Evidence for Antiferroelectric Behavior in KNbO 3 /KTaO 3 Superlattices

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Evidence for Antiferroelectric Behavior in KNbO3/KTaO3 Superlattices J. Sigman1, H. M. Christen2, P. H. Fleming2, L. A. Boatner2, and D. P. Norton1 1 Department of Materials Science and Engineering, University of Florida, P.O. Box 116400, Rhines Hall, Gainesville, FL 32611. 2 Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831. ABSTRACT The dielectric response in artificially layered 1x1 KTaO3/KNbO3 perovskite superlattice structures is reported. While KTaO3 and KNbO3 are ferroelectric or paraelectric, respectively, superlattices appear antiferroelectric based on an increase in dielectric constant with applied dc bias. This “positive tunability” in dielectric response occurs at the same temperature region where a structural phase transition is observed. This dielectric behavior is inconsistent with the nonlinear response for either paraelectric or ferroelectric materials. However, an increase in the dielectric constant with applied electric field is consistent with antiferroelectric behavior. The antiferroelectric ordering correlates with cation modulation imposed by the superlattice. INTRODUCTION The dielectric properties of insulating materials are determined by the polarization behavior. Random alignment of dipoles yields a paraelectric response; parallel or anti-parallel ordering results in ferroelectric or antiferroelectric behavior, respectively. Controlling the nature of dipole polarization is key to tailoring material performance. The manipulation of dielectric properties can be achieved via doping or control of microstructure. In thin films, the formation of artificially layered structures offers opportunities to probe and manipulate dielectric material properties. In this paper, we report on the dielectric properties of K(NbxTa1-x)O3 based superlattice structures. Among the dielectric materials, the mixed-oxide compound K(NbxTa1-x)O3 (or KTN) is a near-ideal material for both understanding and manipulating the dielectric properties of perovskites.[1-3] KTN is similar to (Sr,Ba)TiO3 in that, as the composition is altered, the solid solution exhibits a continuous transition from a paraelectric to a ferroelectric material. Pure KTaO3 is cubic (a300 K = 3.9885 Å) and paraelectric at all temperatures. Decreasing the temperature results in a softening of the TOI transverse optic, zone-center phonon branch. The soft mode is stabilized by zero-point quantum fluctuations, however, so that KTaO3 does not undergo a ferroelectric transition but remains cubic and paraelectric down to 0 K. Hence KTaO3 exhibits so-called “incipient ferroelectric” or “quantum ferroelectric” behavior.[4,5] Pure KNbO3, in contrast, exhibits a first-order ferroelectric phase transition accompanied by a change from the cubic to the tetragonal structure at 701 K (ap = 4.02 Å along [100]p and [010]p directions, and cp = 3.97 Å along the [001]p). Upon further cooling, the structure changes to orthorhombic at 498 K, with lattice parameters a = 5.696 Å, b = 5.7213 Å, and c = 3.9739 Å. For the ferroelectric state