Critical slowing down in betaine phosphate as studied by nuclear magnetic relaxation

Proton and phosphorus spin-lattice relaxation time (T₁) measurements were applied to study the antiferroelectric phase transition in betaine phosphate (CH₃)₃NCH₂COO·H₃PO₄ at 86 K. In the whole temperature range studied, the T₁ or ¹H was governed by a dipolar fluctuation caused by the methyl- and the whole butyl-group reorientations. In the vicinity of the phase transition the T₁ of ³¹P shows a sharp dip which is a characteristic of critical slowing down. The analysis of the T₁ based on the order-disorder mechanism with respect to the position of the acidic protons leads to the critical exponent for the correlation length v=0.5.     

Raman scatteribg and phase transitions in Cd2Nb2O7

Raman scattering and electric permittivity of Cd₂Nb₂O₇ were studied between 4 and 230 K. Phase transitions at T₆ = 44 K and T₇ = 19 K have been discovered. It is hggested that T₆ = 80 K and T = 44 K correspond to normal-incohmensurate and ihcommensurate-commensurate phase transition points respectively.     

Ruman scattering at phase transitions in NH4HSeO4

Raman scattering of NH₄SeO₄ was investigated in the region of 0-1000 cm^-1 at phase transitions between 70 and 296 K. It was observed that at T₂, T₃, T₄, T₅ the intensity of Raman lines and its temperature dependence changed. It is suggested that between T₅ = 98 K and 296 K triclinic deformation of the lattice is delicate and below T₅ the unit cell belongs to the space group P₁.     

Thermodynamic Theory of Single-Crystal Lead Titanate with Consideration of Elastic Boundary Conditions

A phenomenological free-energy function including the effects of elastic boundary conditions was presented and used to investigate the single-domain, single-crystal properties of the ferroelectric perovskite, PbTiO₃. In particular, the effects of tensile and compressive hydrostatic stress on the spontaneous polarization, Curie point, dielectric susceptibility, and piezoelectric property coefficients were examined. The calculated shift of the Curie point with hydrostatic stress, along with the entropy and enthalpy of the ferroelectric-paraelectric phase transition, was found to be in good agreement with experimental measurements. The isothermal variation of the relative dielectric susceptibility and piezoelectric coefficients with hydrostatic stress exhibited the expected behavior near the ferroelectric-paraelectric phase transition.     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_0.67(PbTiO₃)_0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε'₁₁₁ (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.     

Heat Capacity and Structural Relaxation of Enthalpy in As2Se3 Glass

The evolution of enthalpy of As₂Se₃ glass during structural relaxation in the glass transition region was measured via differential scanning calorimetry for two types of time-temperature programs: rate-heating at 10 K/min following a cool at a constant rate (-20 to -0.31 K/min) and isothermal annealing following a temperature step from an equilibrium state. The rate-heating data yield kinetic parameters for the structural relaxation which predict accurately the evolution of the enthalpy during isothermal annealing. The glass heat capacities were independent of cooling rate within experimental precision (≤0.2%). In this respect, As₂Se₃ is unlike previously studied glasses whose heat capacities are more dependent on thermal history.     

Internal Friction, Dielectric Loss, and Ionic Conductivity of Tetragonal ZrO2-3% Y2O3 (Y-TZP)

The defect structure in 3 mol% Y-TZP was studied by correlated internal friction, dielectric loss, and ionic conductivity experiments. A prominent mechanical and dielectric loss peak occurs in the temperature range between 380 and 550 K that depends on the frequency of measurement. The relaxation parameters were determined as H_m = 90 ± 3 kJ·mol⁻¹, τ_∞= (1.0_+1.5^−0.6) × 10₋₁₄ s for the mechanical relaxation and H_d = 84 ± 3 kJ·mol⁻¹, τ_∞= (1.6_+1.7^−0.9) × 10^–13 s for the dielectric relaxation. The ionic conductivity below 790 K is controlled by an activation enthalpy of H _σ= 89 ± 3 kJ·mol⁻¹; at higher temperatures H _σ= 60 ± 3 kJ·mol^–1. An atomistic model is presented which assumes that oxygen vacancies are trapped by yttrium ions forming anisotropic complexes which—by reorientation—cause anelastic and dielectric relaxation. At higher temperatures (>790 K) these complexes are dissociated, which leads to the reduced activation enthalpy for ionic conductivity.     

New Phase Transitions in Perovskite Oxides: BiFeO3, SrSnO3, and Pb(Fe2/3W1/3)1/2Ti1/2O3

This paper reports on two new aspects of oxide perovskites: the first part is devoted to new phase transitions, especially at high temperatures; multiferroics such as BiFeO₃ and Pb–Fe–W–titanate are emphasized but nonmagnetic materials such as SrSnO₃ are included. The work summarized on bismuth ferrite emphasizes its metal–insulator transition near 1200 K (atmospheric pressure) and 47 GPa (room temperature); that on SrSnO₃ emphasizes order–disorder phase transitions; and that on lead–iron tungstate–titanate exhibits a classic second-order ferroelectric phase transition, of which rather few are known in the literature. The second part of the paper presents a discussion of constant phase elements for oxide perovskite ceramics; this is a modern way of characterizing their dielectric relaxation, particularly near phase transition temperatures.     

Weak ferroelectricity in monoclinic RbD2PO4 crystals

Ferroelectric phase transition was detected in monoclinic RbD₂ PO₄ at T_C3 =313K in addition to the known antiferroelectric phase transition at T_C2 =319K. The almost saturated value of the spontaneous polarisation is about P_sy = 6.10^-4 μC/cm² at 280K.     

Field-Induced Strain in Single-Crystal BaTiO3

The electric-field-induced strain in single-crystal BaTiO₃ was investigated. For crystals relatively free of twinning, a longitudinal strain of 0.35% can be induced just above the ferroelectric-paraelectric phase transition temperature (T_c1) primarily by field-forced paraelectric-ferroelectric phase transition. For heavily twinned crystals, 90° domain reorientation under the applied electric field plays an important role in the induced strain below T_c1, and an induced strain of 0.6% is observed a few degrees below T_c1. Above T_c1, the electrostrictive property measured by a weak excitation field is purely intrinsic. When the excitation field is large, so that a field-forced paraelectric-ferroelectric phase transition is involved, the x₃₃/P²₃ value (where x₃₃ and P₃ are the induced strain and polarization along the z axis, respectively) is intrinsic at higher temperature, but may be modified at temperatures just at and slightly above T_c1 by residual 90° twin structure.     

Ferroelectric Behavior in Barium Zirconium Metaniobate

An unusual type of ferroelectric behavior has been observed in barium zirconium metaniobate, BaNb_1.5Zr_0.25O_5.25. The orthorhombic, pseudotetragonal single crystal displays dielectric hysteresis only in its c direction, with normal dielectric properties in the a-b plane. No evidence of a fist- or second-order ferroelectric-paraelectric transition has been observed up to 1400°C. The crystal melts incongruently at about 1525°C. A maximum in the dielectric constant occurs at 80°C. when measured at 10³ c.p.s. but is not accompanied by any effect in the spontaneous polarization. The temperature of this k' maximum has the frequency dependence of a dielectric relaxation process with an average activation energy of 0.2 e.v. These properties are consistent with the idea of a higher than second-order ferroelectric-paraelectric transition in this material. The dielectric behavior is explicable in terms of 180°"dipole aggregates" having a wide range of activation energies of reversal. There are suggestions in the technical literature of similar behavior in other materials.     

Dielectric Properties of Pb(Fe2/3W1/3)O3–PbTiO3 Solid-Solution Ceramics

The dielectric properties of (1− x )Pb(Fe_2/3W_1/3)O₃· x PbTiO₃ solid solutions were investigated from 10² to 10⁶ Hz in the temperature range 150–600 K. The phase transition of Pb(Fe_2/3W_1/3)O₃ (PFW) was shifted by PbTiO₃ (PT) additions to higher temperatures at a rate of 6.3 K/mol% of PT. The temperature dependence of dielectric permittivity showed a sharper transition as the PT content increased. Dielectric measurements in a wide temperature range showed the presence of a second set of dielectric peaks at higher temperatures (350–600 K), besides the ferroelectric–paraelectric phase transition. This second set of peaks vanished when the samples were annealed in nitrogen. The activation energy values for the second relaxation varied between 0.50 and 0.63 eV, in agreement with the conduction activation energy determined for each sample. This relaxation is apparently related to electron holes.     

Calorimetric study of the ferroelectric phase transitions in CsLiWO4 crystal

The specific heat of pseudoproper ferroelectric-ferroelastic CsLiWO₄ was measured in the temperature range from 90 to 305 K. The enthalpy and entropy changes at phase transitions were determined. Interpretation of the data associated with the transition G_o → G₁ is done in terms of the thermodynamical theory.     

Kinetics of Enthalpy Relaxation at the Glass Transition in Ternary Tellurite Glasses

The kinetics of enthalpy relaxation (recovery) at the glass transition in x K₂O·(20− x )MgO·80TeO₂ glasses has been examined from heat capacity measurements using differential scanning calorimetry to clarify the features of the structural relaxation in ternary TeO₂-based glasses. Ternary glasses such as 10K₂O·10MgO·80TeO₂ show high thermal resistance against crystallization compared with binary glasses. The degree of fragility m estimated from the activation energy for viscous flow E _η and the glass transition temperature T _g is m = 55–62, indicating a fragile character in TeO₂-based glasses. Large heat capacity changes of 43.1–48.2 J·mol⁻¹·K⁻¹ are also observed at the glass transition. The activation energy for enthalpy relaxation Δ H is evaluated from the cooling rate dependence of the limiting fictive temperature, and values of Δ H = 897–1268 kJ·mol⁻¹ are obtained. Negative deviation from additivity in Δ H is also observed. Values of the Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from Δ H and T _g are 0.33–0.42 K⁻¹. It has been proposed that ternary glasses have more homogeneous and constrained glass structure compared with binary glasses.     

Electric Relaxation Studies of Defects and Defect Associates in Dilute Ceria-Lanthanum Oxide Solid Solutions

Electric modulus formalism was used to study relaxation processes in the 0.75-mol%-La₂O₃-doped CeO₂ system between 387 and 470 K in the frequency range 5 to 10⁷ Hz. Two relaxation processes were observed in the isothermal studies: one at low frequencies due to long-range migration of free oxygen vacancies     (process A) and one at high frequencies due to charge reorientation relaxation of (La_CeV_o'defect associates (process B). The relaxation processes were analyzed using a nonexponential decay function (φ(t) = exp[—(t/τ_oβ] for 0<β1) of the electric field. The activation enthalpy estimated for process A is 0.72 eV and for process B is 0.55 eV.     

Dielectric and Structural Investigations of the System BaTiO3-BaHfO3

Dielectric measurements and X-ray diffraction studies were made in the system BaTiO₃-BaHfO₃ for polycrystalline specimens containing from 0 to 30 mole % BaHfO₃. The maximum in the dielectric constant for each composition in creased as the BaHfO₃ concentration was in creased to 16 mole % and then decreased with further BaHfO₃ additions. Room-temperature lattice constants and hysteresis loops were meas ured for all compositions. A partial solid-state phase diagram is suggested for the solid solution area of the system. The phase diagram and the dielectric behavior of the system up to 16 mole % BaHfO₃ are explained on the basis of Devonshire's thermodynamic theory. The ferroelectric-paraelectric transition in the composition containing 16 mole % BaHfO₃ was essentially of second order and occurred between a ferroelectric rhombohedral phase and a paraelectric cubic phase.     

Connection between structural phase transitions and localized states in glasslike solids

Experimental investigations of the glassy dielectric absorption maximum at low temperatures between 10 K ≤ T_M ≤ 100 K at 14 different ferroelectrics are presented. From these investigations one can conclude that a connection between localized states, which cause this absorption maximum, and the structural phase transition ferroelectric-paraelectric exists.     

三元低共熔离子液体中CO2的吸收

由N, N-二甲基乙酰胺、氯化胆碱、乙二醇或丙三醇以不同的摩尔比(1:1:3, 1:1:4)合成了一系列三元低共熔离子液体(n_DMA:n_CC:n_{ethylene glycol}=1:1:3, 1:1:4, n_DMA:n_CC:n_glycerol=1:1:3, 1:1:4)。在293.15～323.15 K温度下, 间隔10℃, 0～600.0 kPa压力范围内, 用等温饱和法测量了CO₂在三元体系中的溶解度。CO₂在体系中的溶解度随压力增大呈线性增大趋势, 随温度升高而减小。计算了亨利常数, 结果表明, CO₂在由N, N-二甲基乙酰胺, 氯化胆碱, 乙二醇以摩尔比1:1:3合成的三元体系, 温度为293.15 K下, 亨利常数最小, 最小值为2.174 MPa·kg·mol^-1。报道了关于CO₂吸收的热动力学性质, 包括焓变、熵变、Gibbs自由能变。其中, 焓变为负值, 说明此吸收为放热过程。     

Analysis of Structural Relaxation in Glass Using Rate Heating Data

A method was developed to determine the kinetic parameters controlling structural relaxation in the glass transition region from data acquired during continuous heating or cooling. The nonexponential character of the relaxation is accounted for by assuming an equilibrium isothermal relaxation function of the form exp [−( t /T_oe) ^ß ], where T_oe is a relaxation time and 0< β 1. The data are linearized using the method of Narayanaswamy, and the continuous temperature variation during heating or cooling is dealt with by invoking the superposition principle. The analysis yields the kinetic parameters A , the relaxation-time preexponential term; Δ h ^★, the relaxation-time activation enthalpy; x , a term describing the relative effects of temperature and structure on the rate of relaxation; and β. The method was applied to analysis of the variation of the enthalpy of vitreous B₂O₃ during rate heating through the transition region following rate cooling through the same region at a variety of rates.     

Tetragonal-to-Cubic Structural Phase Transition in Pollucite by Low-Temperature X-ray Powder Diffraction

The tetragonal-to-cubic structural phase transition in pollucite (CsAlSi₂O₆) was investigated using low-temperature X-ray powder diffraction in the temperature range of 93 to 298 K, and the lattice constants were refined with Rietveld analysis. It was found that CsAlSi₂O₆ had a tetragonal phase with a space group of I 4₁/acd in the temperature range of 93 to 248 K, a = 1.36337(4) nm, c = 1.36988(6) nm at 248 K, and underwent a phase transition from tetragonal to cubic with a space group of Ia-3 in the temperature range of 248 to 273 K, a = 1.36645(3) nm at 273 K.