Critical roles of Mn-ions in enhancing the insulation, piezoelectricity and multiferroicity of BiFeO₃-based lead-free high temperature ceramics

A lead-free multiferroic ceramic of BiFe_0.96Sc_0.04O₃-BaTiO₃ is a type of ABO₃ perovskite structure, belonging to the R3c space group, but exhibiting poor insulation and weak multiferroicity. In this work, the critical roles of Mn-ions in tailoring the electrical and magnetic properties of BiFeO₃-based materials are revealed: the introduction of MnO₂ into BiFe_0.96Sc_0.04O₃-BaTiO₃ induces a dramatic improvement in insulation, piezoelectricity and multiferroicity. New compositions of BiFe_0.96Sc_0.04O₃-BaTiO₃ + x mol% MnO₂ were synthesized by a conventional solid-state reaction method. All the ceramics possess a perovskite structure, and a morphotropic phase boundary (MPB) of rhombohedral and monoclinic phases is formed at x = 0.5-1.0. The formation of and is noticeably suppressed and the resistivity of the ceramics is increased by ∼100 times after the addition of 0.5-1.0 mol% MnO₂, which make the ceramic polarizable and thus give strong ferroelectricity and considerable piezoelectricity. The ceramics with the MPB composition exhibit high electrical insulation (R = 1.2-1.7 × 10¹⁰ Ω cm), good piezoelectricity (d₃₃ = 123-143 pC N^-1, k_p = 0.34-0.35), strong ferroelectricity (P_r = 13.1-17.6 μC cm^-2), high Curie temperature (590-596°C) and excellent temperature stability of piezoelectric and ferroelectric properties. These improvements are greatly associated with the contribution of Mn ions in the ceramics. Surprisingly, sharply enhanced ferromagnetism with M_r = 0.4946 emu g^-1 and M_s = 1.0298 emu g^-1 is obtained in the ceramic with x = 7.0, almost one thousand times larger than that of an un-doped ceramic. The origin of unusual ferromagnetism is associated with significant changes in magnetic ordering caused by Mn doping. The high magnetoelectric effect (α₃₃ = 429.6 mV cm^-1 Oe^-1) is obtained after the addition of 2.0 mol% Mn ions. Our study suggests that the present ceramics may have potential applications in advanced memory devices as promising lead-free high temperature piezoelectric and multiferroic materials.