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.
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