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

La and Nb codoping of ferroelectric five-layer Aurivillius phase Sr2Bi4Ti5O18 (T-c similar to 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, Delta T similar to 0.3 K at 355 K with a temperature span similar to 15 K, under Delta E = 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.