Broad-band dielectric spectroscopy of Ba 2NaNb 5O 15 single crystal

Barium sodium niobate (BNN) single crystals are studied by IR spectroscopy, time-domain THz transmission spectroscopy, HF coaxial wave-guide technique and LF dielectric spectroscopy to cover the frequency range 10²-10¹⁴ Hz in a wide temperature interval. The dielectric response parallel and perpendicular to the polar c-axis is discussed. The ferroelectric transition at T _c = 830 K is driven by a relaxational soft mode coupled with another central-mode type relaxation which both gradually disappear on cooling in the ferroelectric phase. Below T _i the parameters of the expected IR active amplitudon were estimated. The low-temperature permittivity increase on cooling for the field direction has been explained by an incipient proper ferroelectric-ferroelastic transition driven by an IR and Raman active B₂-symmetry soft mode. Received 24 August 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: buixader@fzu.cz     

Spontaneous twisting of an incommensurable improper ferroelastic Ba2NaNb5O15

The spontaneous twisting of a y-orientation single-crystal sample of barium-sodium niobate Ba₂NaNb₅O₁₅ at a phase transition and into an incommensurable ferroelastic phase is investigated. Anomalies are found in the temperature dependence of the torsional deformation at temperatures 453, 533, and 573 K and anomalous hysteresis of the temperature dependences of the torsional deformation, measured on heating and cooling of the sample. The results obtained are discussed on the basis of an analysis of the temperature dependence of incommensurable 1q and 2q structures and ferroelastic twins. Fiz. Tverd. Tela (St. Petersburg) 41, 1848–1850 (October 1999)     

Time evolution of polar regions in (Sr0.65Ba0.35)2NaNb5 O 15 single crystal

The linear, second-order and third-order dielectric susceptibilities χ₁, χ₂, χ₃ of a (Sr_0.65Ba_0.35)₂NaNb₅O₁₅ (SBNN) single crystal have been measured. Strong relaxation was found in all investigated dielectric susceptibilities near the phase transition. The time dependence of linear and third-order dielectric susceptibility can be explained as the effect of polar nano-clusters evolution. Near the phase transition, the net polarization of the crystal, monitored by second-order dielectric susceptibility, changes both its value and orientation. Measurements of the pyroelectric effect indicate that the non-zero net polarization in SBNN crystals appears spontaneously. In addition, the ferroelectric phase transition in SBNN showed many features of a first-order phase transition.     

Influence of irradiation defects on the incommensurate phase of Ba2NaNb5O15

Preliminary observations of the incommensurate phase of barium sodium niobate under irradiation are reported here. Defects induce the stabilization of the incommensurate phase at room temperature although the effect of temperature on the incommensurability δ is still visible.     

Energy oscillations in a one-dimensional harmonic crystal on an elastic substrate

A one-dimensional harmonic crystal on an elastic substrate is considered as a stochastic system into which randomness is introduced through initial conditions. The use of the particle velocity and displacement covariances reduces the stochastic problem to a closed deterministic problem for statistical characteristics of particle pairs. An equation of rapid motion that describes oscillations of potential and kinetic energy components of the system has been derived and solved. The obtained solutions are used to determine the character and to estimate the time of decay of the transient process that brings the system to thermodynamic equilibrium.     

The decay of131Ba

The decay of¹³¹Ba has been studied by Ge(Li) detector supplemented by Ge(Li)-Ge(Li) fast-slow coincidence and NaI(Tl)-NaI(Tl) sum-coincidence spectrometer. The existence of 31 gamma rays and the levels at 78.8, 123.5, 133.4, 215.7, 373.0, 585.0, 620.0, 696.3 and 1084.4 keV have been confirmed. No evidences are found for the level at 1170 keV and 215.7?78.8 and 696.3?373.0 transitions. Some transitions are found to be connected with the impurities which were attributed to¹³¹Ba decay in earlier studies. The relative intensities and conversion coefficients have been determined and multipolarities of some of the transitions infered from the comparison of the conversion coefficients with the theoretical values.     

Inelastic light scattering studies of diffuse phase transition in ferroelectric Sr1.9Ca0.1NaNb5O15 thin films

The dynamic behavior of highly oriented ferroelectric Sr_1.9Ca_0.1NaNb₅O₁₅ thin films has been investigated by Brillouin light scattering over a wide temperature range between 25 and 450 °C. The temperature variations of the full‐width at half‐maximum (FWHM) of the central peak (CP) are found to exhibit marked changes around the Curie temperature (T_c = 246 °C). The CP reveals a relaxational mode behavior, which is attributed to thermally activated fast relaxation of polar nanoregions (PNRs). The width of the CP decreases steadily upon cooling from ∼400 °C to T_c and shows little subsequent change within the ferroelectric state, indicating an increase of the number of PNRs and a slowing down of their dynamics. By using a modified superparaelectric model, the activation energy (H₀) of the relaxation processes and the Burns temperature (T_B) are determined to be 2939 and 385 °C, respectively. The evidence for this diffuse phase transition provided by Brillouin scattering is in agreement with results obtained from the temperature‐dependent dielectric permittivity, refractive index, and Raman spectra. Copyright © 2011 John Wiley & Sons, Ltd.     

The character of cooperative low-frequency oscillations in a model of a biopolymer

The oscillations in a model of a biopolymer are calculated under various excitation conditions. Relatively slow cooperative motions, which can be used to simulate conformational transitions in a biological macromolecule from one stable state to another, are analyzed.     

The effect of low-frequency oscillations on cardio-respiratory synchronization

We show that the transitions which occur between close orders of synchronization in the cardiorespiratory system are mainly due to modulation of the cardiac and respiratory processes by low-frequency components. The experimental evidence is derived from recordings on healthy subjects at rest and during exercise. Exercise acts as a perturbation of the system that alters the mean cardiac and respiratory frequencies and changes the amount of their modulation by low-frequency oscillations. The conclusion is supported by numerical evidence based on a model of phase-coupled oscillators, with white noise and lowfrequency noise. Both the experimental and numerical approaches confirm that low-frequency oscillations play a significant role in the transitional behavior between close orders of synchronization.     

Low-temperature specific heat of Er2Ba4Cu7O15-δ

The magnetic ordering of Er ions in Er₂Ba₄Cu₇O_15-δ (δ = 0:7 and δ = 0:08) has been studied by low temperature heat capacity measurements. The Néel temperature T _N of Er₂Ba₄Cu₇O_15-δ is almost independent of the oxygen concentration (TN = 0:54K for δ = 0:08 and T _N = 0:50K for δ = 0:7). While an anisotropic two-dimensional (2D) Ising model describes the experimental data very well for Er₂Ba₄Cu₇O_14:92, this model cannot be applied to the oxygen reduced sample Er₂Ba₄Cu₇O_14:3. By using a model consisting of 1D-Ising chains and of 2D-Ising clusters of Er³⁺ ions we could fit the specific heat data of Er₂Ba₄Cu₇O_14:3 accurately. Our measurements show clearly that with the reduction of the oxygen content the in-plane anisotropy of the magnetic exchange coupling increases.     

High-Q oscillations in a layered elastic medium

Conditions of the existence of long-lived acoustic resonances that occur in a layered medium because of its oscillation with low damping factors are considered. A priori theoretical estimates relating the distribution of resonance frequencies in the complex plane to the parameters of inhomogeneity of the layered system are obtained. A scheme of resonance calculations in numerical modeling is described. Examples of geophysical media with long-lived resonances are presented.     

Identification of a low-frequency elastic behaviour in liquid water

This article deals with the identification of solid-like properties measured at room temperature at a sub-millimetre length scale in liquid water. At a macroscopic scale, normal liquids (i.e.?above melting temperature), and in particular water, are typically and empirically defined by the absence of shear elasticity, in contrast to solids or plastic fluids that require a stress threshold for flowing. A novel method optimizing the transmission of the shear stress to the sample enables a more complete probe of the mechanical response of liquids. It reveals that glass formers and viscous alkanes actually exhibit finite macroscopic shear elasticity away from any phase transition. This protocol is here applied for the first time to liquid water at room temperature, revealing, at the sub-millimetre scale, a low-frequency solid-like property.     

Ba5-xLaxTixNb4-xO15和Ba5-xLaxTixTa4-xO15陶瓷的振动光谱和微波介电性能

采用高温固相合成法制备Ba5-xLaxTixNb4-xO15和Ba5-xLaxTixTa4-xO15(x=1,2,3)陶瓷,通过XRD、Raman测试方法表征其结构,并测试其微波介电性能。拉曼光谱显示:两体系中[NbO6]/[TaO6]氧八面体的A1g(O)模都随着x的增加向低波数移动,此模在铌酸盐体系中强度逐渐升高,在钽酸盐体系中峰劈裂加剧。介电性能结果显示:Nb/Ta体系样品的Q×f值随x增加而增加。因此:随La3 ,Ti4 取代量的增加,B位阳离子的结构有序度逐渐增加,Q×f值也随之增加,这都与[NbO6]/[TaO6]的伸缩振动频率A1g(O)的变化有关。     

Hydration behavior of Ba2Sc2O5 with an oxygen-deficient perovskite structure

The hydration behavior of the barium scandates with oxygen-deficient perovskite-related structures of tetragonal Ba₂Sc₂O₅ and cubic BaScO_3−δ (δ ∼ 0.5) was studied. Both the tetragonal Ba₂Sc₂O₅ and cubic BaScO_3−δ absorbed water and formed Ba₂Sc₂O₅·0.60H₂O and BaScO_3−δ·0.37H₂O, respectively, during cooling in a humidified atmosphere. The hydration and the dehydration temperatures for the barium scandates were ∼ 533 K and ∼ 553 K, respectively; these temperatures were 30–70 K lower than that for Ba₂In₂O₅. A distinct difference in the hydration behavior of the structurally different tetragonal Ba₂Sc₂O₅ and cubic BaScO_3−δ was not observed. The Ba₂Sc₂O₅ shows protonic conduction below 673K and p-type conduction above 773K in a humidified atmosphere.