Large electrocaloric strength in Bi(Mg0.5Ti0.5)O3-modified tetragonal-structured Bi0.5Na0.5TiO3–12.5BaTiO3 ceramics

•2.5 mol.% Bi(Mg0.5Ti0.5)O3 sets as a bridge between nonergodic and ergodic relaxors in tetragonal Bi0.5Na0.5TiO3-12.5BaTiO3.•A microstructure existing P4mm ferroelectric nanodomains in weakly-polar P4bm matrix was constructed.•2.5 mol.% Bi(Mg0.5Ti0.5)O3-modified Bi0.5Na0.5TiO3-12.5BaTiO3 shows aΔT of 0.80 K @ 4 kV/mm a ΔT/ΔE of 0.020 K∙cm/kV. [Display omitted] Taking the advantage of the low transition barrier between P4bm and P4mm distortions, tetragonal structured 0.875Bi0.5Na0.5TiO3–0.125BaTiO3 was designed to construct an electrocaloric ceramics with large electrocaloric strength by adding the third compound Bi(Mg0.5Ti0.5)O3 (BMT). The experiments show that, through adding 2.5 mol.% BMT, a large ΔT of 0.80 K was directly observed under 4 kV/mm; especially, a high ΔT/ΔE of 0.020–0.023 K cm/kV was achieved, approximate to BaTiO3-based and some lead-based relaxors. The high electrocaloric effect is attributed to that the 2.5 mol.% BMT added sample bridges the nonergodic ferroelectric and ergodic relaxor states, and exhibits a microstructure coexisting P4bm and P4mm symmetries. Our design provides a new route in probing lead-free BNT-based electrocaloric materials apart from the MPB.