KDP-type Ferroelectrics Research Articles

Light scattering spectra of polarization fluctuations and models of the phase transition in KDP type ferroelectrics

Abstract Recent light scattering studies in KH2PO4 type hydrogen-bonded crystals are reviewed. Dynamical properties of these crystals have been discussed in terms of the coupled tunneling proton-optical phonon model up to date. Light scattering spectra in several GHz and cm−1 regions with various scattering geometries observed in KH2PO4, KD2PO4 and their mixed crystals, however, revealed that all these spectra are well analysed in terms of a coupled polarization relaxation mode-acoustic phonon system. Since most of the spontaneous polarization is due to shifts of K, P and O ions, relaxation motions of H2PO4 dipoles arc concluded to be the origin of these low-frequency spectra. Models of the phase transition mechanism are discussed on the basis of these results and other recent experimental studies including the explanation of large isotope effects in these crystals.

Polarization relaxation spectra of (H2PO4)-1 dipoles in KDP type ferroelectrics

A brief review is made on recent light scattering spectra observed in KDP1-x DKDP mixed crystals. On the basis of the fact that the spectra are well analysed in terms of the coupled polarization relaxation mode-acoustic phonon system, the order-disorder model of (H2PO4)- dipoles is presented. The origin of the large isotope effects is discussed without the introduction of the tunneling model.

The proton potential in short hydrogen bonds

The state of knowledge of the proton potential in systems for which double minimum and symmetrical, single minimum type curves have been envisaged is critically reviewed. In particular, the proposed model potentials for carboxylic acids, for KDP type ferroelectrics and for the shortest hydrogen bonds are confronted with experimental data, particularly from vibrational spectra. The early attempts at explaining the complex features in the AH stretching region by level splitting in one-dimensional double minimum potentials have not been successful. The interest in the profile of the potential surface is shifted from this aspect to its role in anharmonic interaction based general band-shaping mechanisms.

New paramagnetic probes for the cationic dynamics in the KDP-type ferroelectrics and antiferroelectrics

Abstract Paramagnetic probes have provided valuable, quantitative information on the role of the anionic fragments (PO4 3− and AsO4 3−) in the KH2PO4-type of ferroelectrics and antiferroelectrics. However, parallel studies of the cations (K+, Rb+, Cs+ and NH4 +) have not yet been possible, the progress being hampered mainly by the lack of suitable paramagnetic probes which could accurately simulate the cationic behavior. Previously used probes are Fe3+, Cr3+, Cu2+ and Tl2+, all of which have an excess of positive charge compared with that of the substituted cations. This excess charge would necessarily lead to some structural deformation of the surroundings, and hence all these previously used ions might not be considered as faithful probes. We have now succeeded in stabilizing a variety of monovalent, paramagnetic ions in the KDP-type of ferroelectrics and antiferroelectrics. The structure and the dynamics of these probes have been studied via the temperature and angular dependence of the Zeeman, the hyp...

Isotope effects in Rochelle salt

Isotope effects on deuteration both in the static and dynamical behaviour of Rochelle salt are theoretically investigated on the basis of a two-sublattice Ising model with asymmetric double-well potential. Spontaneous polarization and static dielectric constant are numerically calculated for several values of the tunnelling integral. The dynamical susceptibility is derived from a Bloch-type equation, and the shifts of the relaxation times are calculated from this equation. The notable difference in the dynamical behaviour between Rochelle salt and KDP-type ferroelectrics is also discussed.

Central peak dynamics of Kobayashi's model for KDP-type ferroelectrics

The dynamics of central peak KDP-type crystal described by Kobayashi's model, which takes into account the coupling between optical phonons and pseudo-spins, is studied by means of the continued fraction approximation. The intensities and the ratio of the central peak to the soft mode shape linewidth are calculated and the obtained results fall into the range of the experimental results.

Mechanism of Instabilities of the Coupled Acoustic and Optical Modes in KDP Type Ferroelectrics

Mechanism of Instabilities of the Coupled Acoustic and Optical Modes in KDP Type Ferroelectrics

Order-disorder character of the hydrogen bond in KH2PO4

A discussion is presented on some of the available evidence for the shape of the local effective potential for the hydrogen motion in KDP-type ferroelectrics, particularly regarding the existence or nonexistence of a central barrier in that potential. For this purpose a one-minimum square-well potential model is considered and compared with the corresponding tunneling model. The comparison of results so obtained for both models with available experimental data on the isotope dependence of the critical temperature, Curie constant, and maximum spontaneous polarization at absolute zero leads to the conclusion that the accuracy of the experimental data together with the uncertainties on the theoretical calculations do not yet permit a discrimination in favor of an order-disorder double-well character for these ferroelectric crystals.

Estimating the barrier to proton or deuteron tunnelling in hydrogen-bonded crystals

Abstract We represent a symmetric double minimum proton potential by two Morse functions placed back-to-back. The wave-functions and energy eigenvalues are determined by asymptotic analysis. Using as input the crystallographic atom positions and the frequencies of two infrared transitions we can deduce the well parameters for CrOOD. Using the same well parameters (but somewhat different atom positions) for CrOOH, we derive an energy structure in good agreement with the observed mid infrared, far infrared and neutron inelastic scattering spectra. The model should be useful for estimating the tunnelling integral in KDP-type ferroelectrics, and its dependence on the oxygen-oxygen separation.

Polarization Fluctuation Spectra in KDP type Ferroelectrics Studied by Light Scattering

Polarization Fluctuation Spectra in KDP type Ferroelectrics Studied by Light Scattering

The photovoltaic and photorefractive effects in KDP-type ferroelectrics

By illuminating KDP-type ferroelectrics (KH2PO4, K(D x H1−x )2PO4,x⋟0.7) in the region of the intrinsic optical absorption edge in the absence of an external field the steady state photocurrent and photo emf, being a few orders of magnitude higher than the energy gap, are observed. Under the same conditions the photorefractive effect (“optical damage”) is observed.

Influence of the heavy ions on the spontaneous polarization of KDP-type ferroelectrics

In KDP the contribution of the sybsystems: heavy ions, light ions and electrons to the spontaneous polarization is investigated through CNDO/2 calculations. It is shown that - in contrast to the accepted current view - the light ions play a dominant role.

On the Dynamics of KDP Type Ferroelectrics. I: Formulation and Acoustic Anomaly

On the Dynamics of KDP Type Ferroelectrics. I: Formulation and Acoustic Anomaly

On the soft acoustic mode of KDP type ferroelectrics

KDP type ferroelectrics are investigated on the pseudo spin-phonon coupled model with the use of the Green's function. Results are consistent with experiments on Raman scattering and Brillouin scattering.

Analysis of critical nuclear magnetic relaxation in KDP-type ferroelectrics

Abstract The extensive literature available on anomalies in NMR relaxation rate, near Tc, in KDP-type ferroelectrics, is analyzed both in terms of the “collective model-fluctuation dissipation theorem,” method, as well as in terms of critical indices. The first method gives a non-interacting dipolar relaxation time TC ∼ 5(10−13) sec, in reasonable agreement with available dielectric relaxation results. While previously reported data does not show unambiguous evidence for “true” critical regime behavior, new proton relaxation data in KH2ASO4 is better described by power law rather than logarithmic behavior. This may be a result of true critical effects.

The model of phase transitions in sodium trihydrogen selenite, NaH3(SeO3)2

Methods developed in the theory of KDP-type ferroelectrics are applied to the STS-type crystals. The double-minima potential for hydrogen at the OH-bond is assumed and two locations (states) of proton are described by the spin model. Using the ferroelectric-phase structure data some conclusions on parameters of the model spin hamiltonian may be reached. Also the entropy of transition is found to be in good agreement with the experiment. The structure of β-phase, existing in STS in a certain temperature range, is predicted. The thermodynamics of transitions is investigated on the basis of the suggested hamiltonian, using the cluster approximation.In spite of the existence of many unknown constants, a number of qualitative results follows naturally from the suggested model. They include the existence of the intermediate p-phase, the peculiar change of polarization in the transition region, the influence of electric fields etc.

Theory of coherent neutron scattering by hydrogen-bonded ferroelectrics at low temperatures. II. Scattering characteristics and various conceptions of the tunneling quasispin model

To check experimentally the dispersion law of the polarization (“quasispinwave-like”) mode the characteristics of quasielastic neutron scattering at small angles are studied in KDP-type ferroelectrics. The various conceptions of the tunneling quasispin model, the nature of the low-temperature polar phase, and the vibration of tunneling particles along the axis of anisotropy are discussed with reference to the coherent neutron scattering.

On the properties of the KDP type ferroelectrics with impurities

The properties of KDP type ferroelectrics with impurities (D in KH2PO4 and H in KD2PO4) are considered using the method of zero-temperature Green function. The linewidth of the polarization waves caused by their scattering on such impurities as well as the local frequency of impurity itself are calculated. The obtained results can be tested by measuring an intensity of the coherent (for H in KD2PO4) or incoherent (for D in KH2PO4) inelastic scattering respectively of slow neutrons scattered by such impure ferroelectrics. [Russian Text Ignored].

Models of the phase transition in kdp type ferroelectrics

Recent typical models proposed for KDP type ferroelectrics, the Ising spin models, the tunneling proton model, and the 'mixed' proton and phonon model (the modified tunneling model), are briefly reviewed and compared with experimental facts. The relations between these models are correctly shown paying attention to the expressions for the electric polarization in them. Experimental and theoretical work necessary for deciding upon a definite final model are suggested.