Ferroelectric-ferroelastic Phase Transition Research Articles

Comparative study of the phase transitions and spectral properties of NH2(CH3)2Me1-хСrx(SO4)2×6H2O (Me = Al, Ga) ferroelectrics

This article is devoted to study of Cr3+ ion complexes in NH2(CH3)2Me1-xCrx(SO4)2 × 6H2O, Me = Al, Ga (DMAMe1-xCrxS) crystals. On the basis of spectroscopic data the energy diagram and the parameters of the crystal field corresponding to the above mentioned complexes were specified. It was shown that isomorphous substitution of the metal ion practically does not affect the temperature of ferroelectric-ferroelastic phase transition in NH2(CH3)2Ga1-xCrx(SO4)2 × 6H2O contrary to the case of NH2(CH3)2Al1-xCrx(SO4)2 × 6H2O crystal. The structure and temperature changes of the absorption spectra of the above mentioned solid solutions are explained in terms of the internal electronic transitions in [Сr(H2O)6]3+ octahedral complexes. The symmetry and distortion of the complexes within the structure of all investigated crystals were analyzed on the basis of the Raman spectroscopy data.

Microstructure and impedance properties of La, Ce multi-rare earth ions doped Na0.5Bi2.5Nb2O9 Aurivillius type ceramics

(Li, Ce, La) multi-doped Na0.5Bi2.5Nb2O9 ceramics were prepared by a conventional solid state sintering method. Microstructure of sintered ceramics was examined by the X-ray diffraction and scanning electron microscopy. The introduction of voluminous La3+ cations decreases the degree of lattice distortion, leading a lower Curie-temperature. The detailed electrical conduction behaviors were determined by the electrochemical impedance spectra (EIS). The electrical properties were investigated as a function of frequency (102–106 Hz) at various temperatures (350–750 °C). The underlying physical mechanism of electrical properties at high temperature for multi-rare-earth doping NBN-based was studied via an assistance of EIS techniques. There were a set of Debye-like relaxation peaks in the imaginary part of impedance and electric modulus data. Oxygen vacancies are responsible for both ac conduction and dielectric relaxation processes. A discontinuous variation of the exponent parameter n is found owing to a ferroelectric-ferroelastic phase transition around 550 °C.

Combined Piezoelectrooptic Effect in Rochelle Salt at the Phase Transition

The results of study of piezoelectrooptic (PEO) effect in the course of ferroelectric-ferroelastic phase transition in Rochelle salt crystals are presented. The coefficient of the combined effect is obtained from measurements of the changes in the electrooptic coefficients under the action of mechanical stress and the changes in the piezooptic coefficients under the action of electric field (or spontaneous polarization). It is shown experimentally that the values of both coefficients are the same, as predicted by the theory. The temperature dependence of the coefficient of combined PEO effect is obtained. Its anomalous behaviour at the phase transition appears due to the dielectric permitivity anomaly. It is also demonstrated that the change in the piezooptic coefficients at the phase transition in Rochelle salt crystals may be satisfactorily explained as a result of linear and quadratic PEO effect induced by spontaneous polarization.

Specific heat under uniaxial stress

An experimental approach for the adaptation of conventional calorimetric techniques for measurements under uniaxial stress is reviewed. Special attention is focused on the description of the use of the a.c. method for measuring specific heat under uniaxial stress. The application of isoperibolic calorimetry to specific heat measurements under uniaxial stress is described in detail. The method has been tested at low stress by studying the influence of uniaxial stress on the change of dimensionality of the order parameter of strontium titanate. Measurements of the specific heat and heat power exchanged by a deuterated dihydrogen potassium phosphate sample at the ferroelectric–ferroelastic phase transition are also included. These measurements show the advantages of this method for the study of first-order transitions under uniaxial stress.

Comment on "Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in

Comment on "Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in

Thermal hysteresis effects in the ferroelectric-ferroelastic phase transition in

The phase-transition sequence undergone by [N(${\mathrm{CH}}_{3}$${)}_{4}$${]}_{2}$${\mathrm{ZnI}}_{4}$ has been studied by adiabatic calorimetry. The specific-heat results show the P${112}_{1}$/a-P${2}_{1}$ca phase transformation around 215 K to take place in two independent steps. The corresponding specific-heat anomalies differ by only \ensuremath{\approxeq}0.4 \ifmmode^\circ\else\textdegree\fi{}C on heating, and \ensuremath{\approxeq}3 \ifmmode^\circ\else\textdegree\fi{}C on cooling. An unusual thermal hysteresis effect has been found to be present, which leads to the reversal in the order of appearance of the two specific-heat peaks. This fact has been unambiguously confirmed by means of an exhaustive experimental specific-heat study and by the use of the thermodynamic-functions values associated with this anomalous phase transformation.

Ferroelectric-ferroelastic phase transition in K3Fe5F15and derivatives

Abstract The ferroelectric-ferroelastic phase transition predicted in K3Fe5F15 from the previously determined atomic positions has been demonstrated experimentally. K3Fe5F15 is the first ferroelectric fluoride with a structure derived from that of tetragonal tungsten bronze. The transition is accompanied by reversible order-disorder among the Fe2+ and Fe3+ cations located near the centers of their fluorine octahedra. The predicted Curie temperature (Tc ) is 535 K whereas that determined by x-ray diffraction, optical, calorimetric and dielectric measurements is 490 K. The Curie temperature, spontaneous strain and entropy change in solid solutions of K3-x Fe5F15 (0 ≤ x ≤ 0.2) are found to decrease with increasing values of x as the transition becomes increasingly diffuse. The broadened transition may be related to a statistical distribution in the ionic ratio population.

Mössbauer study of Fe2+/Fe3+ order–disorder and electron delocalization in K3Fe5F15 at the 490-K phase transition

The prediction made in J. Appl. Phys. 65, 3987 (1989) that the ferroelectric–ferroelastic phase transition at Tc =490 K in K3Fe5F15 will be accompanied by a change from order to disorder among the Fe2+/Fe3+ ions has been confirmed by Mössbauer spectroscopy. The thermal dependence of both the Fe3+ quadrupolar splitting and the Fe3+ isomer shift exhibits a smooth decrease with increasing temperature to Tc, each with a sharp and reproducible change in slope above Tc. The average Fe2+ quadrupolar splitting (QS) similarly undergoes a major decrease with increasing temperature, with a clear change in slope above Tc. Furthermore, the thermal dependence of the Fe2+ QS distribution shows that the Fe2+/Fe3+ order required crystallographically below Tc is not conserved at higher temperatures. The proportion of Fe2+ present below Tc is close to the stoichiometric 60% value; the proportion decreases above Tc rapidly with temperature and becomes less than 50% by 543 K, consistent with increasing delocalization of an Fe2+ electron.

Dielectric properties of ferroelectric-ferroelastic Cd2Nb2O7under uniaxial pressure

Abstract Dielectric constant and spontaneous polarization P(T) of Cd2Nb2O7 under uniaxial pressure X≤4 MPa were measured. It was found that (1) phase transition (PT) temperature Ti. between two phases of low ionic conductivity is apparently frequency dependent (Ti.=330 K at 1 MHz and 353 K at 1 kHz), (2) ferroelastic PT at T1=205 K is the first-order PT close to the second-order, (3) the velocity of variation of ferroelectric-ferroelastic PT temperature (T2=196 K at X=0) is dT2/dX=4.3 K/MPa, which is higher than in other ferroelastics.

Investigation of the ferroelectric-ferroelastic phase transition in KH2PO4 and RbH2PO4 by means of γ-ray diffractometry

Without application of an external electric field, the temperature dependence of the shear angle uxy, which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma -radiation in KH2PO4 and RbH2PO4. Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a first-order one. The modified Slater's theory provides for RbDP a good fit to the experimental data uxy(T) in the whole temperature range investigated. The fact that beta =0.5 near Tc can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.

Critical behavior near the ferroelectric-ferroelastic phase transition of tanane

Critical behavior near the ferroelectric-ferroelastic phase transition of tanane