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

A hydrothermal method was used to epitaxially grow 70-nm-thick tetragonal (Bi,K)TiO3 films with stoichiometric compositions at 240 & DEG;C on (001)(c)SrRuO3//(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 10(3)-10(5) Hz, while the dielectric loss, tan delta, was below 5%. Well-saturated ferroelectric polarization-electric field (P-E) hysteresis loops were observed, and the remanent polarization (P-r) was 84 mu C/cm(2). In addition, the effective piezoelectric constant, d(33,eff.), was estimated to be 85 pm/V by time-resolved x-ray diffraction measured under an applied electric field. The P-r and d(33,eff.) values were larger than those reported for polar-axis-oriented epitaxial tetragonal Pb(Zr0.35Ti0.65)O-3 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 epsilon(r) and large d(33,eff.) led to an improvement in the figures of merit [d(33,eff.)/epsilon(r) and (d(33,eff.))(2)/epsilon(r)], the most important index in sensor and energy-harvester applications.