Recently, two-dimensional van der Waals ferroelectrics have been receiving much interest with continuous exploration of the underlying physics and device applications. While α-In2Se3 in an atomically thin crystal form is believed to have nonzero out-of-plane polarization, its ferroelectric (FE) instability in competition with the antiferroelectric (AFE) ground state is highly concerned. Along this line, a bilayer α-In2Se3 structure should be a good object for clarifying this issue since it is the simplest 2D lattice accommodating an AFE state, possibly allowing the AFE–FE competition. In this work, we employ the first-principles calculation to investigate the lattice and electronic structures of bilayer α-In2Se3, and special attention is paid to the FE instability in competition with the AFE ground state. It is found that the energy difference between the AFE ground state and FE state is small, thereby allowing an electric field modulation of the AFE–FE inter-conversion. More importantly, the Hyed–Scuseria–Ernzerhof algorithm predicts that the FE state is indeed semiconducting rather than metallic, removing the inconsistency between experimental observation and theoretical prediction. The spin–orbital coupling effect can further enlarge the bandgap and drive the indirect-to-direct bandgap transition, and thus appears to be an important ingredient of the underlying physics.
BiFeO3 represents the most extensively investigated multiferroic due to its fascinating ferroelectric domain structures, large polarization, and multiferroic coupling, among many other emergent phenomena. Nevertheless, much less concern with the piezoelectricity has been raised while all these well addressed properties are identified in thin film BiFeO3, and bulk ceramic BiFeO3 has never been given priority of attention. In this paper, we report our experiments on the ferroelectric and piezoelectric properties as well as domain structures of BiFeO3 bulk ceramics synthesized by rapid hot-press sintering. It is revealed that these properties are strongly dependent on the microstructural quality, and the largest piezoelectric coefficient d33 = 55 pC/N with electric polarization as large as 45 μC/cm2 is obtained for the sample sintered at 800 °C, while they are only 30 pC/N and 14 μC/cm2 for the samples sintered in normal conditions at 800 °C. The two-level hierarchical stripe-like and irregular dendrite-like domain structures are observed in these hot-press sintered samples. It is suggested that the enhanced piezoelectric property is ascribed to the two-level hierarchical stripe-like domain structure which may respond more easily to electrical and strain stimuli than those irregular dendrite-like domains. The enhanced remnant polarization should be owing to the improved sample quality and large grains in the properly hot-press sintered samples.
Magnetically induced ferroelectric polarization in rare-earth RMn2O5 manganites is believed to originate from the symmetric exchange striction associated with a specific antiferromagnetic phase in the low temperature (T) region and would be irrelevant with electropoling in the high-T paramagnetic-paraelectric phase region. In this work, we demonstrate that low-T pyroelectric polarization of GdMn2O5 single crystals along the b axis in the antiferromagnetic phase exhibits remarkable dependence on the electropoling history imposed in the high-T paramagnetic-paraelectric phase. In particular, the high-T electropoling results in a reversal of ferroelectric polarization in the low-T region, which can be flopped back by the electropoling being sustained in the low-T ferroelectric region. The existence of an electrically polarizable magnetic cluster state in the high-T paramagnetic-paraelectric region is proposed based on a combination of experimental observations and first-principles calculations. An intrinsic correlation between the low-T antiferromagnetic ordering and the high-T polarizable state is discussed. The present experiments unveil the emergent phenomena on multiferroicity of RMn2O5 and suggest an alternative scenario for electrocontrol of magnetism.
A ferroelectric thin film epitaxially deposited on a substrate is usually subjected to the mixed mechanical boundary conditions that can neither be treated as purely fixed-strain boundaries nor purely fixed-stress ones, thus causing the inconsistency or inaccuracy for choice of the Helmholtz or Gibbs thermodynamic potential. It would be of interest to clarify this inconsistency and set up some rules for such a choice. In this work, we discuss various roadmaps to construct two types of modified thermodynamic potentials under the epitaxial thin film boundary conditions. The equivalence of these thermodynamic potentials is then established. Subsequently, a set of misfit strain–strain phase diagrams by choosing K0.5Na0.5NbO3 thin films as an example of realistic calculations are constructed to check this equivalence. Finally, some scenarios on choosing various thermodynamic potentials for ferroelectric thin films are discussed.
BiFeO3 has been receiving continuous attention for its excellent ferroelectric and multiferroic properties. Nevertheless, the piezoelectricity as a complementary property of ferroelectricity remains less addressed for BiFeO3 at least in a single phase form. In this work, we investigate the piezoelectric behaviors of bulk Bi1−xNdxFeO3 ceramics, given that the Nd-substitution may trigger structural phase transitions from the R3c phase to the Pna21 phase and eventually toward the non-polar Pnma phase. It is revealed that the piezoelectric coefficient d33 does increase with the increasing Nd content x inside the R3c phase region. However, no d33-enhancement across the R3c–Pna21 boundary region and the Pna21–Pnma boundary region is identified, suggesting no positive correlation between the piezoelectric coefficient and the possible morphotropic phase boundaries. The observed d33-enhancement inside the R3c phase region should be attributed to the remarkably reduced domain size and release of pinned domain wall motion by defects.
Charge-ordered layered manganites ReA2Mn2O7 (Re = rare-earth species and A = Ca, Sr, Ba, etc.) are believed to offer a number of fascinating electronic and magnetic properties, including the long-time claimed but not yet confirmed ferroelectricity associated with charge-ordering. Experimental observations of the charge-order induced transport and electrically polar behaviors have been insufficient. In this work, we synthesize the La(Ca0.8Sr0.2)2Mn2O7 (LCSMO) single crystal and investigate its structural, magnetic, and dielectric properties. It is revealed that LCSMO undergoes two consecutive charge-ordering transitions upon decreasing temperature T before entering an antiferromagnetic state in the low-T range. The first charge-order transition occurs at temperature TCO1 ∼ 314 K from the high-T paramagnetic state. This charge-order state (CO1 state) is transferred into another charge-order state (CO2 state) by a sequence starting from ∼290 K, and the resultant CO2 state is dynamic and polar-like. The dynamic behaviors of this polar-like CO2 state is confirmed by the remarkable dielectric relaxation associated with this state. The present work provides a connection between the charge-ordering and electrically polar response in LCSMO, while ferroelectricity remains yet to be an issue.
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