Electrocaloric effect near room temperature in lead-free Aurivillius phase Sr2Bi4Ti5O18 upon La and Nb codoping

Abstract

La and Nb codoping of ferroelectric five-layer Aurivillius phase Sr2Bi4Ti5O18 (TC ~ 560 K), induces compositional disorder and diffuse relaxor transitions at much lower temperatures than the parent compound, while preserving the orthorhombic structure. For moderate La and Nb concentrations, the ferroelectric properties are enhanced in the vicinity of room temperature, and so is the electrocaloric effect as measured directly with a homemade quasi-adiabatic calorimeter. The adiabatic temperature change reaches, e.g., in Sr2Bi3.44La0.5Ti4.8Nb0.2O18, ΔT ⁓ 0.3 K at 355 K with a temperature span ⁓ 15 K, under ΔE = 37 kV/cm. Besides, a large field dispersion of the electrocaloric effect is found, ascribed to field-induced transitions from short-range-ordered relaxor to long-range-ordered ferroelectric states. Indirect estimations of the electrocaloric effect from electric polarization data are not able to quantitatively predict the direct results and emphasize the need of applying direct measurement methods, especially for relaxor ferroelectrics.