Second harmonic generation in order-disorder ferroelectrics

We investigate the temperature dependence of second harmonic generation in order-disorder ferroelectrics. We obtain the qualitative temperature dependence of the second harmonic amplitude; the amplitude goes to zero when the temperature reaches the Curie point. We take into account the disruption of wave number harmony when the temperature is changed. We show the second harmonic is largest at the temperature where the wave numbers are synchronized and there is no detuning. The theoretical conclusions reached here are confirmed by experimental data. Volgogradskii State University, Volgograd. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 55–58, January, 1999.     

Microscopic theory of the electro-acoustic echo in order-disorder antiferroelectrics

A microscopic theory of the electro-acoustic echo (EAE) is proposed for the case in which two pulses of a variable electric field act on an antiferroelectric. This theory augments the phenomenological theory proposed for the purpose of interpreting experiments on the main regularities of the electro-acoustic echo in order-disorder antiferroelectrics. The deuterization effect and “pre-polarization” effect are explained. The shape of the echo signal is derived analytically and it is shown that this shape depends on the time interval between pump pulses. Fiz. Tverd. Tela (St. Petersburg) 40, 118–121 (January 1998)     

Dynamics of the nonlinear excitation of a wave packet in order-disorder ferroelectrics

Microscopic pseudospin formalism is used to examine the main laws governing the nonlinear excitation of a packet of pseudospin waves in ferroelectrics of isomorphous KDP. The nonlinear frequency shift of the packet is determined along with its temperature dependence. A qualitative analysis is made, of the dynamics of the “center of gravity” of the packet. Volgograd University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 98–102, August, 1997.     

Calculation of the susceptibility of an eight-site order-disorder model for ferroelectrics

The contribution of the diffusion of the titanium ion to the dielectric tensor, is estimated for all phases of BaTiO₃ in thermodynamical model; the values obtained for the tetragonal phase are in good agreement with the difference between the phonon contribution and the capacitance measurements, for both directions parallel and perpendicular to the ferroelectric axis.     

Displacive and order-disorder behavior of KDP-type ferroelectrics on the local scale

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KH₂PO₄ or KDP-type ferroelectrics, was used to investigate the properties of these crystals on the local scale. Results calculated by molecular dynamics (MD) simulation show that both order-disorder and displacive characteristics of one PO₄ dipole are present in KDP and KD₂PO₄ (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.     

Theory of electroacoustic waves in cloudy atmosphere

A general system of equations of motion of a mixture of weakly charged gas and a dropwise liquid is obtained. Vapor condensation on the drops, electrization of the gas, as well as coagulation of drops are taken into account.     

Theory of ion scattering from single crystals

The scattering of He⁺, Ne⁺ and Ar⁺ ions at 600 eV from Ni(110) in the [11̄0] direction is modeled using classical dynamics. The distributions of final scattering angles of the primary ions are displayed as contour plots over the surface impact zone. From the contour plots the regions of the surface that give rise to scattering at specific angles can be isolated. The majority of the inplane scattering arises from collisions of the primary ion with second layer or “valley” atoms. However, to correctly reproduce the experimental energy distribution curves of Heiland and Taglauer (J. Vacuum Sci. Technol. 9 (1971) 620), we must include a simple collision time correction to account for neutralization of the ion beam. This analysis predicts that the ions which collide with second layer atoms of the solid are preferentially neutralized. The energy distributions due to first layer collisions agree well with experiment. We find that a full molecular three dimensional model is needed to describe all of the ion scattering events since for most of the collisions, the ion is simultaneously interacting with at least two atoms of the solid. However, in agreement with other workers we find only a single collision is responsible for the “binary” peak in the energy distribution. In addition the relative scattering intensity at different angles is dependent on having a three-dimensional solid.     

Optical echo in photonic crystals

The dynamics of a photonic wavepacket in the effective oscillator potential is studied. The oscillator potential is constructed on the base of one-dimensional photonic crystal with a period of unit cell adiabatically varied in space. The structure has a locally equidistant discrete spectrum. This leads to an echo effect, i.e., the periodical reconstruction of the packet shape. The effect can be observed in the nonlinear response of the system. Numerical estimations for porous-silicon based structures are presented for a femtosecond Ti:sapphire laser pump. The text was submitted by the authors in English.     

Theory of order-disorder transformation in binary systems of the Cu-Au type-II

In the Kikuchi approximation [1,2] the temperature and concentration dependence of long- and short-range order parameters in f.c.c. alloys which order with the formation of L1₂ and L1₀ superstructures are calculated. The values of atomic order parameters for the L1₂ superstructure found in this way are in close agreement with experimental data.     

Theory of order-disorder transformation in binary systems of the Cu-Au type-I

The phase diagram of f.c.c. alloys which order with formation of L1₂ and L1₀ superstructures is calculated by using the Kikuchi approximation. For both superstructures the order-disorder transformation is a first-order transition. There are three regions of ordered phases. The phase region boundaries are significantly different from those found by Shockley and Li. The maximum of the order-disorder transition temperature in the case of the L1₂ superstructure is displaced from the stoichiometric composition AB₃ to a composition having a greater concentration of A. The following correlation was found between the ordering energy v=2v_AB-v_AA-v_BB and the critical order-disorder transition temperature T_c: for the L1₂ superstructure, v=2·059kT_c; for the L1₀ superstructure, v=2·113kT_c (k=the Boltzmann constant).     

Influence of hydrostatic pressure on the polarization dynamics of order-disorder ferroelectrics: AgNa(NO2)2

The characteristic ferroelectric dispersion of the complex dielectric constant of the ferroelectric AgNa(NO₂)₂ is measured as function of temperature and hydrostatic pressure. For the first time the temperature and pressure dependences of the static order parameter susceptibility and the order parameter relaxation time could be reproducibly determined. The results are interpreted in terms of a kinetic Ising model, whose parameters show a characteristic pressure dependence. The results are discussed on the basis of thermodynamic relations and related to the dynamical behaviour of other order-disorder ferroelectrics.     

Antiferroelectric and ferroelectric phase transitions of the displacive and order-disorder type in PbZrO3 and PbZr1-xTi x O3 single crystals

For PbZrO₃ and PbZr_0.99Ti_0.01O₃ single crystals the first order Raman light scattering spectra in the paraelectric phase have been analysed. In lead zirconate two kinds of crystals, with and without a transient phase, were investigated. It was found that temperature dependence of the defect-induced soft mode frequency in the paraelectric phase has a negligible contribution to the low-frequency dielectric response. From the point of view of strong dielectric relaxation in the range of 10⁶–10⁹ Hz, which originates in a disorder of the lead sites, crossover of the order-disorder and displacive type of antiferroelectric phase transition in lead zirconate is considered: the phase transition mechanism is neither purely displacive nor order-disorder. Contribution of physical phenomena responsible for the ε(ω,T) dielectric response at low frequencies of an external electric field are described.     

Slow molecular motions and solid echo in hydrate crystals

The temperature dependences of the solid-echo amplitude have been measured in stilbite single crystal at temperatures of slow diffusion motions of water molecules. The obtained experimental data agree well with the derived theoretical expressions describing the two-pulse response of the isolated two-spin system performing jumps between two equilibrium positions.     

Theory of switching of multiaxial ferroelectrics (the main stages)

The kinetics of switching of multiaxial ferroelectric crystals with 180° and 90° domains under conditions of normal and layer-by-layer domain growth is studied using the multidimensional theory of first-order phase transitions. The main stages of the process of switching of a multiaxial ferroelectric are considered under the assumption that repolarized cylinder-shaped domains grow three-dimensionally. A closed set of equations describing the kinetics of switching is derived with allowance for a change in repolarization in the course of the phase transition. Equations for the time-dependent switching current are derived. The main characteristics of switching are compared qualitatively with the experimental data on barium titanate.     

(1)H and (87)Rb nuclear magnetic resonance study of the order-disorder phase transition of RbHSeO(4) single crystals

The ferroelectric phase transition at T(C2) (=370K) in RbHSeO(4) has been studied by (1)H and (87)Rb solid-state NMR. Although not large, the spin-lattice relaxation time, T(1), and the spin-spin relaxation time, T(2), of rubidium and of the alpha- and beta-type protons show distinct change near the phase transition. The intensity of the signal due to the alpha-type protons decreases with increasing temperature, and the intensity of alpha-type protons is quite weak above 330K: at a temperature which is about 40K lower than the phase transition temperature, the ordering of the alpha-type protons occurs. The alpha-type protons in the ferroelectric phase lead to a noticeable change in the proton magnetic resonance spectra. Our study of the (1)H spectra shows that the ferroelectric phase transition in RbHSeO(4) is of order-disorder type and is due to the ordering of protons in hydrogen bonds.