Polar-axis-oriented epitaxial tetragonal (Bi,K)TiO3 films with large remanent polarization deposited below Curie temperature by a hydrothermal method

Abstract

A hydrothermal method was used to epitaxially grow 70-nm-thick tetragonal (Bi,K)TiO3 films with stoichiometric compositions at 240 °C on (001)cSrRuO3//(001)SrTiO3 substrates. Crystal structure analysis revealed that the obtained (Bi,K)TiO3 films had a polar-axis orientation attributable to the matching of the in-plane lattice with the SrRuO3 underlayer. Large coherent displacement of A-site ions along the polar direction observed by transmission electron microscopy may have induced the large tetragonal distortion of c/a = 1.046. The dielectric constant was about 100 and was almost frequency independent in the range of 103–105 Hz, while the dielectric loss, tan δ, was below 5%. Well-saturated ferroelectric polarization−electric field (P−E) hysteresis loops were observed, and the remanent polarization (Pr) was 84 μC/cm2. In addition, the effective piezoelectric constant, d33,eff., was estimated to be 85 pm/V by time-resolved x-ray diffraction measured under an applied electric field. The Pr and d33,eff. values were larger than those reported for polar-axis-oriented epitaxial tetragonal Pb(Zr0.35Ti0.65)O3 films, indicating that the polar-axis-oriented tetragonal (Bi,K)TiO3 film has high potential as a lead-free material for various applications. Specifically, the small εr and large d33,eff. led to an improvement in the figures of merit [d33,eff./εr and (d33,eff.)2/εr], the most important index in sensor and energy-harvester applications.