The thermodynamic functions of SbSBr crystal

Using density functional theory methods, the phonon density of states, Helmholtz free energy, internal energy, and entropy of ferroelectric and paraelectric phases are investigated. The temperature dependence of the free energy indicates that vibrational entropy contributes to the destabilization of the ferroelectric phase. The vibrational entropy of Sb, S, and Br atoms is attributed to the stabilization of ferroelectric SbSBr at the temperature T _c. Calculations indicated that SbSBr in ferroelectric phase become more stable than in paraelectric phase at temperatures lower than 22.8 K. The calculated temperature of ferroelectric phase transition is in good agreement with the experimental data.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb_0.50Sr_0.50TiO₃ thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO₃ perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f     

Phase transition and linear thermal expansion of (Pb1− x Ba x )ZrO3 ceramics

(Pb_{1? x} Ba _x )ZrO₃ ceramics for the composition range 0?≤?x?≤?0.30 were prepared by the mixed oxide solid state reaction method. Phase transition was studied by dielectric and dilatometric measurements. The ferroelectric to paraelectric phase transition temperature was progressively shifted to a lower temperature by replacing lead with barium. The x?=?0.20 sample showed the maximum dielectric constant of 16,300 at the transition temperature. For compositions 0?≤?x?≤?0.075, the antiferroelectric to ferroelectric phase transition exhibited a large linear thermal expansion. However, the antiferroelectric to ferroelectric phase transition did not exist for 0.10?≤?x?≤?0.30 samples. A phase diagram for PBZ ceramics prepared by the conventional mixed oxide method was also present.     

Spontaneous polarization and repolarization in crystals of the perovskite type

Based on molecular kinetic concepts, we have theoretically studied the processes of polarization and repolarization in ABO₃ crystals with a perovskite structure. We have calculated the free energy taking into account the possible displacements of the A atoms of the transition element from the equilibrium positions and the ordering of the dipole moments; we have found the dependence of the free energy for different phases of the crystal on temperature, the dipole order parameter, and the energy constants. We have obtained the thermodynamic equilibrium equation determining the temperature dependence of the degree of dipole order; we have found the Curie-Weiss transition temperature from the paraelectric to the ferroelectric state. Analysis of the equations obtained allowed us to provide a basis for the polarization and repolarization processes observed experimentally in BaTiO₃ and KNbO₃ crystals and to estimate the phase transition temperatures.Dnepropetrovsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 8–14, May, 1995.     

Depolarization field and properties of thin ferroelectric films with inclusion of the electrode effect

The influence of metallic electrodes on the properties of thin ferroelectric films is considered in the framework of the Ginzburg-Landau phenomenological theory. The contribution of the electrodes with different screening lengths l _s of carriers in the electrode material is included in the free-energy functional. The critical temperature T _cl , the critical thickness of the film, and the critical screening length of the electrode at which the ferroelectric phase transforms into the paraelectric phase are calculated. The Euler-Lagrange equation for the polarization P is solved by the direct variational method. The results demonstrate that the film properties can be calculated by minimizing the free energy, which has a standard form but involves the coefficient of the term P². This coefficient depends not only on the temperature but also on the film thickness, the surface and correlation effects, and the electrode characteristics. The calculations of the polarization, the dielectric susceptibility, the pyroelectric coefficient, and the depolarization field show that the ferroelectric state of the film can be destroyed using electrodes from a material whose screening length exceeds a critical value. This means that the electrodes being in operation can induce a transition from the ferroelectric phase to the paraelectric phase. The quantitative criteria obtained indicate that the phase state and properties of thin ferroelectric films can be controlled by choosing the appropriate electrode material.     

Ferroelectric phase transition in PbHPO4 studied by IR-spectroscopy of internal modes

The spectrum of the internal modes in PbHPO₄ from 300 to 1100 cm^-1 is measured in reflection between 77 K and 370 K, and mode frequencies are determined by Kramers Kronig analysis. The spectrum is the ferroelectric phase is in agreement with space group Pc. The temperature dependence of the modes can be attributed to proton disorder. There is no indication of PO₄³-distortion in the phase transition and local ferroelectric order still exists in the paraelectric phase.     

Dielectric relaxation in ferroelectric (CH3)2NH2AL(so4)2 · 6H2O crystal

Abstract

This paper presents the results of the investigation of dielectric dispersion and ultrasonic velocity in the ferroelectric (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal. The crystal shows a critical slowing down process of polarization with an extremely long relaxation time of the dipole system (τ = 1.6 · 10^?7s at the phase transition point). The dielectric response over the frequency range up to 56 GHz in the paraelectric phase can be well described in terms of a monodispersive Debye-type formula. The activation energy of dipoles in the paraelectric phase is 0.11 eV = 8.5 kT_c . The results show that the proper ferroelectric phase transition is nearly critical and of the order-disorder type.     

High-pressure dielectric studies of a novel hydrogen-bonded ferroelectric (NH4)2H2P2O6

The hydrostatic pressure effect on the dielectric properties of (NH₄)₂H₂P₂O₆ ferroelectric crystal was studied for pressures from 0.1 MPa to 360 MPa and for temperatures from 100 to 190 K. The pressure–temperature phase diagram obtained is linear with increasing pressure. The paraelectric–ferroelectric phase transition temperature decreases with increasing pressure with the pressure coefficient dT_c/dp=?5.16×10^?2 K MPa^?1. Additionally, the pressure dependences of Curie–Weiss constants for the crystal in paraelectric (C₊) and ferroelectric (C_?) phases are evaluated and discussed. The possible mechanism of paraelectric–ferroelectric phase transition is also discussed.     

Preparation and properties of Ba1−xCaxTiO3 nanopowders obtained by mechanochemical synthesis

ABSTRACT

The calcium-substituted barium titanate nanopowders Ba_1?xCa_xTiO₃ (0.2 ≤ x ≤ 0.3) have been obtained at room temperature by mechanochemical synthesis. The formation of the perovskite phase was controlled by X-ray diffraction studies at various milling duration. The powders possess the perovskite crystallographic structure directly after milling longer than 10 h. The dielectric properties of the ceramics obtained by sintering of the nanopowders were investigated in the temperature range between 300 and 500 K. The temperature dependence of permittivity exhibited a single anomaly, which corresponds to the ferroelectric–paraelectric phase transition.     

X-ray study of the phase transition in RbHSeO4

The ferroelectric phase transition in RbHSeO₄ has been examined by precise measurements of the temperature dependence of the unit-cell parameters using the Bond technique. The transition, at T_c = 370.6 K, is of first order and the triclinic unit cell of the ferroelectric phase transforms to an equitranslational monoclinic cell in the paraelectric phase. In a temperature region of several K below T_c reflections belonging to both phases have been observed.     

Ferroelectric phase transitions of the 0.32PIN-0.345PMN-0.335PT single crystals studied by temperature-dependent Raman spectroscopy,dielectric and ferroelectric performance

The ferroelectric phase transition characteristics of the 0.32Pb(In_1/2Nb_1/2)O₃-0.345Pb(Mg_1/3Nb_2/3)O₃-0.335PbTiO₃ (0.32PIN-0.345PMN-0.335PT) single crystals were studied by the temperature-dependent Raman spectroscopy and some electrical properties. Ferroelectric monoclinic phase was confirmed at room temperature by the numbers of the Raman modes. Successive ferroelectric phase transitions, i.e. ferroelectric monoclinic phase to ferroelectric tetragonal phase transition (FE_M-FE_T) and ferroelectric tetragonal phase to paraelectric cubic phase transition (FE_T-P_C), are evidenced by the anomalies of Raman modes line width, peaks intensity and their ratios around T_M-T and T_C/T_m temperatures. The temperature dependent permittivity derivative ξ = d?/dT not only provides further evidence of the successive ferroelectric phase transitions, but also demonstrates the second-order transition characteristic of the FE_M-FE_T phase transition and the first-order transition feature of the FE_T-P_C phase transition. The FE_M-FE_T phase transition is also confirmed by the abnormal narrowing of the P-E loops, decrease of the P_r and E_c values, and extremums of the pyroelectric performance.     

Size-induced appearance of ferroelectricity in thin antiferroelectric films

In the paper we consider size effects on phase transitions and polar properties of thin antiferroelectric films. We extend the phenomenological approach proposed by Kittel for thin films allowing for gradient (correlation) energy and depolarization field energy. Surface piezoelectric effect as well as misfit strain appear due to lattice constants mismatch between the film and its substrate. Direct variational method is used to derive the free energy with renormalized coefficients depending on the film thickness. Obtained free energy expression allows the calculation of phase diagrams and all electro-physical properties by a conventional minimization procedure. Approximate analytical expressions for the paraelectric–antiferroelectric–ferroelectric transition temperature dependences on film thickness, polarization gradient coefficient, and extrapolation lengths were obtained. The thickness dependence of the electric field critical value that causes antiferroelectric–ferroelectric phase transition was calculated. Under favorable conditions the antiferroelectric phase at first transforms into ferroelectric one and then into paraelectric phase with the decrease of the film thickness. Proposed theoretical consideration explains the experimental results obtained in antiferroelectric PbZrO₃ thin films.     

Calculation of phase diagrams for solid solutions of ferroelectrics

The integral equations for calculating ferroelectric and antiferroelectric phase transition temperatures, order parameters, and critical concentrations of solid solution components are derived. The electric dipoles randomly distributed in the system are treated as sources of random fields. The random field distribution function is constructed taking into account the contribution of nonlinear effects and the differences in the dipole orientations for different solid solution components. The dependence of the phase transition temperature on the composition of a binary solid solution in the ferroelectric-antiferroelectric and ferroelectric-paraelectric systems is calculated. Numerical calculations are carried out for the PbTi_xZr_1?x O₃ and BaZr_xTi_1?x O₃ solid solutions. The results obtained are in good agreement with the experimental phase diagrams of these systems. Analysis of the results indicates that any solid solution containing ferroelectric (antiferroelectric) and paraelectric components transforms into a relaxor state at sufficiently high concentrations of the paraelectric component.     

23Na nuclear magnetic resonance study of the phase transition in ammonium rochelle salt,NaNH4C4H4O6 • 4H2O

The satellite NMR spectrum of ²³Na in ammonium Rochelle salt (NaNH₄C₄H₄O₆ ? 4H₂O) is investigated near the transition temperature. Each line in the paraelectric phase splits discontinuously into four lines at the transition temperature; this fact is compatible with the first-order phase transition and reveals the existence of the superstructure in the ferroelectric phase.     

Internal energy and parameters of the order-disorder phase transition in titanium monoxide TiO y

Quantum-mechanical ab initio calculations are used to simulate the free energy functions for titanium monoxide TiO _y . The effect of the long-range order of the Ti₅O₅ type superstructure on the internal energy of the compound is studied by the supercell method. The dependences of the configuration entropy and free energy on the long-range order parameter are determined. It is found that the order-disorder phase transition in titanium monoxide must occur in accordance with the mechanism of the first-order phase transition with a critical value of the long-range order parameter of 0.971. The calculated parameters of the phase transition are compared with the experimental data and the results obtained using the model of point charges and by calculating the Madelung energy. It is concluded that the short-range order and the phonon entropy must be taken into account in calculating the equilibrium phase diagrams for strongly nonstoichiometric compounds.     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

Influence of magnetic field on the paraelectric to ferroelectric phase transition in BaTiO3

The first experimental results on the effect of a strong magnetic field on the stability of the ferroelectric phase in perovskites are presented. In BaTiO₃ a magnetic field of 20 T increases the paraelectric to ferroelectric phase transition temperature by about 0.3 K for E 6 B and by about 0.2 K for E ⊥ B.     

Thermodynamic properties of ZnSe under pressure and with variation in temperature

The thermodynamic properties of Zn Se are obtained by using quasi-harmonic Debye model embedded in Gibbscode for pressure range 0–10 GPa and for temperature range 0–1000 K. Helmholtz free energy, internal energy, entropy,Debye temperature, and specific heat are calculated. The thermal expansion coefficient along with Gruneisen parameter are also calculated at room temperature for the pressure range. It is found that internal energy is pressure dependent at low temperature, whereas entropy and Helmholtz free energy are pressure sensitive at high temperature. At ambient conditions,the obtained results are found to be in close agreement to available theoretical and experimental data.     

Observation of87Rb NMR satellite transitions in the commensurate and incommensurate phases of Rb2ZnCl4

It is shown that the first order quadrupole split NMR satellite transition frequencies of the⁸⁷Rb nucleus can be detected in the paraelectric, incommensurate and ferroelectric phases of Rb₂ZnCl₄. From rotation patterns the electric field gradient tensor at the Rb sites is determined for the paraelectric phase. The data demonstrate a considerable influence of the structural changes in the incommensurate and ferroelectric phases on the observed NMR transition frequencies. For some crystal orientations the satellite transitions are followed through the incommensurate into the ferroelectric phase. Whereas in the former typical quasi continuous spectra are observed in the latter several sharp lines appear. The results are discussed in relation to the structural changes at the phase transitions.     

氢键铁电体相变的格林函数理论(Ⅱ)

在文献[1]的基础上,计算了包括四体相互作用项的氢键铁电系统自由能,给出了确定铁电一级相变时序参数跃变和居里温度的表示式。通过对自由能在相变温度附近行为的详细分析,表明了微观理论结果与朗道热力学理论完全一致,并给出了朗道自由能表示式中各系数与微观相互作用参数之间的关系,使各系数的物理意义更加明确。再通过与PbHPO₄,PbDPO₄和H₂C₄O₄等晶体的实验结果比较,进一步证明本文理论能成功地处理氢键铁电体的一级相变和二级相变问题。 关键词：