Ferroelectric Phase Transition In KDP Research Articles

Monte Carlo studies of the ferroelectric phase transition in KDP

The order-disorder type ferroelectric phase transition in KDP is described by the pseudospin model. Four-particle cluster approximation provides its solution which fits well with the experimental data. Here we check for the first time how this approximation is exact in the absence of proton tunneling and long-range dipole interactions by means of Monte Carlo simulations. Polarization, dielectric susceptibility and heat capacity analytical curves are reproduced well by our simulations and the difference in critical temperature turns out to be much less than a percent.

Comment on the interpretation of Raman spectrum of KDP

ABSTRACTControversial views of the Raman spectrum related to the mechanism of the ferroelectric phase transition of KDP (KH2PO4) are discussed. In the model proposed by Tominaga etal. (Solid St. Commun. 48 (1983) 265), it was concluded that PO4 tetrahedra are distorted to polar C2 symmetry already above Tc. Here, it is shown that the C2 symmetry is simply that of a local ion. Therefore, the Raman spectra do not give any evidence of the "PO4 orientational order-disorder model" in which protons are not involved at all in the phase transition. The assignment of the overdamped soft mode in this model is also discussed.

Pressure effect on the ferroelectric phase transition in KDP in the cluster approximation of the proton-tunneling model

ABSTRACTWe investigate the pressure dependence of the ferroelectric phase transition in KDP and DKDP in frame of the four-particle cluster approximation of the proton-tunneling model on the assumption of the dipole-dipole interaction nature of the cluster approximation parameters. The obtained values of the derivative somewhat differ from the experimental ones. However, the calculated tricritical point coordinates and the pressure at which the transition vanishes appear to be close to experimental values.

Indirect non-electrostatic mechanism of the proton-proton interaction and proton-lattice coupling in KH2PO4 ferroelectrics

It has been shown that the mechanism previously proposed for the effective interaction of protons in MH2 AO4 ferroelectrics, such as KH2PO4 (KDP), is applicable to the description of the proton-lattice coupling. The expression derived for the total Hamiltonian of the coupled proton-lattice system, along with the single-mode approximation, also allows the consideration of several modes involved in the ferroelectric phase transition. The use of the proposed approach is illustrated by the example of the numerical evaluation of the effect of the proton-lattice coupling on the critical temperature T c of the ferroelectric phase transition in KDP and deuterated KDP.

The Isotope-Effect in the Phase Transition of KH2PO4: New Insights from Ab Initio Path-Integral Simulations

We investigate the quantum-mechanical localization of 1H and 2H isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics simulations to obtain the nuclear real-space and momentum-space densities n(R) and n(k) of 1H and 2H, of which the latter densitites are compared to experimental neutron compton scattering data. Our results suggest a qualitative difference in the nature of the paraelectric phase in KDP between the two isotopes. Whereas both paraelectric states result from quantum delocalization, the essential difference is the change from a probably coherent to incoherent tunneling behavior of the hydrogen atoms across the hydrogen-bonds.

The influence of an electric field on the latent heat of the ferroelectric phase transition inKDP

The specific heat, heat flux (DTA trace) and dielectric constant of KDP ferroelectric crystalhave been measured simultaneously for various electric fields with a conduction calorimeter.The specific heat presents a strong anomaly, but these simultaneous measurements allow usto evaluate the latent heat accurately. The latent heat decreases with field, and the value ofthe critical electric field—that where the latent heat disappears—is estimated to be(0.44 ± 0.03) kV cm−1. Incidentally, we have measured simultaneously the dielectric permittivity, which suggeststhat latent heat is developed as domains are growing.

Forty crystal structure analyses of KH2PO4 in the paraelectric phase

A resolving power of the structure analysis is governed by the accuracy of the diffraction intensity measurement. This fact implies that adjacent disordered positions can be distinguishable on the histogram in position derived from a number of split-atom structure analyses. To find the true P atom position (s) of KH2PO4 (KDP) in the paraelectric phase along this line, we collected forty X-ray diffraction intensity sets and carried out the split-atom structure refinement. Analyzing the derived histograms of the P atom position for several structure models, we conclude that the average structure model is most probable to indicate the ferroelectric phase transition of KDP is of the displacive-type.

An Automated System for Measuring Elastic Properties by the Piezoelectric Resonance Method and Its Applications to Elastic Phase Transitions

Experiments have been performed to construct an automated system for measuring elastic constants of piezoelectric substances by the piezoelectric resonance method. The measuring system was automated by introducing a network analyzer controlled through a GP-IB interface by a personal computer. The system was applied for investigating the ferroelectric phase transition of KDP and the ferroelastic phase transition of LAT. Excellent data were obtained for both compounds, and it was confirmed that the measuring system was applicable to the investigation of the phase transitions with large elastic anomalies.

A Relaxational Central-Mode in KDP

The dynamics of the ferroelectric phase transition of KDP has been studied by the use of a high-resolution light-scattering system, which is composed of a Fabry-Perot interferometer and a double monochromator, in the frequency range from 0 to 10 cm-1. The spectra observed in the x(yx)y geometry show a broad central component which grows up near the phase transition temperture Tc. This component is analyzed by using a Debye-type susceptibility function which includes the polarization relaxation time τ and the static susceptibility χ(0). It is found that τ shows a critical slowing down and χ(0) obeys the Curie-Weiss law. The origin of the central component is assigned to fluctuating polarization which becomes divergent near Tc. The phase transition in KDP is well explained by an order-disorder mechanism of the polarization.

Central component of y(xy)x Raman spectra in paraelectric KH 2PO 4

The very low frequency Raman spectra (∼±20cm −1) of y( xy) x geometry in a paraelectric KH 2PO 4 (KDP) have been found to change drastically near the ferroelectric transition temperature ( T c =122.2K). The temperature dependence of TI −1 0 have been found to be proportional to ( T−117K) for T≲ 200 K, where I o is the integrated intensity of the both Stokes and anti-Stokes responses in the ± 50cm −1 region. They very low frequency y( xy) x Raman response which had been neglected in the previous analyses has been found to play the important role in the ferroelectric phase transition of KDP.

Logarithmic Anomaly in the Debye‐Waller Factor and Thermal Expansion Coefficient at Ferroelectric Phase Transition in KDP

AbstractTemperature dependence of the thermal expansion coefficient and the Debye‐Waller factor along the polar axes in KDP were investigated by means of X‐ray diffraction. A logarithmic anomaly was observed in the range from Tc up to 6 K above Tc.

Acoustic longitudinal mode coupling to the order parameter and its effect on the ferroelectric phase transition in RbH2PO4

The resonant part of the electrostrictive coupling of the acoustic longitudinal model, which propagates along the polar axis, to the order parameter, has been taken into account in the pseudo-spin model for the ferroelectric phase transition in KDP. Using a Green function method and an RPA-type decoupling scheme, an expression is derived for the velocity of the acoustic mode in the ferroelectric phase. The results are compared with the available experimental data on RbH2PO4. The effect of this coupling on the thermodynamic behaviour at the phase transition is discussed.

A Two Doublet Model for the Phase Transition in KDP

By assuming that each of the ground and lowest excited levels of the proton (deuteron) on the hydrogen bond in KDP composes a doublet, the pseudospin hamiltonian for the proton (deuteron) system with tunneling motion, which is assumed to occur in the doublet of excited level, is presented to investigate the ferroelectric phase transition in KDP. On the basis of this hamiltonian, the first and second order transitions, which depend on the pressure, and therefore the existence of a tricritical point are explained on the basis of a mean field approximation, including the isotope effect in the transition. These are discussed in terms of the energy separation between the two doublets, the tunneling frequency at the excited doublet and the parameters of coupling between protons (deuterons) on different bonds.