Polarization mechanism of ferroelectric lattice instability in crystals

It is shown that the intercell dipole-dipole (polarization) interaction lowers the frequencies of the polar transverse-optical (TO) lattice vibrational modes (to a greater extent, the larger the corresponding dipole oscillator strengths) and can be a cause of ferroelectric instability in crystals. A model-independent method for determining the frequencies, eigenvectors, and dipole oscillator strengths of all the polar TO modes for a crystal with a suppressed polarization interaction from the known values of these quantities for the corresponding real crystal is proposed. The results of the corresponding calculations for several compounds with perovskite structure (SrTiO₃, BaTiO₃, KTaO₃, KNbO₃, and KCoF₃) and rutile structure (TiO₂, SnO₂, SiO₂, CoF₂, and NiF₂) are presented, and the nature of the ferroelectric soft mode in oxides with crystal structures of these two types is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 687–693 (April 1997)     

Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals

The specific features of diffuse X-ray scattering in BaTiO₃, KNbO₃, and PbTiP₃ perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO₃. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO₃ and KNbO₃ crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO₃, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO₃, KNbO₃, and PbTiO₃ are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.     

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.     

The lifetime of the “ferroelectric” phonon in some perovskite crystals at low temperatures

The lifetime of a low-lying transverse optic phonon with the wave vectork=0 (ferroelectric phonon) in perovskite crystals having the Curie point at low temperatures is investigated. Using a phenomenological Hamiltonian it is shown that mainly the process, in which the ferroelectric phonon and the transverse acoustic phonon are destroyed in order to create another optic phonon from the same branch, determines the lifetime of a ferroelectric phonon. The general result is specialized for SrTiO₃, and the temperature dependence of, exhibiting anomalies in comparison with normal ionic crystals, is discussed.The author thanks Dr. M. Marvan of the Physical Faculty of Charles University in Prague and Dr. P. Glogar and Dr. M. Trlifaj of the Institute of Physics, Czechoslovak Academy of Sciences, in Prague for valuable remarks.     

Mechanoluminescence of ferroelectric crystals

Mechanoluminescence spectra of triglycine sulfate, guanidine aluminium sulfate, rochelle salt, diglycine manganous chloride dihydrate, lithium ammonium tartate monohydrate and potassium dihydrogen phosphate crystals resemble the discharge spectra of nitrogen gases. Glycine silver nitrate, thiourea, ammonium sulfate and azobenzene crystals, the structure of which is centrosymmetric at room temperature, do not show mechanoluminescence. Following the role of crystal structure, the mechanoluminescence excitation is discussed on the basis of the piezo-electrification of the new surfaces created during fracture of the crystals.     

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.     

Nonmonotonic TC trends in bi-based ferroelectric perovskite solid solutions

We use first-principles density functional theory calculations to investigate the strongly nonlinear compositional trends in ferroelectric BiBO3-PbTiO3 solid solutions for a variety of cations on the perovskite B site. We demonstrate that previously tabulated crystal chemical parameters (extracted from other Pb-based perovskite alloys [Grinberg et al., J. Appl. Phys. 98, 094111 (2005)]) permit accurate prediction of cation displacements in these new Bi-Pb alloys. We find that observed transition temperatures in these materials are well correlated with computed polarization magnitudes. The presented model for coupling between compositional variation and cation displacements explains the highly nonlinear and often nonmonotonic dependence of the Curie temperature (T(C)) on composition observed in these solid solutions.     

Colour centre-free perovskite single crystals

Yb³⁺:YAlO₃ (YAP) and Yb³⁺:GdAlO₃ (GAP) are interesting 1 μm high-power laser media thanks to their very good thermo-mechanical properties. However, as-grown perovskite single crystals exhibit colour centres. Parasitic thermal load generated by these centres is deleterious for high-power laser action and can lead to crystal damages. Moreover these defects decrease Yb³⁺ lifetime. They are related to trapped holes on the oxygen network. In the present work, several schemes to remove colour centres are presented. Attention is focused on cerium codoping, thermal annealing under reducing atmosphere and growth of non-stoechiometric compounds.     

Photorefractive effect in ferroelectric liquid crystals

In this paper, we review recent progress of research on the photorefractive effect of ferroelectric liquid crystals. The photorefractive effect is a phenomenon that forms a dynamic hologram in a material. The interference of two laser beams in a photorefractive material establishes a refractive index grating. This phenomenon is applicable to a wide range of devices related to diffraction optics including 3D displays, optical amplification, optical tomography, novelty filters, and phase conjugate wave generators. Ferroelectric liquid crystals are considered as a candidate for practical photorefractive materials. A refractive index grating formation time of 8–10 ms and a large gain coefficient are easily obtained in photorefractive ferroelectric liquid crystals.     

Microscopic band-mixing calculations in154,156Gd

A many-body microscopic band-mixing formulation of variation after projection of angular momentum and conservation of nucleon number is used to study the yrast band and first excitedK ^π =0⁺ band in the doubly even nuclei^154,156Gd. The computed energy spectra and the interband and intra-band B (E2) values are in good agreement with corresponding experimental data. Connection with the phenomenological model of Stephens and Simon is discussed, bringing out the role played by thei _13/2 neutron pair in the microscopic formalism.     

Echo response in ferroelectric liquid crystals

It is proposed that the kinetics and dynamics of ferroelectric liquid crystals may be studied by using the echo responses to the pulsed action of a microwave electric field. For ferroelectric liquid crystals, with homogeneously oriented molecules, calculations are made of the echo signals for two pulses and it is shown that these have values which are quite easily measured. Fiz. Tverd. Tela (St. Petersburg) 40, 1370–1372 (July 1998)     

Optical autowaves in photorefractive ferroelectric crystals

The effect of appearance of traveling structures, referred to as scattering autowaves, under illumination of photorefractive media by coherent light under certain conditions (power, wavelength, and focusing of pump radiation) is theoretically described. A model is proposed that takes into account both longitudinal and transverse photovoltaic currents in crystals with a spatially isotropic distribution of seed permittivity fluctuations.     

Pulsed laser deposition of perovskite relaxor ferroelectric thin films

Structural, dielectric and ferroelectric properties of thin films of La-doped lead zirconate titanate (PLZT) and sodium bismuth titanate-barium titanate (NBT-BT) perovskite relaxor ferroelectric have been investigated. PLZT films were deposited on Pt/Si substrates in oxygen atmosphere by pulsed laser deposition (PLD) and radio frequency (RF) discharge-assisted PLD, using sintered targets with different La content and Zr/Ti ratio, near or at the boundary relaxor ferroelectric. The films are polycrystalline with perovskite cubic or slightly rhombohedral structure. A slim ferroelectric hysteresis loop, typical for relaxors, has been measured for all film sets. Dielectric characterization shows a large value of capacitance tunability and low dielectric loss. However, common problems related to lead diffusion into the metallic electrode layer do not allow one to obtain high capacitance values, due to the formation of an interface layer with low dielectric constant. Lead-free NBT-BT thin films have been deposited on single crystal (1 0 0)-MgO substrates starting from targets with composition at the morphotropic phase boundary between rhombohedral and tetragonal phase. Films deposited by PLD are polycrystalline perovskite with a slight (1 0 0) orientation. Capacitance measurements were performed using interdigital metallic electrodes deposited on the film's top surface and showed high relative dielectric constant, on the order of 1300.     

Structural study of the ferroelectric instability in SnPSe

The structural change occurring in between the paraelectric and the ferroelectric phases is investigated by means of X-ray diffraction. Details of the structure in both phases are obtained and the role of the lone pair is discussed in the light of structural data. In agreement with the 2/ m to m symmetry lowering, polar displacements are found within the m plane away from a particular crystallographic direction but antiparallel displacements occur also. These results are discussed in the frame of the phenomenological theory which predicts a particular temperature dependence of the dielectric polarization in this crystal. Received: 10 July 1998     

Secondary ferroic properties of ferroelectric crystals

Twenty five types of structural phase transitions that are accompanied by the appearance of a spontaneous optical activity in the low-temperature phase are analyzed. A technique is proposed for describing the enantiomorphic domain structure (orientation states) in these crystals.     

Microscopic calculations of charge-exchange nuclear modes

Microscopic calculations of nuclear states excited by means of charge-exchange reactions and involving spin and isospin degrees of freedom, in particular, of the Gamow—Teller and the spin—dipole resonances, are discussed. The framework is a fully self-consistent nonrelativistic spherical quasiparticle random-phase approximation constructed on top of the Hartree—Fock—Bardeen—Cooper—Schrieffer approach. Our results are compared with available experimental data, and a critical discussion is attempted. The text was submitted by the authors in English.     

Magnetoplastic effect in NaNO2 ferroelectric crystals

This paper reports on a laser interferometric study of the effect of a dc magnetic field (MF) on the rate of plastic deformation (creep) $\dot \varepsilon$ of NaNO₂ ferroelectric crystals under compression. It is established that the application of a dc MF to a loaded specimen results in an increase in the creep rate and that removal of the MF brings about a decrease in $\dot \varepsilon$ . Subjecting an unloaded specimen to a dc MF beforehand also affects its strain rate under the subsequent loading. The observed magnetoplastic effect is most clearly pronounced within a certain $\dot \varepsilon$ interval, and the magnitude of this effect for the NaNO₂ ferroelectric is several times larger than that for LiF crystals.     

Soft mode dynamics of perovskite type crystals

The soft mode dynamics and related properties of perovskite, ABO₃-type crystals have been studied using the operator form of the model Hamiltonian proposed by Pytte. The correlations have been evaluated using the double time thermal Green’s function technique and Dyson’s equation. Without any decoupling, the higher order correlations, appearing in the dynamical equation, have been evaluated using the renormalized Hamiltonian. The dielectric properties are directly related to the optical soft mode. The phonon width and shift have been calculated for different structural phases. The analysis of the temperature dependence of microwave loss tangent and dielectric constant explains the experimental results.