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.

Preparation of {100}-oriented (1−x)(Bi,K)TiO3−xCaTiO3 solid-solution epitaxial films by the hydrothermal method and their ferroelectric and piezoelectric properties

“…This is basically the same result as our previous reports for a film with x = 0, (Bi,K)TiO 3 . 22) In the case of the film with x = 0.27 located in Region 2, an additional spot originating from Peak II was observed together with the spots observed for the films with x = 0.12 located in Region 1, as shown in Figs. 3(c) and 3(d).…”

“…© 2023 The Japan Society of Applied Physics displacement. 22) The larger displacement of Bi ions than that of K ions would contribute to a large tetragonality. This suggests that the unique structure observed in (Bi,K)TiO 3 films is kept up to x = 0.12 despite the formation of the solid solution with CaTiO 3 .…”

“…20,21) In 2022, our group reported on the growth of polar-axis-oriented epitaxial (Bi,K)TiO 3 film on (100) c SrRuO 3 //(100)SrTiO 3 substrate at a low deposition temperature of 240 °C by the hydrothermal method. 22) Their c/a ratio and P r are 1.046, and 84 μC cm −2 , respectively. The c/a ratio and P r value are larger than those previously reported 16) for (Bi,K)TiO 3 ceramics.…”

“…The enhanced tetragonality of the (Bi,K)TiO 3 film makes it possible to relate it to the unique arrangement of the A-site cations, Bi and K in (Bi,K)TiO 3 . 22) Furthermore, Huang et al succeeded in growing perfect solid-solution films of (Bi,K)TiO 3 with ferroelectric rhombohedral (Bi,Na)TiO 3 . 23) X-ray diffraction (XRD) analysis revealed a continuous change in the c/a ratio against the K/(K + Na) ratio of the films.…”

Automated Experiments of Local Non‐Linear Behavior in Ferroelectric Materials

“…Amplitude, phase, and frequency images show domains with a lateral scale of hundreds of nanometers with variable piezoresponses. Imaging, domain writing, hysteresis loop measurements, and electric field cycling on the macroscopic device [41][42][43][44][45][46][47] were recorded.…”