Antiferroelectric Order Research Articles

Room-temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab initio molecular dynamics perspective

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to ``freely'' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite-temperature ab initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long-range antiferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures, the motions are less correlated.

Existence of polar switching in the nematic and orthogonal smectic phases in novel four-ring bent-core compounds

Bent-core liquid crystals have set the first example of forming polar superstructures from achiral molecules. Polar switching studies in smectic phases have revealed several exciting sub-phases which have never been observed in rod-like liquid crystals. In this study, mesomorphic and polar switching properties of three bent-core compounds belonging to a homologous series have been investigated using polarizing optical microscopy (POM), differential scanning calorimetry, XRD studies, electro-optics, and dielectric spectroscopy. These achiral, unsymmetrical four-ring bent-core liquid crystals with a polar fluoro substituent at one end and n-alkoxy chain at the other terminal end possess azo, ester, and imine linkages between the four phenyl rings and different lateral substituents. The compounds 16-F and 18-F exhibit orthogonal smectic phase with antiferroelectric polar order, and additionally, the compound 16-F exhibits a short range nematic phase with a polar order. The compound 7-F exhibits broad enantiotropic nematic mesomorphism without appearance of any smectic ordering at low temperature. The smectic and nematic phases were identified by their optical textures observed by POM. Distinct polarization current peaks under triangular wave voltage are observed for all the compounds in the entire mesophase range. Relaxation phenomena corresponding to polar associations of the molecules are observed in dielectric spectroscopy. The cybotactic nature of the nematic phase is established by the XRD and electro-optic studies of 16-F. Although it is rather difficult to form mesophase when the number of aromatic rings is reduced from five, we have successfully demonstrated the bent-core compounds with four-ring which exhibit orthogonal smectic phases as well as nematic mesomorphism with unusual cybotactic signature.

Pinched hysteresis loop in defect-free ferroelectric materials

In addition to the single polarization-versus-electric field hysteresis loop that is characteristic of ferroelectrics and the double hysteresis loop that is known to occur in antiferroelectrics, a third kind of polarization-versus-electric field function has been reported in several systems. This third kind is commonly termed the ``pinched'' loop due to its unusual shape, and is typically believed to originate from the pinning of domain walls interacting with defects. Here, using an atomistic effective Hamiltonian scheme, we demonstrate that such a belief has to be broadened since our simulations also yield pinched loops in defect-free ferroelectric materials, as a result of the occurrence of intermediate modulated phases exhibiting an inhomogeneous dipolar pattern leading to the coexistence of both ferroelectric and antiferroelectric orders.

Continuous control of local magnetic moment by applied electric field in multiferroicsBa2CoGe2O7

${\mathrm{Ba}}_{2}{\mathrm{CoGe}}_{2}{\mathrm{O}}_{7}$ exhibits a collinear-antiferromagnetic structure with the easy axis along $\ensuremath{\langle}100\ensuremath{\rangle}$ directions and an antiferroelectric order with the polarization axis along the [001] direction. By applying the electric field the magnetic moment rotates from $\ensuremath{\langle}100\ensuremath{\rangle}$ to [110] directions and, simultaneously, the antiferroelectric state changes to the ferroelectric state gradually. This magnetoelectric effect, i.e., continuous control of the local magnetic moment by the electric field, is quantitatively explained by the Hamiltonian including the dielectric energy.

Tunneling Motion and Antiferroelectric Ordering of Lithium Cations Trapped inside Carbon Cages

Dielectric and X-ray diffraction measurements of [Li@C60](PF6) single crystals reveal the motion of the Li+ cations inside the C60 cages at low temperature. An increase in the dielectric permittivity below 100 K is consistent with a combined tunneling and hopping motion of the Li+ cation between two positions inside the C60 cage. A phase transition accompanied by a decrease in the dielectric permittivity at TC = 24 K is explained by an antiferroelectric ordering of the Li+ cations. The Li+ ordering is caused by interactions among electric dipole moments formed between the Li+ cations inside and the PF6− anions outside the C60 cages. The electric dipole moments that are switched by the Li+ tunneling and interact with each other are potential qubits in a quantum computer using electric dipole moments.

Phase transitions to dipolar clusters and charge density waves in high Tc superconductors

We show that doping of hole charge carriers leads to formation of electric dipolar clusters in cuprates. They are created by many-body interactions between the dopant ion outside and holes inside the CuO planes. Because of the two-fold degeneracy holes in the CuO plane cluster into four-particles resonance valence bond plaquettes bound with dopant ions. Such dipoles may order into charge-density waves (CDW) or stripes or form a disordered state depending on doping and temperature. The lowest energy of the ordered system corresponds to a local anti-ferroelectric ordering. The mobility of individual disordered dipoles is very low at low temperatures and they prefer first to bind into dipole-dipole pairs. Electromagnetic radiation interacts strongly with electric dipoles and when the sample is subjected to it the mobility changes significantly. This leads to a fractal growth of dipolar clusters. The existence of electric dipoles and CDW induce two phase transitions with increasing temperature, melting of the ordered state and disappearance of the dipolar state. Ferroelectricity at low doping is a natural consequence of such dipole moments. We develop a theory based on two-level systems and dipole-dipole interaction to explain the behavior of the polarization as a function of temperature and electric field.

Theory of antiferroelectric phase transitions

At variance with structural ferroic phase transitions which give rise to macroscopic tensors coupled to macroscopic fields, criteria defining antiferroelectric (AFE) phase transitions are still under discussion due to the absence of specific symmetry properties characterizing their existence. They are recognized by the proximity of a ferroelectric (FE) phase induced under applied electric field, with a double hysteresis loop relating the induced polarization to the electric field and a typical anomaly of the dielectric permittivity. Here, we show that there exist indeed symmetry criteria defining AFE transitions. They relate the local symmetry of the polar crystallographic sites emerging at an AFE phase transition with the macroscopic symmetry of the AFE phase. The dielectric properties of AFE transitions are deduced from a Landau theoretical model in which ferroelectric and ferrielectric phases are shown to stabilize as the result of specific symmetry-allowed couplings of the AFE order- parameter with the field-induced polarization.

Refining the phase diagram of PbZr1−x−ySnxTiyO3 ceramics with 0.40≤x≤0.54 by crystal structural, dielectric response and hysteresis loop investigations

Ceramics of PbZr1−x−ySnxTiyO3 (PZST100x/100y) with 0.40≤x≤0.54 were synthesized via conventional solid state reaction and their crystal structures, dielectric response, and hysteresis loops were systematically investigated. A more precise range of the antiferroelectric/ferroelectric phase boundary of PZST100x/100y ceramics with 0.40≤x≤0.54 was established compared with the results reported by Jaffe. It was found that the introduction of Ti4+stabilized the ferroelectric order, and Sn4+substitution enhanced the antiferroelectric order. Thus, the system properties could be tailored by altering Sn and Ti concentration. In addition, the hydrostatic pressure-induced ferroelectric-to-antiferroelectric phase transition was also studied. It has been clarified that the critical transition pressure (Pt) increased with an increasing Ti content at a fixed Sn content.

Role of Ferroelectric Nanodomains in the Transport Properties of Perovskite Solar Cells

Metropolis Monte Carlo simulations are used to construct minimal energy configurations by electrostatic coupling of rotating dipoles associated with each unit cell of a perovskite CH3NH3PbI3 crystal. Short-range antiferroelectric order is found, whereas at scales of 8-10 nm, we observe the formation of nanodomains, strongly influencing the electrostatics of the device. The models are coupled to drift-diffusion simulations to study the actual role of nanodomains in the I-V characteristics, especially focusing on charge separation and recombination losses. We demonstrate that holes and electrons separate into different nanodomains following different current pathways. From our analysis we can conclude that even antiferroelectric ordering can ultimately lead to an increase of photoconversion efficiencies thanks to a decrease of trap-assisted recombination losses and the formation of good current percolation patterns along domain edges.

Structural, thermal, dielectric and phonon properties of perovskite-like imidazolium magnesium formate.

We report the synthesis and characterisation of a magnesium formate framework templated by protonated imidazole. Single-crystal X-ray diffraction data showed that this compound crystallizes in the monoclinic structure in the P21/n space group with lattice parameters a = 12.1246(4) Å, b = 12.2087(5) Å, c = 12.4991(4) Å and β = 91.39(1)°. The antiparallel arrangement of the dipole moments associated with imidazolium cations suggests the antiferroelectric character of the room-temperature phase. The studied compound undergoes a structural phase transition at 451 K associated with a halving of the c lattice parameter and the disappearance of the antiferroelectric order. The monoclinic symmetry is preserved and the new metrics are a = 12.261(7) Å, b = 12.290(4) Å, c = 6.280(4) Å, and β = 90.62(5)°. Raman and IR data are consistent with the X-ray diffraction data. They also indicate that the disorder of imidazolium cations plays a significant role in the mechanism of the phase transition. Dielectric data show that the phase transition is associated with a relaxor nature of electric ordering. We also report high-pressure Raman scattering studies of this compound that revealed the presence of two pressure-induced phase transitions near 3 and 7 GPa. The first transition is most likely associated with a rearrangement of the imidazolium cations without any significant distortion of these cations and the magnesium formate framework, whereas the second transition leads to strong distortion of both the framework and imidazolium cations. High-pressure data also show that imidazolium magnesium formate does not show any signs of amorphization up to 11.4 GPa.

Cooperative ordering in lattices of interacting two-level dipoles

We investigate the cooperative behavior of regular monolayers of driven two-level dipoles, using classical electrodynamics simulations. The dipolar response results from the interference of many cooperative eigenmodes, each frequency-shifted from the single resonant dipole case, and with a modified lifetime, due to the interactions between dipoles. Of particular interest is the kagome lattice, where the semiregular geometry permits simultaneous excitation of two dominant modes, one strongly subradiant, leading to an electromagnetically induced transparencylike interference in a two-level system. The interfering modes are associated with ferroelectric and antiferroelectric ordering in alternate lattice rows with long-range interactions.

Comparison of Lanthanum and Bismuth modification of lead Zirconate-Lead Titanate PZT—A structural and dielectric study

Lead Zirconate Titanate (PZT) ceramics Pb1−1.5xBixVPb′′0.5xZr0.90Ti0.10O3 containing Bismuth (x=0.005–0.12) were structurally and dielectrically characterized. Results are compared to Lanthanum modified Pb1−1.5xLaxVPb′′0.5xZr0.90Ti0.10O3, (x=0.01–0.08) ceramics. For Bi a solubility limit is observed at x=0.04. The Lanthanum-doped series proved to be single phase in the investigated range. Lanthanum doping leads to a continuous reduction of cell volume causing a change from rhombohedral to orthorhombic symmetry at x=0.03 accompanied by switching from ferroelectric to antiferroelectric behaviour. Such a change was not observed with Bismuth. This might be due to the fact that a critical cell volume necessary for structural transition could not be achieved within the solubility range.For Lanthanum doped PZT with x=0.03 orthorhombic structure was determined already at room temperature, but with ferroelectric polarization curve, which switched to antiferroelectric polarization at 120°C. This raises new questions on the appearance and stability of antiferroelectric ordering.

Dipolar clusters and ferroelectricity in high Tc superconductors

In this paper, we show that doping of hole charge carriers induces formation of resonance plaquettes (RPs) having electric dipolar moments and fluctuating stripes in cuprates. A single RP is created by many-body interactions between the dopant ion or a charge fluctuation outside and holes inside the CuO plane. In such a process, Coulomb interacting holes in the CuO plane are self-organized into four-particles resonance valence bond plaquettes bound with dopants or polarons located in the spacer layer between CuO planes. Such RPs have ordered and disordered phases. They are ordered into charge density waves (CDW) or stripes only at certain conditions. The lowest energy of the ordered phase corresponds to a local antiferroelectric ordering. The RPs mobility is very low at low temperatures and they are bound into dipole–dipole pairs. Electromagnetic radiation interacts strongly with RPs electric dipoles and when the sample is subjected to it, the mobility changes significantly. This leads to a fractal growth of dipolar RP clusters. The existence of electric dipoles and CDW reveal a series of new phenomena such as ferroelectricity, strong light and microwave absorption and the field induced superconductivity.

Bond network topology and antiferroelectric order in cuprice CuOH.

Using density functional theory (DFT) and a graph theory based approach, we investigated the topology of bond network in CuOH(s) (cuprice) considering only symmetry-distinct structures. In parallel, we conducted the synthesis and X-ray diffraction characterization of the compound and used the combined theoretical-experimental effort to validate the lowest energy structure obtained with DFT. The ground-state structure of CuOH(s) consists of compact trilayers of CuOH connected to each other via hydrogen bonds, where the inner layer of each trilayer is composed entirely of Cu atoms. Each trilayer is a dense fabric made of two interlocked arrays of polymer [CuOH]n chains. This structure corresponds to an antiferroelectric configuration where the dipole moments of CuOH molecules belonging to adjacent arrays are antiparallel and are arranged in the same way as the water molecules in ice-VIII. It is shown that a collective electrostatic interaction is the main driving force for the cation ordering while the local atomic configuration is maintained. These findings and the possibility of synthesizing exfoliated two-dimensional cuprice are important for some technological applications.

Double hysteresis loop in BaTi1−xHfxO3 ferroelectric ceramics

Hafnium doped barium titanate ceramics were prepared by the conventional solid-state reaction method. The structural, microstructure and ferroelectric properties were analyzed with varying Hf concentration. It is found that the crystal structure of the solid solution remains in tetragonal phase with the decrease of tetragonality by increasing of Hf ion concentration. The grain growth habit is significantly depending upon Hf ion concentration. The remnant polarization and coercive field is decreased along with narrowing of the central portion of the ferroelectric hysteresis loop with the increasing of Hf ion concentration. The double hysteresis loops are associated with the onset of an antiferroelectric order and it is increased with increasing of Hf ion concentration.

What Are ThresholdLess AntiFerroelectric (TLAF) LCs?–Disordered SmC*-Like Phase with qT = 1/2 in a Wide Temperature Range

In the MC881-MC452 binary mixture system developed by MGC, the near-IR Bragg reflection peak is observed in a wide temperature range from 80°C down to room temperature or lower for a narrow concentration region between 54.99 and 57.50 wt.%; the phase responsible for this near-IR Bragg reflection peak can be identified as disordered SmC*-like phase with qT = 1/2 – hereafter designated as SmC*D(1/2) – which has the TLAF property in the bulk. Four intriguing mechanisms are operative: (1) The critical concentration exists, above which the free energy of SmC*A never becomes lower than that of SmC*; (2) The disclination-assisted thermal linkages assure the thermal equilibrium between ferroelectric and antiferroelectric orderings; (3) The long-range interlayer interactions in the quasi-molecular model stabilize the qT=1/2 state in a wide temperature range; and (4) A ferroelectric hexatic phase exists on the low temperature side of SmC*A and the frustration between them stabilizes another qT=1/2 state which may coalesce into the ordinary one in the intermediate concentration region. In this way coalesced SmC*D(1/2) emerges stably. Because of the disclination-assisted thermal linkages, SmC*D(1/2) must be disordered and soft with respect to the tilting directions and senses of the in-layer directors even in the bulk; electric-field-induced switching is expected to occur layer by layer but not cooperatively and hence shows the TLAF property.

Electric Field Induced Transitions in Polar Liquid Crystals with Frustrating Interlayer Interaction

Using Landau theory of phase transitions, behaviour of antiferroelectric liquid crystal and first order transition to the nonpolar SmA phase were studied in electric field. Influence of frustrating interaction on structure above the temperature of the transition into a nonpolar phase was investigated. We have calculated an electric field - temperature phase diagram, including structures with spatial variation of both the phase and the modulus of the order parameter.

Electric-field-induced local structural phenomena in Pb-based ABO3-type relaxor ferroelectrics.

Lead-based ABO3-type relaxors and related systems have numerous applications in modern technical devices because of their remarkably high dielectric permittivity and piezoelectric/electroelastic and electro-optic coefficients. However, lead is not desired from an environmental point of view, and to switch to alternative alkali-, Ba-, or Bi-based relaxor systems, one must understand in great detail the structural mesoscopic order and coupling processes responsible for the outstanding performance and multifunctionality of the exemplar Pb-based compounds. To elucidate the type of ferroic coupling, three relaxor compounds PbSc0.5Ta0.5O3 (PST), Pb0.78Ba0.22Sc0.5Ta0.5O3 (PST-Ba), and PbSc0.5Nb0.5O3 (PSN), were studied by polarized Raman scattering and acoustic emission at different temperatures and under an external electric field. The results reveal the coexistence of mesoscopic-scale ferroelectric and antiferroelectric coupling, which are predominantly related to B-site cations and A-site Pb cations, respectively. This suggests that the polar structural state of relaxors is frustrated ferrielectric. The presence of A-site cations with affinity to off-center is significant for the development of mesoscopic-scale antiferroelectric order coexisting with the mesoscopic-scale ferroelectric order.

High-temperature ferroelectricity and strong magnetoelectric effects in a hybrid organic-inorganic perovskite framework

The coexistence of ferroic ordering such as magnetic and ferroelectric or antiferroelectric or-dering, called multiferroicity, is a rare phenomenon in sin-gle-phase materials [1–5]. In addition to coexistence, their cross coupling known as magnetoelectric (ME) coupling effects, provide great opportunities for the mutual control of magnetism by electric fields and electric polarization by magnetic fields. While the majority of multiferroic materi-als are inorganic compounds and composites, there are cur-rently increasing interests in searching multiferroics in or-ganic materials. Compared to inorganic compounds, the high tunability of organic materials in building blocks and molecular functionalities may open up new routes to lead-free compounds exhibiting strong polarization. One possi-ble route toward organic multiferroics is to combine the inorganic magnetism from metal ions and organic ferro-electricity into a single structure. The hybrid organic–inorganic materials such as metal-organic frameworks (MOFs) represent a promising candidate of such a strategy. The MOFs are ordered crystalline structures composed of inorganic metal ions and organic linkers [6–10]. They have been known to show diverse chemical and physical properties arising from the organic–inorganic duality [11–16]. Especially, dense MOFs with the ABX

Phase diagram and incommensurate antiferroelectric structure in (Pb1−1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 ceramics discovered by band-to-band optical transitions

Optical properties and phase transitions of (Pb1−1.5xLax)(Zr0.42Sn0.40Ti0.18)O3 (PLZST 100x/42/40/18) ceramics with different compositions have been investigated by temperature dependent spectroscopic ellipsometry. Two interband critical points (Ecp1 and Ecp2) located at about 3.9 and 5.1 eV can be obtained by fitting standard line shapes to the second derivatives of the complex dielectric functions. Based on the band-to-band transitions, the phase diagram of PLZST ceramics can be well presented. Moreover, a peculiar incommensurate antiferroelectric state has been found to exist above the temperature of the normal commensurate antiferroelectric tetragonal structure. It can be stable below Curie temperature, evolving slowly with decreasing temperature towards the commensurate structure, which is due to strong pinning of incommensurate domain walls. The phenomena can result from a competition between ferroelectric ordering and antiferroelectric ordering caused by the lanthanum modification.