoelectric 0.9(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)–0.1SrTiO3 thin films doped with x mol% La (abbreviated as BNT–BKT–ST–xLa) (x=0, 0.5, 1.0, 1.5) were prepared by a sol–gel method on Pt(1 1 1)/Ti/SiO2/Si substrates. The effects of La content on the microstructure, ferroelectric and piezoelectric properties were investigated systematically. The X-ray diffraction patterns indicated that all films had a single-phase perovskite structure. Meanwhile, we found that lanthanum content had great influence on the grain size and electrical properties. The thin film with a composition of BNT–BKT–ST–0.5La showed the optimal electrical properties with a remnant polarization, dielectric constant and effective piezoelectric constant of 10 μC/cm2, 560, and 83 pm/V, respectively. The results indicated that 0.5 mol% La-doped BNT–BKT–ST thin films would be of great interest for lead-free piezoelectric devices.