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

La and Nb codoping of the five-layer Aurivillius phase Sr2Bi4Ti5O18 induces compositional disorder and diffuse relaxor transitions at much lower temperatures than the undoped ferroelectric compound (560 K), 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 their electrocaloric effect measured directly with a homemade quasi-adiabatic calorimeter. The adiabatic temperature change reaches, e.g., ??T ??? 0.3 K at 355 K with a temperature span ??? 15 K, under ??E = 37 kV/cm. The large field dispersion of the effect is 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.