Defect-engineered BT-based ceramics: correlating phase coexistence, electrocaloric response and energy storage...

The effects of Fe and Mn substitution on the structural, dielectric, ferroelectric, electrocaloric, and energy storage properties of Ba_0.95Ca_0.05Ti_0.908M_0.002Sn_0.08Zr_0.01O₃ (BCTSZ-M, M = Fe and Mn) ceramics were systematically investigated. The ceramics were synthesized using a conventional solid-state reaction method. Rietveld refinement of the X-ray diffraction patterns revealed that the Fe-doped composition (BCTSZ-Fe) exhibits a coexistence of orthorhombic (Amm2) and tetragonal (P4mm) phases, whereas the Mn-doped composition (BCTSZ-Mn) shows the coexistence of rhombohedral (R3m) and orthorhombic (Amm2) phases. These polymorphic phase structures are expected to facilitate polarization rotation and domain-wall motion, thereby promoting enhanced functional properties. Microstructural analysis revealed a strong dopant-dependent grain refinement, while Raman spectroscopy combined with Fano analysis highlighted pronounced phonon - defect interactions, structural disorder, and electron-phonon coupling induced by Fe and Mn substitution. Dielectric measurements showed high permittivity values for both compositions, with ε _r,max reaching 10 262 for BCTSZ-Fe and 11 911 for BCTSZ-Mn. The diffuseness of the phase transition was confirmed using the modified Curie-Weiss law. Ferroelectric analysis revealed well-saturated P-E hysteresis loops, indicating stable polarization switching. Energy storage performance demonstrated a recoverable energy density of 0.0976 J cm^-3 and an efficiency of ∼74.4% for BCTSZ-Fe, compared to 0.0929 J cm^-3 and ∼61.8% for BCTSZ-Mn under 30 kV cm^-1. The improved performance of the Fe-doped ceramic is attributed to defect dipole formation and domain wall pinning, whereas Mn doping enhances polarization at the expense of higher hysteresis losses. The electrocaloric effect was evaluated using both indirect (Maxwell relation) and direct measurement methods. The results reveal significant temperature and entropy changes near the phase transition region, with a maximum electrocaloric responsivity of ∼0.25 K mm kV^-1 for the Mn-doped ceramic. The good agreement between direct and indirect measurements confirms the reliability of the obtained results. While Mn substitution enhances electrocaloric strength, Fe doping ensures a more stable and broader thermal response. These findings demonstrate that dopant-controlled defect chemistry and phase coexistence provide an effective strategy for optimizing multifunctional properties in BCTSZ ceramics, making them promising candidates for dielectric capacitors and solid-state electrocaloric cooling operating under moderate electric fields.