KDP-type Ferroelectrics Research Articles

Contribution of vibronic interactions to the thermodynamics of the phase transition in H-bonded ferroelectrics: Proton-lattice coupling effects in crystals of the KDP family

A model Hamiltonian for microscopic description of structural phase transitions in KDP type ferroelectrics was considered in terms of the vibronic theory of heteroligand systems (VTHS). The Hamiltonian contains three terms that describe the proton interactions, the potential energy of lattice oscillators, and the relationship between these subsystems. In the second order perturbation theory, the configuration energies of the Bethe cluster method retained the Ising form, with the pseudospin Hamiltonian parameters explicitly depending on the electronic structure characteristics and the vibronic constants of the AO4-containing structural units. The Ising version of the theory was used for obtaining the numerical estimations of the thermodynamic properties, in particular, the critical temperature Tc as a function of all the parameters of the pseudospin Hamiltonian.

Dielectric properties of KDP-type ferroelectric crystals in the presence of external electric field

Considering external electric field as well as third- and fourth-order phonon anharmonic interaction terms in the pseudospin-lattice coupled mode (PLCM) model Hamiltonian for KDP-type ferroelectrics, expressions for field-dependent shift, width, renormalized soft mode frequency, Curie temperature, dielectric constant and dielectric loss are evaluated. For the calculation, method of statistical double-time temperature-dependent Green’s function has been used. By fitting model values of physical quantities, temperature and electric field dependences of soft mode frequency, dielectric constant and loss have been calculated which compare well with experimental results of Baumgartner [8] and Choi and Lockwood [9]. Both dielectric constant and loss decrease with electric field.

Correlation between hydrogen bond geometry and phase transition temperature in KDP-type ferroelectrics

Molecular dynamics simulation of the modified strong dipole-proton coupling model is used to show that ferroelectric phase transition temperature, T c, in KH 2PO 4 (KDP) and KD 2PO 4 (DKDP) linearly depends on hydrogen bond length, R O−O, and separation between peaks in proton (deuteron) thermal distribution in the paraelectric phase, δ. Separation between lines for KDP and DKDP is larger in the T c- R O−O dependence than in the T c- δ dependence indicating equal effect of different R O−O and different proton and deuteron masses on the whole change of T c between KDP and DKDP and establishing δ as a more important determinant of T c in these crystals.

Development and Application of the Modified Strong Dipole-Proton Coupling Model for KDP-Type Crystals

The modified strong dipole-proton coupling (MSDPC) model, which predicted several static and dynamic dielectric properties of KDP-type ferroelectrics, was used to investigate local properties of these crystals. The results showed both order-disorder and displacive characteristics of pure KDP and DKDP, as in many other experimental studies (NMR, neutron scattering) of local properties. Paramagnetic center SeO4 3− introduced in KDP and DKDP lattice as a local probe in EPR technique was treated within MSDPC model as a “soft” impurity. Simulated dipole reorientational dynamics of the impurity showed characteristics similar to the slow local dynamics of the probe detected in an EPR experiment. Additionally, the MSDPC model was adapted to describe dielectric properties of KDP-type antiferroelectrics. Antiferroelectric phase transition of the first order and isotopic shift in Tc were reproduced.

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 KH 2PO 4 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 PO 4 dipole are present in KDP and KD 2PO 4 (DKDP). These results correlate with experimental data from NMR and neutron scattering studies of local properties.

Femtosecond time resolved impulsive stimulated Raman scattering in KDP-type Ferroelectrics

Femtosecond time resolved impulsive stimulated Raman scattering has been applied to the low lying B2 mode of KH2PO4 and KD2PO4. The time dependent signals of both crystals show an exponential decay, meaning that both modes are relaxational in nature. The crossing angle of the exciting beams was varied to study the wavenumber dependence of the excited mode. The relaxation time of the individual dipoles of the relaxational mode was calculated to be τ = 0·131 ps for KH2PO4 and 1·06 ps for KD2PO4. Here τ0 is the relaxation time of individual dipoles which is given in the equation of the relaxation time τ = τ0 T 0/(T–T 0).

水素結合における構造物性と機能 トンネリングモデルの歴史と現状

A historical review is given on the proton tunneling model of phase transitions in hydrogen-bonded crystals making the main reference to KDP type ferroelectrics. The model has been shown to be inconsistent with experimental results in Japan. This procedure is reviewed in the present paper and will be in other papers in this issue. The validity of the tunnneling model in the present stage of study on structural phase transitions is discussed.

Anisotropic Neutron Critical Scattering in DKDP Originating from the Ice Rule by Six-States Rotator Model

A theoretical origin of the anisotropic neutron critical scattering observed in DKDP is studied on the basis of the H 2 PO 4 dipole order-disorder model and the ice rule constraint in KDP type ferroelectrics. A radical D 2 PO 4 is assumed to take six Slater configurations described by a six states rotator. The diffused scattering intensity is given in terms of neutron scattering structure factors for respective six configurations and correlation functions among six states rotators. The scattering intensity contour in the reciprocal lattice space is numerically computed and shown to be similar to the experimental results even if the rigid ice constraint is released to some extent. Discussions are made on the controversy between the proton tunneling model and the H 2 PO 4 dipole order-disorder model from these results.

Structural origin of tricritical point in KDP-type ferroelectrics

Abstract At ambient pressure the phase transition in hydrogen-bonded ferroelectric crystals of the (KDP stands for KH2PO4) is a combined effect of atomic displacements and of proton disordering in hydrogen bonds. It is shown, that the occurrence of tricritical point in these materials result from the competition between the structural features connected with the hydrogen bonds, favouring angular displacements of structural units from the positions required by the paraelectric symmetry, and van der Waals and electrostatic interactions favouring the symmetric arrangement. The displacements, contributing to the first order character of the phase transition, are reduced at elevated pressures. The tricritical point of the phase transition in KDP had been evaluated from structural data.

Pre-fractal domain patterns optically observed in KDP-type ferroelectrics KD 2PO 4 and RbH 2PO 4

Lamellar ferroelectric domain structures periodic along the normal to 180° walls have been optically observed in unelectroded c plates of KD 2PO 4 (DKDP) and RbH 2PO 4 (RDP) between parallel polarizers. It is found that DKDP and RDP show the pre-fractal domain patterns (PFDPs) which are characterized by the pentad Cantor sets along the wall normal. PFDPs of DKDP and RDP are the same as those of isostructural KH 2PO 4 (KDP). This clarifies that PFDPs are formed in the presence of both depolarization field and surface energy density σ of walls irrespective of the amount of σ which is proportional to the square root of the Curie temperature T c.

Diffused scattering originated from the ICE rule

Abstract The effect of the ice rule constraint on the critical diffused scattering from deuterons is theoretically studied in KD2PO4 from the statistical mechanical viewpoint by the 4-cluster approximation. The anisotropic intensity contour in the reciprocal lattice space observed in neutron scattering has been realized in a similar shape. The relation to the controversy on models of phase transition in KDP type ferroelectrics is discussed.

Electron nuclear double resonance evidence for bifurcated hydrogen bonds in KH 2PO 4: Possible role in the ferroelectric transition

Electron nuclear double resonance measurements on OH⋯O protons in KH 2PO 4, using SeO 3− 4 as a probe, demonstrate that the superhyperfine tensor for the “far” (O⋯H) proton is dominated by dipolar interaction with the unpaired electron in the 2 p z orbital of an oxygen to which a “close”, i.e., O-H, proton is attached. This result implies that each hydrogen is involved in a bifurcated hydrogen bond. This conclusion was supported by a reanalysis of the earlier reported ENDOR data that was obtained using the AsO 4− 4 radical as a probe in KH 2AsO 4, in particular, by the fact the isotropic part of the tensor for the “far” protons is sizable (2.8 MHz) and has same sign (negative) as that of the “close” proton. Moreover, the various O⋯H distances and bond angles for the proposed bifurcated hydrogen bonding are comparable to the literature values for these parameters for carbohydrates. These findings provide the basis for a new contribution toward the role of hydrogens in the mechanism of phase transition in the KDP-type ferroelectrics.

Anharmonic effects in potassium-dihydrogen-phosphate-type ferroelectrics.

A Green's-function technique is used to study the influence of spin-phonon interactions in the Ising model with a transverse field (describing KDP-type ferroelectrics) including higher-order anharmonic terms. The renormalized spin-wave and phonon energy, the damping of the spin waves, and the phonon damping have been evaluated. The contribution of the anharmonicity is at low temperatures very small, but at temperatures near the ferroelectric critical temperature ${\mathit{T}}_{\mathit{c}}$, and above ${\mathit{T}}_{\mathit{c}}$, it increases strongly.

Thermodynamical characteristics of thin ferroelectric films

Abstract Thermodynamic characteristics of the KDP type ferroelectrics are analysed in the paper. Dispersion laws of the ferroelectric excitations in the thin ferroelectric films are considered. The ordering parameter was investigated at the low temperatures and at the temperatures near the phase transition temperature. The analysis shows that the spontaneous polarization in the film decreases more slowly with increase of the temperature than in the bulk structure. This conclusion is important for practical application because the thin films keep the ferroelectric properties at significantly higher temperatures than the bulk structures.

Kinematic levels and dielectric permittivity of KDP-type ferroelectrics

An analysis of the dielectric permittivity of KDP-type ferroelectrics has been performed in the vicinity of the transition point, in the framework of RPA applied to the Ising model in a transverse field with the inclusion of kinematical levels. The results agree with the experimental curves obtained for KDP crystals.

Mechanism of domain freezing in KDP type ferroelectrics

Abstract The dielectric relaxation time τ of KH2PO4 and CsH2AsO4 was investigated in the ferroelectric phase through measurements of the complex dielectric constants in the frequency range between 103 Hz and 109 Hz. Anomalous increase in τ near the domain freezing temperature Tf , considered together with the relation between the free dielectric constant ec and the the elastic stiffness C 66 E , suggests that dipolar reorientation in the domain walls has a close relation to the hardening of the crystal around Tf , which originates the domain freezing.

Relaxation Time for KDP‐Type Ferroelectrics in an External Electric Field

physica status solidi (b)Volume 151, Issue 2 p. K209-K212 Short Note Relaxation Time for KDP-Type Ferroelectrics in an External Electric Field Julia M. Wesselinowa, Julia M. Wesselinowa Department of Physics, University of Sofia Search for more papers by this author Julia M. Wesselinowa, Julia M. Wesselinowa Department of Physics, University of Sofia Search for more papers by this author First published: 1 February 1989 https://doi.org/10.1002/pssb.2221510261 Blvd. Anton Ivanov 5, BG-1126 Sofia, Bulgaria. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Volume151, Issue21 February 1989Pages K209-K212 RelatedInformation

Study of electrogyration in KDP-type ferroelectrics†

Using special technique for optical activity measuring in the usual birefringence presence the spontaneous and induced electrogyration effects in some KDP-type crystals are studied. According to the quadratic electrogyration coefficients temperature dependences and their theoretical interpretation, the phase transition in these crystals is ascertained to be dependent on order parameter and polarization coupling strength (proton-phonon coupling). The latter is determined mainly by the ionic composition without any significant dependence on deuteration.

Paramagnetic resonance studies of phase transitions in condensed materials

Abstract The use of electron paramagnetic resonance (EPR) as a tool to study various kinds of phase transitions in condensed materials is discussed. The subject is reviewed by considering representative examples of studies in the different kinds of phase transitions. Structural phase transitions in solids, dynamics of formation of Slater configurations in KDP type ferroelectrics, temperature dependence of ordering in liquid crystals, studies of the dynamics of chain motion in glass to rubber transitions in polymer systems as well as magnetic transitions are discussed.

Order-disorder model of PO4dipoles in kdp based on recent Raman spectroscopic studies

Abstract Raman spectra of KDP type ferroelectrics have been analysed from a new standpoint. The local and momentary site symmetry of PO4 tetrahedra have been concluded to be C2 even above the transition temperature. The order-disorder model of PO4 dipoles is presented and the relation to the conventional proton order-disorder model is discussed.