B23-O-08Microstructures in improper ferroelectric compounds revealed by electron microscopy
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Abstract:
In improper ferroelectric materials, the order parameter of the phase transition is not the polarization but another physical quantity, where transformation properties are different from those of the polarization. The spontaneous polarization arises in the phase transition as a secondary effect [ 1 ]. For example, geometric ferroelectric such as the hexagonal manganites are improper ferroelectrics in which geometric structural constrains induce ferroelectric polarization [ 2 ]. An another example is a hybrid improper ferroelectricity such as (Ca,Sr) 3 Ti 2 O 7 , whichresults from the combination of two or more non-ferroelectric structural order parameters [ 3 ]. The coupling between the spontaneous ferroelectric polarization and other physical quantities should result in unique domain structures in the improper ferroelectric materials. Figure 1 is a domain structure in the improper ferroelectric compound BaAl 2 O 4 with the hexagonal structure [ 4 ]. Characteristic structural antiphase domains are observed, in which structurally modulated superstructure with 2 a × 2 a × c . In the presentation, unique charged domain walls found in some improper ferroelectric compounds will be reported.Keywords:
Superstructure
The authors of paper commented claim that trimethylammonium tetrachlorozincate crystal shows at 282 K the ferroelectric-paraelectric phase transition. But no ferroelectric hysteresis loop was observed below this temperature. Moreover, in the low-temperature phase the ferroelectric domain walls should exist giving dielectric relaxation in a low frequency electric field. The authors conclude that the phase transition is of the second order. This conclusion is contrary to the DSC data where the phase transition has a strong first order character. In the whole measured temperature range the dielectric loss is 100 times higher than the real part of dielectric constant.
Hysteresis
Atmospheric temperature range
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It is shown that the constant current method, in which a dielectric is charged with a constant current while the voltage is monitored, allows one to determine the dependence of the stable ferroelectric polarization with the electric field. The determination is based on two successive experiments separated in time by a short-circuit period: a charging process in which polarization switching occurs followed by a recharging with the same current polarity. Analysis of the recharging experiments for poly(vinylidene fluoride), PVDF, shows that the polarization appearing in it is a metastable ferroelectric polarization, due to the reorientation of ferroelectric polarization lost during the short-circuit period. The method was applied to measure the ferroelectric polarization in PVDF samples with different β-phase contents and in an exploratory way for a few other ferroelectric polymers.
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Ferroelectric Polymers
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Valley polarization and ferroelectricity are the two basic concepts in electronic device applications. However, the coexistence of these two scenarios in one material has not been reported. Here, using first-principles calculations, we demonstrated that the two-dimensional GaAsC6 monolayer which is a hybrid structure of GaAs and graphene has a pair of inequivalent valleys with opposite Berry curvatures and an intrinsic out-of-plane spontaneous electric polarization. It also has a direct band gap of about 1.937 eV and a high carrier mobility of about 1.80 × 105 cm2 V-1 s-1, which are promising for electronic device applications. The integration of valley polarization and ferroelectricity in a single material offers a promising platform for the design of electronic devices.
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Superstructure
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Hexagonal Lattice
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The ferroelectric phase transition characteristics of the 0.32Pb(In1/2Nb1/2)O3-0.345Pb(Mg1/3Nb2/3)O3-0.335PbTiO3 (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FEM-FET) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FET-PC), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around TM-T and TC/Tm temperatures. The temperature dependent permittivity derivative ξ = dϵ/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FEM-FET phase transition and the first-order transition feature of the FET-PC phase transition. The FEM-FET phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the Pr and Ec values, and extremums of the pyroelectric performance.
Tetragonal crystal system
Monoclinic crystal system
Transition temperature
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The Oxygen-stoichiometric sample YBaCo4O7 was measured to be a hexagonal lattice with the parameters of a=0.629(5) nm,c=1.023(5) nm by electron diffraction.And we investigated a superstructure in oxygen non-stoichiometric material YBaCo4O8.5,which is a 3×3×1 hexagonal structure(as=bs=1.090(4) nm,cs=1.002(6) nm) with respect to its parent hexagonal structure.This study advances the understanding of the structural properties for YBaCo4O7+δ and related transition-metal oxide systems.
Superstructure
Stoichiometry
Lattice constant
Hexagonal Lattice
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High pressure ultrasonic and D - E hystresis measurements were performed on a vinylidene fluoride-trifluoroethylene (VDF/TrFE) copolymer with 54 mol% VDF content to investigate the pressure effect on physical properties accompanying wiht a ferroelectric phase transition. Differing from the results at atmospheric pressure where the ferro-to-paraelectric phase transition proceeded simply in one-step, this copolymer exhibited at 350 MPa two-step temperature variations in ultrasonic velocity and absorption, and remanent polarization upon the ferroelectric phase transition. These results agree well with the previous results of DTA and X-ray diffraction experiments, suggesting that the changes in the nature of the ferroelectric phase transition is attributed to the pressure-induced structural transformation.
Transition temperature
Ferroelectric Polymers
Ferroics
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Hydrothermal synthesis yielded crystals of heptabarium dodecafluoride dichloride, Ba7F12Cl2, displaying a new 2a,c hexagonal superstructure with P-6 symmetry. The superstructure results from the disorder of Ba2+ cations over two adjacent tricapped trigonal prismatic sites located in channels parallel to the c axis.
Superstructure
Trigonal crystal system
Hydrothermal Synthesis
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