coatings Article Structural and Electric Properties of Epitaxial Na 0.5 Bi 0.5 TiO 3 -Based Thin Films Bruno Magalhaes 1,2, *, Stefan Engelhardt 1,2 , Christian Molin 3 , Sylvia E. Gebhardt 3 , Kornelius Nielsch 1,2 and Ruben Hühne 1   Citation: Magalhaes, B.; Engelhardt, S.; Molin, C.; Gebhardt, S.E.; Nielsch, K.; Hühne, R. Structural and Electric Properties of Epitaxial Na 0.5 Bi 0.5 TiO 3 - Based Thin Films. Coatings 2021, 11, 651. https://doi.org/10.3390/ coatings11060651 Academic Editor: Andrey Tumarkin Received: 11 May 2021 Accepted: 27 May 2021 Published: 28 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Leibniz-IFW Dresden, Institute for Metallic Materials, 01069 Dresden, Germany; s.engelhardt@ifw-dresden.de (S.E.); k.nielsch@ifw-dresden.de (K.N.); r.huehne@ifw-dresden.de (R.H.) 2 Institute of Material Science, Faculty of Mechanical Science and Engineering, TU Dresden, 01062 Dresden, Germany 3 Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstraße 28, 01277 Dresden, Germany; christian.molin@ikts.fraunhofer.de (C.M.); sylvia.gebhardt@ikts.fraunhofer.de (S.E.G.) * Correspondence: b.magalhaes@ifw-dresden.de Abstract: Substantial efforts are dedicated worldwide to use lead-free materials for environmentally friendly processes in electrocaloric cooling. Whereas investigations on bulk materials showed that Na 0.5 Bi 0.5 TiO 3 (NBT)-based compounds might be suitable for such applications, our aim is to clarify the feasibility of epitaxial NBT-based thin films for more detailed investigations on the correlation between the composition, microstructure, and functional properties. Therefore, NBT-based thin films were grown by pulsed laser deposition on different single crystalline substrates using a thin epitaxial La 0.5 Sr 0.5 CoO 3 layer as the bottom electrode for subsequent electric measurements. Structural characterization revealed an undisturbed epitaxial growth of NBT on lattice-matching substrates with a columnar microstructure, but high roughness and increasing grain size with larger film thickness. Dielectric measurements indicate a shift of the phase transition to lower temperatures compared to bulk samples as well as a reduced permittivity and increased losses at higher temperatures. Whereas polarization loops taken at −100 ◦ C revealed a distinct ferroelectric behavior, room temperature data showed a significant resistive contribution in these measurements. Leakage current studies confirmed a non-negligible conductivity between the electrodes, thus preventing an indirect characterization of the electrocaloric properties of these films. Keywords: ferroelectrics; epitaxy; thin films; pulsed laser deposition 1. Introduction Materials with distinct ferroelectric properties enable the direct conversion between thermal, mechanical, and electrical energies, which are utilized in devices such as actuators, transducers or sensors [1–3]. More recently, the conversion of electrical to thermal energy by using the so-called electrocaloric effect (ECE) received renewed interest due to its potential for realizing energy-efficient solid-state cooling devices [4,5]. Starting with the work of Mischenko et al. on PbZr 0.95 Ti 0.05 O 3 [6], giant electrocaloric effects were measured in thin films. Whereas a number of other lead-containing materials show large temperature changes due to the ECE [5], the major focus of the present research has moved towards environmentally friendly, lead-free materials. The majority of research was performed on BaTiO 3 -based thin films due to their well-established preparation routes as well as the tunable structural and ferroelectric properties [7–13]. In the quest for alternative materials, Na 0.5 Bi 0.5 TiO 3 (NBT)-based compounds were also tested for their ECE, and significant temperature changes of up to 1.5 K (at 50 kV cm −1 and 135 ◦ C) were found for bulk NBT- based compound [14,15]. Due to the breakdown voltage limitation (∼50 kV cm −1 )[6,16] of bulk materials, the change in temperature might be significantly higher in thin films, as Coatings 2021, 11, 651. https://doi.org/10.3390/coatings11060651 https://www.mdpi.com/journal/coatings