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Achieving frequency- and temperature-independent colossal permittivity (CP) with low dielectric loss is a long-standing challenge for electronic materials, in which the basic issue is understanding the underlying relaxation mechanism. In this paper, taking CaCu3Ti4O12 ceramics as an example, CP was ascribed to electron-trapping behaviors at the edge of a double Schottky barrier (DSB). On the one hand, the widely reported origins of CP, i.e. Maxwell–Wagner relaxation and polaronic relaxation, were identified as two aspects of the same bulk conductivity. This caused the insights derived from the commonly employed impedance and admittance spectra to be revisited. On the other hand, hysteresis between CP and external voltages at low temperatures, which was caused by electron filling of interface states, was predicted and experimentally confirmed. This further supported the proposal that CP arose from electron trapping at the DSB. Moreover, multiple relaxations were foreseen when more than one kind of point defect existed in the depletion layers of a DSB. The establishment of intense ‘effective’ relaxation, which was related to shallow traps, was indispensable for achieving CP, while ‘redundant’ relaxation was induced by deep-level defects, resulting in relatively high dielectric loss. Therefore, proper manipulation of the DSB and its related defect structures was crucial for achieving stable CP with sufficiently low dielectric loss.

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As important multifunctional oxides, BiFeO 3 -PbTiO 3 (BFPT) based perovskite materials have drawn increasing research interests due to their promising roomtemperature magnetoelectric (ME), [1] piezoelectric/ferroelectric, [2,3] photovoltaic, [4] and tuneable thermal expansion performances. [5] The two end members, BiFeO 3 (BFO) and PbTiO 3 (PT), can form complete solid solution in the (1−x)BFO-xPT system, where a morphotropic phase boundary (MPB) with coexisting rhombohedral and tetragonal phases was found in the composition range around x = 0.3. [6] Due to the high Curie temperatures (T C ) in both end members, the T C for the MPB compositions in BFPT is as high as ≈ 630 °C. [7] Moreover, the MPB is of metastable nature in a relatively wide composition range, [8] where a phase transition can be induced by an applied electric field. [1] Such an MPB-related phase transition could result in a giant and stable piezoelectric response. [9] Indeed, enhanced electromechanical effects were obtained recently in the vicinity of MPB of BFPT and Mn-doped BFPT, [1,10] making the BFPT system particularly interesting for high-temperature piezoelectric/ferroelectric applications.In prototype piezoelectrics such as Pb(Zr x Ti 1−x )O 3 (PZT) and Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 (PMN-PT), [11,12] monoclinic phases are found to exist and act as a bridge connecting two phases on both sides of the MPB. In contrast, the MPB compositions of BFPT system show coexistence of two distinct ferroelectric phases inherited from the two respective end members. In addition, it is found in modified BFPT materials [13] that the two different ferroelectric phases also reveal diverse magnetic orderings. In our previous work, the substitution of rare earth Dy for Bi in the BFPT system is proved to induce ferromagnetism in the rhombohedral compositions with magnetic hysteresis loops, while the antiferromagnetic state is retained in the tetragonal compositions [14] at room temperature. These characteristics also make the BFPT system promising materials for the so-called phase-change large ME effects, where large ME Perovskite materials based on BiFeO 3 -PbTiO 3 (BFPT) solid solutions are promising for various applications thanks to the extremely large spontaneous polarization (P s ) and existence of multiferroic morphotropic phase boundary. For applications in piezoelectric and memory devices, complete switching of P s is needed, which is hard to achieve practically. In this work, a simple modified mixed-oxide reaction method is developed allowing to prepare Dy-and Smmodified BFPT ceramics with significantly improved properties. The MPB compositions demonstrate well-saturated ferroelectric hysteresis loops with large switchable remanent polarization of 60 µC cm −2 and enhanced piezoelectric properties with a large-signal piezoelectric coefficient d 33 * = 214 pm V −1 and a direct piezoelectric coefficient d 33 = 128 pC N −1 , which is one and a half times larger than the best d 33 value reported for the BFPT ceramics so far. The Curie temperature reaches...