Structural and Electric Properties of Epitaxial Na0.5Bi0.5TiO3-Based Thin Films

Substantial efforts are dedicated worldwide to use lead-free materials for environmentally friendly processes in electrocaloric cooling. Whereas investigations on bulk materials showed that Na0.5Bi0.5TiO3 (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 La0.5Sr0.5CoO3 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 degrees 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.

Automated summary

While bulk materials research has identified Na0.5Bi0.5TiO3 (NBT)-based compounds as potential candidates for lead-free electrocaloric cooling processes, this study focuses on investigating the feasibility of epitaxial NBT-based thin films for a more detailed analysis of the relationship between composition, microstructure, and functional properties. The thin films exhibited undisturbed epitaxial growth on lattice-matching substrates with columnar microstructure, but showed increased roughness and grain size with thicker films. Dielectric measurements showed a shift in phase transition temperature, reduced permittivity, and increased losses compared to bulk samples.