Nanosecond dynamics of ferroelectric/dielectric superlattices

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.     

Unusual thermodynamic properties and nonergodicity in ferroelectric superlattices

The properties of [Pb(Zr(1-x(1))Ti(x(1)))O(3)](n)/[Pb(Zr(1-x(2))Ti(x(2)))O(3)](n) superlattices, with a 2n period, are simulated using an ab initio based approach. The x(1) and x(2) compositions are chosen to be located across the morphotropic phase boundary of the corresponding disordered alloys, while the (x(1)+x(2))/2 average composition lies inside this boundary. These superlattices exhibit an unusual thermodynamic phase transition sequence, including a triclinic ground state. They also have the kind of peculiar free-energy landscape yielding nonergodicity. The effects responsible for these anomalies are discussed.     

Thermodynamic modeling of critical properties of ferroelectric superlattices in nano-scale

Modeling nano-scale ferroelectric superlattices using the Landau free-energy functional approach requires incorporating contributions from the interfacial and depolarization field effects. The choice of the order parameter then becomes a vital issue. In this paper, we compare the predictions of models using the spontaneous polarization as order parameter (SPOP approach) with models using the total polarization as order parameter (TPOP approach). We have comprehensively calculated the critical properties of nano-scale ferroelectric superlattices, such as the phase-transition temperature, critical thickness and Curie–Weiss-type relation using both approaches. We found that all the SPOP results are in excellent agreement with experimental measurements and first-principle calculations in all cases studied here. The TPOP approach, on the other hand, much overestimates the depolarization by underestimating the effect of the dielectric screening and produces results that deviate significantly from the experimental ones. Our results also traced the dependence of the critical properties on the thicknesses of the constituent layers of the ferroelectric superlattices to the interfacial and depolarization field effects.     

Ab initio studies of dielectric,piezoelectric, and elastic properties of BaTiO 3 /SrTiO 3 ferroelectric superlattices

The phonon spectrum; crystal structure of the polar phase; spontaneous polarization; dielectric constant, piezoelectric, and elastic moduli tensors for free_standing and substrate_supported superlattices mBaTiO₃/nSrTiO₃ (with m = n = 1–4) were calculated within the density functional theory. The simulation of properties of the disordered Ba_0.5Sr_0.5TiO₃ solid solution using two special quasirandom SQS-4 structures and their comparison with the properties of the superlattices revealed a tendency of the BaTiO₃-SrTiO₃ system to superstructure ordering and showed that the superlattices are thermodynamically quite stable. The ground state of the free-standing superlattice corresponds to the monoclinic polar phase Cm, which transforms to the tetragonal polar phase P4mm under in-plane compressive strain of the superlattice and to the orthorhombic polar phase Amm2 under in-plane tensile strain. With a change in the in-plane lattice parameter, in the vicinity of boundaries between neighboring polar phases, some optical and acoustic modes soften and some components of the static dielectric constant, piezoelectric, and elastic moduli tensors diverge critically.     

Polarization properties of light reflected from dielectric superlattices

The polarization of the light reflected from a superlattice has been studied theoretically as o function of the angle of incidence and wavelength. The spectral and angular distributions of the ellipticity and azimuth of the polarization of the diffracted light in the vicinity of Bragg resonances have been found. It is shown that near the Brewster angle and on the boundaries of the stophands of the superlattice the effect of an anomalous increase in the ellipticity of the reflected light is manifested. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 2, pp. 199–202, March–April, 2000.     

Dynamical,scattering and dielectric properties of ferroelectric crystals

A unified treatment is given of various properties of ferroelectric crystals in their non-polar phases, typified by BaTiO₃, KH₂PO₄ and NaNO₂. Certain relations are established between measurable dielectric and scattering properties. Critical scattering is discussed in terms of lattice dynamics.     

Correlation of the piezoelectric and dielectric properties of soft ferroelectric ceramics

The dependences of the piezoelectric (g ₃₃) and electrostriction (Q ₃₃) coefficients of nonpolarized soft relaxor ferroelectric ceramics of the PbMg_1/3Nb_2/3O₃-PbTiO₃ system on electric field E ₃ are calculated and interpreted. The nonmonotonic g ₃₃(E ₃) dependence and negative values of Q ₃₃ were revealed in strong electric fields. These features are shown to be the manifestation of correlation between the piezoelectric and dielectric properties.     

Ground state and properties of ferroelectric superlattices based on crystals of the perovskite family

The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO₃/SrTiO₃, PbTiO₃/SrTiO₃, PbTiO₃/PbZrO₃, SrZrO₃/SrTiO₃, PbZrO₃/BaZrO₃, BaTiO₃/BaZrO₃, PbTiO₃/BaTiO₃, BaTiO₃/CaTiO₃, KNbO₃/KTaO₃, and KNbO₃/NaNbO₃) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.     

Domain-enhanced interlayer coupling in ferroelectric/paraelectric superlattices

We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional, we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength Lambda and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.     

Second harmonic generation in generalized Thue-Morse ferroelectric superlattices

In this paper the second harmonic generation (SHG) in generalized Thue-Morse (GTM(m, n)) ferroelectric superlattices is studied. Under the small-signal approximation, the SHG spectra in both real and reciprocal spaces are investigated. It is found that: (1) only when the structure parameters l, l_A, and l_B are all chosen to be proper, can SHG in GTM(m, n) ferroelectric superlattices be generated; (2) for Family A of generalized Thue-Morse, GTM(m, 1) ferroelectric systems, with the increase of parameter m, the intense peaks of SHG concentrate on the long wavelength (the fundamental beam (FB) wavelength is within ), but for Family B of generalized Thue-Morse, GTM(1, n) ferroelectric superlattices, with the increase of parameter n, the intense peaks of SHG concentrate on the middle wavelength ; and (3) for GTM(m, 1) ferroelectric superlattices, the bigger the m, the stronger the relative integral intensity (RII) of SHG would be, but for GTM(1, n) ferroelectric systems, the bigger the n, the weaker the RII of SHG would be.     

Enhanced dielectric properties and energy storage density of interface controlled ferroelectric BCZT-epoxy nanocomposites

Polymer nanocomposites with ferroelectric fillers are promising materials for modern power electronics that include energy storage devices. Ferroelectric filler, Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) nanopowder, was synthesized by sol-gel method. X-ray diffraction (XRD) studies confirmed the phase purity and the particle size distribution was determined by transmission electron microscopy (TEM). Extended aromatic ligand in the form of naphthyl phosphate (NPh) was chosen for surface passivation of BCZT nanoparticles. Surface functionalization was validated by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and impedance spectroscopy using slurry technique. The dielectric constant of surface-passivated BCZT nanopowder was ~155, whereas pristine BCZT nanopowder dielectric constant could not be assessed due to high innate surface conductivity. Furthermore, BCZT–epoxy nanocomposite films were prepared and analyzed by differential scanning calorimetry (DSC), dielectric spectroscopy, dielectric breakdown strength (DBS), and scanning electron microscopy (SEM). Owning to stronger polymer–particle interface, dielectric measurements of 5 vol.% NPh surface functionalized BCZT–epoxy nanocomposites indicated improved DBS and glass transition temperature (T_g), reduced dielectric loss, and enhanced energy storage density compared to untreated BCZT–epoxy composites and pure epoxy. The energy storage density of 30 vol.% NPh surface functionalized BCZT–epoxy nanocomposite of 20 μm film thickness was almost three times that of pure epoxy polymer of identical film thickness.     

Pressure dependence of dielectric properties of the LiNaGe4O9 ferroelectric

Dielectric properties of lithium–sodium–tetragermanate (LNG) LiNaGe₄O₉ crystal were investigated under hydrostatic pressure. The phase diagram and phase transition temperature as a function of pressure was constructed. Up to 200 MPa the dependence of T_C on pressure is linear with negative slope of 8.5 K/GPa. The influence of pressure on the Curie–Weiss law was observed. It was also shown that with increasing pressure the maximum value of the electric permittivity and Curie–Weiss constant are decreasing. The obtained results revealed complex mechanism of phase transition in the ferroelectric LNG.     

Nonlinear dielectric properties of planar structures based on ferroelectric betaine phosphite films

Ferroelectric films of partly deuterated betaine phosphite are grown on NdGaO₃(001) substrates with an interdigitated system of electrodes on their surfaces by evaporation at room temperature. These films have a high capacitance in the ferroelectric phase transition range. The dielectric nonlinearity of the grown structures is studied in small-signal and strong-signal response modes and in the intermediate region between these two modes by measuring the capacitance in a dc bias field, dielectric hysteresis loops, and the Fourier spectra of an output signal in the Sawyer-Tower circuit. In the phase transition range, the capacitance control ratio at a bias voltage U _bias = 40 V is K ? 7. The dielectric nonlinearity of the structures in the paraelectric phase is described by the Landau theory of second-order phase transitions. The additional contribution to the nonlinearity in the ferroelectric phase is related to the motion of domain walls and manifests itself when the input signal amplitude is higher than U _st ～ 0.7–1.0 V. The relaxation times of domain walls are determined from an analysis of the frequency dependences of the dielectric hysteresis.     

Effect of Ni doping on ferroelectric and dielectric properties of strontium barium niobate crystals

The influence of Ni doping on the ferroelectric and dielectric properties have been examined in Sr_0.61Ba_0.39Nb₂O₆ (SBN:61) relaxor crystals. The dopants introduced into SBN:61 crystals promote the switching process by reducing the value of threshold nucleation field, and thus coercive field. We present real-time studies of domain nucleation and growth processes in doped SBN:61 by the nematic liquid crystal (NLC) decoration technique. The broad phase transition and low-frequency dielectric dispersion that are exhibited by doped SBN:61 samples have a strong link to the configuration of the ferroelectrics microdomains, which in turn is strongly determined by Ni ions concentration.     

Divide line between relaxor,diffused ferroelectric,ferroelectric and dielectric

The dielectric spectroscopic studies of relaxor ferroelectric Ba(Ti_0.70Sn_0.30)O₃ (BTS30), diffused ferroelectric Ba(Ti_0.70Sn_0.20)O₃ (BTS20), classical ferroelectric BaTiO₃ (BT0), and normal dielectric Ba(Fe_0.50Ta_0.50O₃) (BFT) were carried out in the temperatures ～100 K–650 K, and frequencies ～100 Hz–1 MHz, which showed relaxor behavior at 130 K with typical 30 K frequency dispersion, perfect diffused ferroelectric character at 190 K, classical ferroelectric, and dielectric relaxation at 450 K–550 K respectively. A divide line was sketched between the relaxors, ferroelectrics and normal dielectrics based on the Maxwell–Wagner space charge model. The impedance spectroscopy and ac conductivity spectroscopy were carried out to explain the relaxor and pseudo relaxor behavior.     

Oxygen vacancy effect on dielectric and hysteretic properties of zigzag ferroelectric iron dioxide nanoribbon

Zigzag FeO₂ nanoribbon defected by the removal of oxygen atoms is simulated using Monte Carlo simulations. All possible arrangements of positions and number of oxygen vacancy are investigated. Temperature dependence of polarization, dielectric susceptibility, internal energy, specific heat and dielectric hysteresis loops are all studied. Results show the presence of second order phase transition and $Q-$ type behavior. Dielectric properties dependence on ribbon's edge, positions and number of oxygen vacancy are discussed in detail. Moreover, single and square hysteresis loops are observed whatever the number of oxygen vacancy in the system.     

Optical activity of dielectric superlattices with defects

Natural optical activity of a nonideal 1D multilayer system is considered phenomenologically and the concentration dependence of its specific rotation angle is simulated numerically. As a model system, the two-sublattice SiO₂ LC structure was chosen. Specific gyrotropy features caused by the corresponding disordering types of the studied systems were revealed. Based on the developed phenomenological theory, the frequency dependence of the specific rotation angle of the polarization plane of linearly polarized light was studied for the case of a molecular-crystal multilayer system whose layers contain pointlike defects. This creates additional possibilities for simulating optically active multilayer composite materials.     

Optical properties of thin metallic film/dielectric superlattices constrained by the size effect

The dispersion relation of the 1D Bloch wave vector accompanied with an incident transverse electromagnetic wave on thin metallic film/dielectric superlattices is crucial to its optical properties. The interference of excited longitudinal plasmons between neighbouring metallic films induces a collective oscillation of the whole superlattice causing coupled plasmon polariton waves. A calculation of the optical reflectance manipulated by the size effect on the dielectric constant induces more allowable bands at < as the specularity parameters p and q decrease, meanwhile the cut-off frequency for band rejection also shows a blue shift.