The direct conversion of electrical energy to mechanical work by a material is relevant to a number of applications. This is illustrated by ferroelectric ‘relaxors’ such as Pb(Mg1/3Nb2/3)O3-PbTiO3 ...(PMN-PT; refs 5, 6): these materials exhibit a giant electromechanical (piezoelectric) response that is finding use in ultrasonic and medical applications, as well as in telecommunications. The origins of this effect are, however, still unclear. Here we show that the giant electromechanical response in PMN-PT (and potentially other ferroelectric relaxors) is the manifestation of critical points that define a line in the phase diagram of this system. Specifically, in the electric-field–temperature–composition phase diagram of PMN-PT (the composition being varied by changing the PT concentration), a first-order paraelectric–ferroelectric phase transition terminates in a line of critical points where the piezoelectric coefficient is maximum. Above this line, supercritical evolution is observed. On approaching the critical point, both the energy cost and the electric field necessary to induce ferroelectric polarization rotations decrease significantly, thus explaining the giant electromechanical response of these relaxors.
Intermolecular hydrogen bonds impede long-range (anti-)ferroelectric order of water. We confine H
O molecules in nanosized cages formed by ions of a dielectric crystal. Arranging them in channels at ...a distance of ~5 Å with an interchannel separation of ~10 Å prevents the formation of hydrogen networks while electric dipole-dipole interactions remain effective. Here, we present measurements of the temperature-dependent dielectric permittivity, pyrocurrent, electric polarization and specific heat that indicate an order-disorder ferroelectric phase transition at T
≈ 3 K in the water dipolar lattice. Ab initio molecular dynamics and classical Monte Carlo simulations reveal that at low temperatures the water molecules form ferroelectric domains in the ab-plane that order antiferroelectrically along the channel direction. This way we achieve the long-standing goal of arranging water molecules in polar order. This is not only of high relevance in various natural systems but might open an avenue towards future applications in biocompatible nanoelectronics.
Far-infrared and terahertz spectroscopy of ferroelectric soft and central modes in thin films on substrates is reviewed. In addition to classical displacive proper ferroelectrics, also incipient and ...relaxor ferroelectrics and multiferroics are discussed. Special attention is paid to differences between the soft-mode behavior in thin films and bulk materials (ceramics and single crystals) and their influence on the low-frequency permittivity. Particularly the effects of the thin film strains and depolarizing electric fields of the probing waves on the grain boundaries are emphasized. The soft-mode spectroscopy is shown to be a very sensitive tool to reveal the thin film quality.
Water is characterized by large molecular electric dipole moments and strong interactions between molecules; however, hydrogen bonds screen the dipole-dipole coupling and suppress the ferroelectric ...order. The situation changes drastically when water is confined: in this case ordering of the molecular dipoles has been predicted, but never unambiguously detected experimentally. In the present study we place separate H
O molecules in the structural channels of a beryl single crystal so that they are located far enough to prevent hydrogen bonding, but close enough to keep the dipole-dipole interaction, resulting in incipient ferroelectricity in the water molecular subsystem. We observe a ferroelectric soft mode that causes Curie-Weiss behaviour of the static permittivity, which saturates below 10 K due to quantum fluctuations. The ferroelectricity of water molecules may play a key role in the functioning of biological systems and find applications in fuel and memory cells, light emitters and other nanoscale electronic devices.
Giant permittivity (GP) materials are of interest as possible capacitors for energy storage. Ferroelectric and related homogeneous materials suffer from strongly temperature dependent dielectric ...properties. Therefore, within the last two decades, the interest has been focused to dielectric studies of various inhomogeneous GP materials, where the essential role was shown to be due to weak but inhomogeneous conductivity. Here we study, within the known models based on effective medium approximation, the broadband dielectric spectra of conducting core - less conducting shell structures, which are expected to be the most advantageous for revealing the GP effects. Examples of core-shell structures, in which the dielectric properties of both components are independent of frequency, are studied analytically and numerically within the generalized brick model. For the simple case of coated-spheres model the dielectric dispersion is of a single Debye-relaxation type and the optimal GP properties are for highly conducting cores and thinnest possible nonconducting and high-permittivity shells. For non-spherical core-shell particles the conditions for macroscopically isotropic composite are briefly discussed.
The dielectric response to infrared waves polarized along the tetragonal axis of a ferroelectric single-domain crystal of BaTiO3 was determined by time-domain THz spectroscopy and Fourier-transform ...infrared reflectivity techniques. In addition to the three well-known polar lattice modes, the experiment shows an additional mode of the relaxation type in the THz spectral region, which accounts for the Curie-Weiss behavior of the c-axis dielectric constant. A comparison of experimental results with ab initio based effective-Hamiltonian simulations allows us to elucidate its relation to the order-disorder model of Comes, Lambert, and Guinier Solid State Commun. 6, 715 (1968)10.1016/0038-1098(68)90571-1.
Fano resonance is a phenomenon in which a discrete state interferes with a continuum of states and has been observed in many areas of science. Here, we report on the prediction of a Fano resonance in ...ferroelectric relaxors, whose properties are poorly understood: an ab initio molecular dynamic scheme reveals such resonance between the bare optical phonon mode of the Zr sublattice (the discrete state) and the bare optical phonon mode of the Ti sublattice (the continuum of states) in disordered lead-free Ba(Zr,Ti)O3. The microscopic origins of the discrete state and continuum of states are discussed in the context of relaxor properties. Furthermore, our simulations suggest that the T* characteristic temperature of relaxor is related to a hardening of the vibrational frequencies associated with fluctuation of the Ti sublattice. Finally, a terahertz relaxation mode reflecting reorientations of Ti dipoles and showing a thermally activated behaviour is predicted, in agreement with previous experiments.
In this paper, we discuss broadband dielectric spectroscopy from mHz up to the infrared range mainly for materials with inhomogeneous weak conductivity, including conductor-dielectric nanocomposites. ...Our discussion is based on the effective medium approximation (EMA) and experiments modeled by this approach are reviewed. We discuss core-shell composites modeled by coated-spheres (Hashin-Shtrikman model) and normal composites with a possible percolation of the conductor component resulting in sharp or smeared percolation threshold of the DC conductivity and diverging static permittivity in the former case. The sharp percolation threshold is modeled by the Bruggeman EMA or by general EMA with arbitrary percolation threshold and arbitrary critical exponents of the DC conductivity and static permittivity. For the case of smeared percolation threshold in the case of complex topologies, we use the Lichtenecker model allowing for partial percolation of both the components. Finally, numerous papers reporting negative permittivity in weakly conducting materials are criticized and concluded to be due to spurious effects.
Dense nanocrystalline BaTiO
3 ceramics with grain size (GS) down to 50
nm were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), impedance spectroscopy and Raman ...spectroscopy. A continuous reduction of the tetragonal distortion towards the pseudo-cubic state was obtained when the GS was reduced. Therefore, even the finest structure (ceramic with average GS of 50
nm) is still non-centrosymmetric. The dielectric constant (
K) shows relative thermal stability in a large range of temperatures and is strongly depressed in the nanocrystalline ceramics, in comparison with the micrometric ones (
K being below 1000 for the ceramic with 50
nm GS). The losses are smaller than 5% in the frequency range of 10
2–10
6
Hz and temperatures below 200
°C. As the GS decreases, the structural phase transitions assume a more diffuse character. A decrease of the Curie temperature with reducing the GS was confirmed by X-ray, calorimetric and permittivity data. The Raman spectra collected for the range 80–800
K provided evidence for the presence of all the crystalline phases of BaTiO
3, as in single-crystal and micrometric ceramics; a few differences can be attributed to GS effects and to the high density of the non-ferroelectric grain boundaries. Evidence for the different phase transitions were provided by the disappearance of some bands and by anomalies in positions and intensities of selected Raman modes. The overall properties of the nanocrystalline BaTiO
3 ceramics can be explained as a combination of intrinsic effects, associated with the decrease of tetragonality and heat of transition with reducing GS, and extrinsic contributions due to the non-ferroelectric grain boundaries causing a “dilution” of the ferroelectric properties.
Searching for giant dipolar defects, suggested as the explanation of the colossal permittivity (CP) in (Nb + In) co-doped rutile, we have recently published (J. Appl. Phys. 119, 154105 (2016) and ...Phys. Rev. Mat., in press) temperature-dependent dielectric spectra from sub-Hz to THz range (including 4-point DC conductivity) of several (Nb + In) co-doped rutile ceramics. Using the model of two leaky capacitor in series below ∼1 MHz, we estimated the depletion layer thickness and conductivity. The spectra below ∼10 GHz were fitted with two Cole-Cole relaxations obeying the Arrhenius law, assigned to the effect of depletion layers and grain boundaries. In this paper, using effective medium models, we have for the first time modelled the spectra as a composite of semiconducting grains and substantially less-conducting grain boundaries and near-electrode depletion layers. The CP effect has been fully explained by the combination of surface and internal barrier-layer capacitor effects, without any measurable contribution from the dipolar defects.
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