High-Quality Epitaxial Five-Layer Aurivillius Films with In-Plane Ferroelectricity for Electrocaloric Cooling

High-quality purely c-axis oriented epitaxial thin films of the Aurivillius phase Sr₂Bi₄Ti₅O₁₈ with n = 5 (Sr,Bi)TiO₃ perovskite-like layers, are grown on SrTiO₃ substrates by pulsed laser deposition. The highest crystalline quality is obtained with a 20 wt.% Bi-excess target and average stacking order values in the proximity of the ideal value n = 5 are attained for an optimum deposition temperature of 650 °C. Scanning transmission electron microscopy reveals regions with n ranging from 4 to 6 around an average thickness of n = 5, in agreement with the X-ray diffraction analysis. Interdigital electrodes are used to probe the in-plane polarization and survey the electrocaloric properties. A maximum adiabatic temperature change of ΔT ∼ 0.95 °C for an electric field of 150 kV cm⁻¹ is observed at ≈135 °C. Larger values are expected at higher temperatures around the ferroelectric Curie temperature, T_C. Since T_C of Sr₂Bi₄Ti₅O₁₈ can be tuned by codoping, the findings pave the way toward a large electrocaloric effect at ambient temperature.