Displacive-type Ferroelectrics Research Articles

Ultrafast Photoinduced Electric-Polarization Switching in a Hydrogen-Bonded Ferroelectric Crystal.

Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching 20 μC/cm^{2}, which originates from the strong coupling of proton and π-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.

Critical scaling analysis for displacive-type organic ferroelectrics around ferroelectric transition

The critical scaling properties of displacive-type organic ferroelectrics, in which the ferroelectric–paraelectric transition is induced by spin-Peierls instability, are investigated by Green’s function theory through the modified Arrott plot, critical isothermal and electrocaloric effect (ECE) analysis around the transition temperature TC. It is shown that the electric entropy change −ΔS follows a power-law dependence of electric field E:−ΔS∼En with n satisfying the Franco equation n(TC)=1+(β−1)/(β+γ)=0.618, wherein the obtained critical exponents β=0.440 and γ=1.030 are not only corroborated by Kouvel–Fisher method, but also confirm the Widom critical relation δ=1+γ/β. The self-consistency and reliability of the obtained critical exponents are further verified by the scaling equations. Additionally, a universal curve of −ΔS is constructed with rescaling temperature and electric field, so that one can extrapolate the ECE in a certain temperature and electric field range, which would be helpful in designing controlled electric refrigeration devices.

Kinetics of nanosize ferroelectrics

We analyze theoretically the finite-temperature polarization dynamic in displacive-type ferroelectrics. In particular we consider the thermally-activated switching time of a single-domain ferroelectric polarization studied by means of the Landau-Khalatnikov equation, extended as to capture thermal fluctuations. The results are compared with the switching time formula that follows from the analytical solution of Pauli master equations. In a second step we focus on the phase diagram of a prototypical ferroelectric as described by the temperature-dependent Landau-Devonshire model including thermal fluctuations. Our simulations show the emergence of phase instability at reduced sizes which we attribute to thermal fluctuations in the order parameter in the respective phase. We conclude that, along with the temperature-dependent potential coefficients, thermal fluctuations should be taken into account to achieve a comprehensive description of the thermal behavior of reduced-size ferroelectrics. To endorse our conclusions, we simulated the electric-field activated switching time for a multi-domain system and compared the results to the predictions of well-established models such as the Kolmogorov-Avrami-Ishibashi.

Enhanced electrocaloric effect in displacive-type organic ferroelectrics

We explore the intrinsic feature of electrocaloric effect (ECE) accompanied by ferroelectric (FE)-paraelectric (PE) transition for displacive-type organic ferroelectrics using Green's function theory. It is demonstrated that decreasing elastic constant K or increasing spin-lattice coupling λ can enhance the ECE, as well as polarization P and transition temperature TC. Indeed, one expects that the optimal operating temperature for solid-state refrigeration is around room temperature, at which the ECE achieves its maximum. As TC is tuned to ∼310 K, it presents larger ECE response and remanent polarization with lower coercive field for smaller K value, suggesting that well flexible displacive-type organic ferroelectrics are excellent candidates both for electric cooling and data storage in the design of nonvolatile FE random-access memories. Furthermore, in an electric field, it provides a bridge between a Widom line that denotes FE-PE crossover above TC and a metaelectric transition line below TC that demonstrates an FE switching behavior with an antiparallel field.

Ab Initio calculations of the lattice dynamics and the ferroelectric instability of the BiFeO3 multiferroic

The pressure dependences of the vibration frequencies of the bismuth ferrite crystal lattice in the ferroelectric and paraelectric rhombohedral phases have been calculated from first principles in terms of the LSDA + U approximation. It has been found that there is a linear relationship between the magnetic moments of iron cations and the band gaps calculated using the same parameters of the correlation interaction. An analysis of the pressure dependences of the frequencies of the transverse optical phonons indicates their high stability in both the paraelectric and ferroelectric phases, which is not typical for classical displacive-type ferroelectrics.

Displacive-type ferroelectricity from magnetic correlations within spin-chain.

Observation of ferroelectricity among non-d0 systems, which was believed for a long time an unrealistic concept, led to various proposals for the mechanisms to explain the same (i.e. magnetically induced ferroelectricity) during last decade. Here, we provide support for ferroelectricity of a displacive-type possibly involving magnetic ions due to short-range magnetic correlations within a spin-chain, through the demonstration of magnetoelectric coupling in a Haldane spin-chain compound Er2BaNiO5 well above its Néel temperature of (TN = ) 32 K. There is a distinct evidence for electric polarization setting in near 60 K around which there is an evidence for short-range magnetic correlations from other experimental methods. Raman studies also establish a softening of phonon modes in the same temperature (T) range and T-dependent x-ray diffraction (XRD) patterns also reveal lattice parameters anomalies. Density-functional theory based calculations establish a displacive component (similar to d0-ness) as the root-cause of ferroelectricity from (magnetic) NiO6 chain, thereby offering a new route to search for similar materials near room temperature to enable applications.

Neutral-Ionic Transition, Ferroelectricity, and Field-Effect Transistors Based on Molecular Donor-Acceptor Compounds

Versatile π-electronic functionality enabled by application of molecular donor(D)-acceptor(A) compounds are overviewed. These compounds are highlighted with respect to the quantum neutral-ionic phase transition, relaxor, displacive-type ferroelectricity, and also to the use in interface engineering of organic field-effect transistors (FETs).

Molecular Donor–Acceptor Compounds as Prospective Organic Electronics Materials

The π-electronic functionalities of molecular donor ( D )–acceptor ( A ) compounds have been attracting subjects for the solid state science as well as for the prospective technological exploitation in organic electronics. In this review, it is shown that the novel neutral–ionic valence instability in the charge-transfer (CT) complexes is closely related to valuable electric properties such as the current-induced resistance switching, quantum phase transition, gigantic dielectric response, and relaxor ferroelectricity. Furthermore, the displacive-type ferroelectricity has recently been developed on the D A combinations in the hydrogen-bonded co-crystals. It is also discussed that the intermolecular CT mechanism in the molecular D A compounds is promising to advance the potential of organic field-effect transistors (FETs) that are envisioned as key components in future organic electronics.

The Microstructural, Dielectric, Pyroelectric and Ferroelectric Properties of SrBi2(Nb1-xVx)2O9(0 ≤ x ≤ 0.3) Ceramics

Layered SrBi2(Nb1−x V x )2O9 (SBVN) ceramics with x ranging from 0 to 0.3 (30 mol%) were fabricated via the conventional sintering technique. A remarkable transformation in the microstructure of SBN ceramic was encountered as the V2O5 content increased from 3–30 mol%. The average grain size of the SBN ceramic containing V2O5 was found to increase with increase in V2O5 content. The dielectric constant (ϵ r ) as well as the dielectric loss (D) increased with increase in grain size (6 μm–17 μm). The ferroelectric to paraelectric transition temperature (T c ) shifted to higher temperature with increasing grain size. The pyroelectric coefficient was positive at 300 K and increased with increasing grain size. Interestingly, the SrBi2(Nb0.7V0.3)2O9 (x = 0.3) ceramics with a grain size of 17 μm exhibited higher remnant polarization (P r ) and lower coercive field (E c ) than those with grains of 7 μm. The Curie-Weiss constant that was obtained for these ceramics was in line with that of displacive type ferroelectrics.

Size driven phase transition of barium titanate nanoparticles prepared by plasma chemical vapor deposition

Size driven phase transition of barium titanate nanoparticles prepared by plasma chemical vapor deposition

Calculation of effective charges of ions in displacive-type ferroelectrics

The procedure of calculating the effective charges of ions in ABO3-type ferroelectrics is considered, which is based on the comparison of two experimentally determined quantities: spontaneous polarization in a crystal and the parameters of the tensor of an electric-field gradient at the quadrupolar nuclei of A ions calculated based on the classical electrostatic model. The determined effective charges of ions in an LiNbO3 crystal are considered.

Optimization of the calculation of a local electric field in crystals by the transient-region method

The specific features of using the method of transient-region for calculating local electric fields and the corresponding potentials are considered on examples of the calculations for NaF, CsCl, and LiNbO3 crystals. It is shown that for crystals with a primitive cubic lattice, this method can provide any given accuracy of the calculated Madelung constant; for displacive-type ferroelectrics (LiNbO3), the method allows the calculation of the local electric field with a very high accuracy. It is emphasized that for each specific object, one has to carefully select the parameters of the convergence function.

Direct evidence ofA-site-deficient strontium bismuth tantalate and its enhanced ferroelectric properties

The Rietveld refinement of synchrotron-radiation diffraction has been performed to investigate the displacive-type ferroelectricity and crystal chemistry for Sr-deficient and Bi-excess strontium bismuth tantalate. The analysis of ${\mathrm{Sr}}_{0.73}{\mathrm{Bi}}_{2.18}{\mathrm{Ta}}_{2}{\mathrm{O}}_{9}$ demonstrates that excess Bi $({\mathrm{Bi}}_{0.18})$ is substituted at the perovskite A site (Sr site) as trivalent ions and that the charge neutrality in the crystal is satisfied through the introduction of cation vacancies at the A site. The substitution of Bi with the A-site vacancies led to a marked increase in Curie temperature ${(T}_{C}),$ and resulted in a twofold remanent polarization without sacrificing the polarization switching properties. The structure refinement revealed that the rotation of ${\mathrm{TaO}}_{6}$ octahedra in the $a\ensuremath{-}b$ plane accompanied with the whole shift of the octahedra along the a axis is largely enhanced by the Bi substitution with A-site vacancies, which is responsible for the higher ${T}_{C}$ and giant polarization.

Preparation, structural characterization and ferroelectric properties of compensated Te-doped n=2 Aurivillius oxides ceramics

New solid solutions Bi 2− x Te x SrNb 2− x B x O 9 (B=Zr, Hf; 0≤ x≤0.5) and Bi 2− y Te y Sr 1− y A y Nb 2O 9 (A=K; 0≤ y≤0.25) have been synthesized and characterized by powder X-ray diffraction methods, thermal analysis and dielectric measurements. Evolution of the unit-cell parameters at room temperature shows that doping the A or B perovskite sites dominates the size of (001) plane in the structure, whereas the c value is largely influenced by the cations which occupy the bismuth position in its layer. Thermogravimetric studies show that these materials are stable up to 800°C for x, y=0.25 and 750°C for x=0.5. Differential thermal analysis curves of x=0.25 compounds exhibit one exothermic peak at 400 and 500°C for zirconium and hafnium phases, respectively. This effect is related to the expansion of their network, without changing the symmetry or space group. All compositions are ferroelectric at room temperature, and exhibit a non-zero d 33 value after poling. Measurements of the dielectric permittivity as a function of the temperature reveal signs of at least two possible phase transitions. The disappearance of the induced spontaneous polarization when heating samples above the lower temperature anomaly in the dielectric permittivity clarifies the T c value of the compounds. Assuming they are displacive-type ferroelectrics, it can be shown that the variation of T c with the composition is an effect of the change of the unit cell size. This result offers the possibility of tailoring a ferroelectric material with the desired thermal working range.

Vanishing of order-disorder type phase transition in DKDP at high pressure

Temperature dependences of the dielectric constant along the ferroelectric c-axis have been measured at various pressures up to 7.9 GPa in DKDP. The vanishing of the ferroelectric state has been observed at 6.3 GPa. The Curie constant C has been saturated to about three fourths of that at atmospheric pressure above 4 GPa even if the transition temperature T becomes 0. The vanishing of Tc with finite C is theoretically derived from the temperature dependence of electric polarizability in displacive type ferroelectrics. The crossover of the order-disorder type to the displacive one under high pressure is discussed on the mechanism of phase transition to explain these experimental results in DKDP.

Calculation of local electric fields in displacive-type ferroelectrics: LiNbO3

The method is suggested for calculating the intensity and potential of a local electric field in displacive-type ferroelectrics which is based on the introduction of a transitional zone between the region of the discrete localization of ions and the homogeneously polarized medium. The method is exemplified on the calculation of the contribution of Li+ ions to the local electric field in the lithium niobate structure. It is shown that the method provides fast convergence of the results in the calculation of the field intensity and high relative accuracy of the calculated electric-field potential inside the crystal. The contribution of the induced dipoles to the local electric field is calculated, and it is shown that the stability of the LiNbO3 structure is provided by considerable anisotropy in the polarizability of Nb-O bonds. The method is applicable to any ionic crystal.

Effect of negative chemical pressure on some displacive-type ferroelectrics

The effect of embedded helium on the phase-transition temperature and properties of displacive-type ferroelectrics has been studied. It is shown that helium embedded in crystals produces an effect opposite to that of hydrostatic pressure, namely, the temperatures of the ferroelectric phase transitions increase, and those of the antiferroelectric ones, decrease.

Two dielectric contributions due to domain/cluster structure in the ferroelectrics with diffused phase transitions

The main dielectric contributions and the structure of dielectric spectrum of the ferroelectrics with diffused phase transitions are considered on the base of PbMg1/3Nb2/3O3 (PMN) crystals investigation. Dielectric spectra of the displacive, order-disorder and diffused phase transitions ferroelectrics are compared. Dielectric properties of the PMN are nontypical for displacive-type ferroelectrics and aren't defined mainly by soft phonon mode. The main polarization mechanisms are caused by the thermoactivated dynamics of the metastable polar phase clusters and polar - nonpolar phase boundaries.

Nature of the ferroelectric phase transition in PbTiO3.

We have measured quantitatively the temperature dependence of the local distortions of ${\mathrm{PbTiO}}_{3}$ crystals below and above the structural ferroelectric phase transition, using x-ray absorption fine-structure (XAFS) measurements. Two probe atoms, Pb and Ti, were used. The results were analyzed by fitting parametrized theoretical XAFS spectra to the experimental results. These measurements provide quantitative ``distortion parameters'' defined as the difference between the distance of the probe to its nearest neighbor measured in the actual structure and that in a centrosymmetric structure with the same unit cell dimensions. At low temperatures the Pb-edge spectra were fit using four shells and included single, double, and triple scattering configurations. The high-temperature fits included only two shells. The Ti-edge spectra were fitted with the first shell only. The Pb and Ti distortions vary relatively little with temperature below the transition and decrease faster near the transition temperature. Above the transition temperature, the Pb and Ti distortions at the peaks of their distortion distribution function (DDF's) are both more than 70% of the corresponding low-temperature values. These results show that an essential element of order-disorder is present even in this nominally pure ferroelectric crystal which displays a soft mode and a dielectric constant typical of displacive-type ferroelectrics. The presence of the local distortions suggests that the displacements should have at least two correlation length scales, one associated with the local distortions, the other with the order parameter.

Enhancement of the electron-lattice interaction due to strong electron correlation.

Effects of strong electron correlation on the electron-lattice interaction are studied by means of the exact diagonalization method for a finite-size cluster system. A simple two-component tight-binding Hamiltonian with the intrasite and intersite Coulomb repulsions is adopted. Upon changing the correlation energy (U) and the energy-level separation (\ensuremath{\Delta}), the system is found to undergo a transition between the ionic insulating phase and the Mott insulating phase. Near the phase boundary the system becomes unstable under a small perturbation and a dramatic enhancement of the electron-lattice interaction is observed. Spin and charge properties are also strongly affected by the lattice distortion in a crossover region. The implication of the calculated results to the origins of the displacive-type ferroelectricity and the superconductivity in transition-metal oxides is discussed.