Acoustic properties of glycine phosphite crystals with an admixture of glycine phosphate

Acoustic and dielectric anomalies in the region of the ferroelectric phase transition in crystals of glycine phosphite (GPI) with a 2 mol % admixture of glycine phosphate (GP) are studied. The acoustic anomalies were found to differ strongly from those observed in nominally pure glycine phosphite crystals. A theoretical analysis of the acoustic and dielectric properties of the crystals was carried out within the model of a pseudoproper ferroelectric phase transition. It is shown that the acoustic anomalies, as well as the temperature dependences of the dielectric constant (for various external electric fields) and pyroelectric current observed in the vicinity of the phase transition in GPI-GP crystals, can be adequately described when the macroscopic polarization present in these crystals above the phase transition temperature is taken into account. The thermodynamic-potential parameters describing electrostriction and the biquadratic relation between the polarization and strain turned out to be close to those characterizing a nominally pure GPI crystal. An irreversible phase transition was observed to occur in GPI-GP crystals at T = 240 K, i.e., above the ferroelectric phase transition temperature.     

Studying the current-voltage characteristics of liquid-crystal langmuir films in the region of the ferroelectric phase transition

The electrophysical properties of hyperfine films of liquid crystals are studied. At temperatures near the phase transition, singularities in the curves of the temperature dependence of the sample capacitance are found. Hypotheses regarding the nature of the phase transition are proposed. A change in the electrical conductivity of the samples based on a TDOB AM BCC liquid crystal is detected upon the ferroelectric phase transition. Hypotheses regarding the causes of the influence of the phase transition on the electrophysical properties of the films are put forward.     

Optical study of the spontaneous ferroelectric phase transition in lead scandoniobate single crystals

A study is reported of the optical transmission and small-angle light scattering (SAS) in ordered and disordered stoichiometric single crystals of lead scandoniobate (PSN) with a zero and a dc electric field applied. It is shown that the spontaneous phase transition occurring in both crystals is accompanied by a sharp peak in SAS intensity, which implies the percolation nature of this transition. A field-temperature phase diagram has been constructed for the PSN single crystals studied in the work.     

Determination of the liquid crystals phase transition temperatures using optical rotation effect

Using optical rotation effect, a sensitive, simple optical analytical system is developed for determining the phase transition temperatures of liquid crystals (LCs). When a monochromatic polarized light passes through LCs sample and analyzer, the light intensity changes with temperature. Especially, during the phase transition process, the intensity varies greatly due to optical rotation effect. The variation of light intensity versus variation of temperature curve shows the phase transition temperatures of LCs clearly. The phase transition temperatures of three cholesteric liquid crystals (ChLCs) and a nematic liquid crystals (NLCs) were detected by this method, and compared with those of the differential scanning calorimetry (DSC) and polarized light microscope (PLM) methods.     

Spectroscopic study of the behavior of the order parameter in model ferroelastic crystals of Hg<Subscript>2</Subscript>Cl<Subscript>2</Subscript>

Information is obtained about the temperature behavior of the order parameter of a phase transition by theoretical and experimental investigation of odd (acoustic and IR-active) phonons that appear in the Raman scattering spectra from the X points of the Brillouin zone (BZ) boundary in the paraphase of Hg₂Cl₂ crystals and are induced by the phase transition, unit-cell doubling, and the X → Γ folding in the BZ. The relevant critical exponents are determined, whose values are in agreement with the results of X-ray diffraction measurements and, within the Landau phenomenological theory of phase transitions, indicate that the phase transition in these crystals is close to the tricritical point.     

Effect of electric field on the percolation phase transition in lead scandotantalate single crystals

The effect of a dc electric field on the phase transition process in lead scandotantalate single crystals differing in the degree of ion ordering has been studied by small-angle light scattering (SAS). The spontaneous phase transition occurring in these crystals is shown to be accompanied by a sharp SAS intensity peak indicating the percolation nature of this transition. A phase diagram in the field-temperature coordinates has been constructed for all the PST crystals studied in the work. The electric field and the temperature variation rate have been found to affect the SAS intensity.     

Features of the spectral intensity of scattered light near the phase transition point in quartz crystals

The isofrequency dependences of inelastic light scattering in quartz crystals near the phase transition point are calculated taking into account both spatially inhomogeneous fluctuations of the order parameter and the finite spectral width of the spectrometer slit. It was found that the theory is in satisfactory agreement with the experimental data on dynamic opalescence observed in quartz crystals during phase transformations.     

Phase transition in diserine sulfate monohydrate crystals

The temperature dependences of the real and imaginary parts of the complex permittivity of diserine sulfate monohydrate crystals and of their electrical response in different crystallographic directions to a change in temperature are investigated. It is confirmed that these crystals undergo a first-order phase transition in the temperature range 260–270 K. This phase transition is assumed to originate from the freezing of the orientational degrees of freedom of the crystallization water molecules involved in these crystals.     

Wetting by solid helium,a model system

This is a review of the wetting properties of solid helium on various solid substrates. Due to its extreme purity and to its very fast growth dynamics, solid helium 4 is often considered as a model system in materials science. Several wetting phenomena have been studied with helium 4 crystals, namely contact angles on solid substrates with variable roughness, wetting on graphite where epitaxial growth takes place, the roughening transition as a function of film thickness, the wetting of grain boundaries by the liquid phase.     

Differential scanning calorimetry study of the phase transition in diserine sulfate monohydrate crystals

Differential scanning calorimetry (DSC) was used under the highest resolution conditions to study the phase transition in diserine sulfate monohydrate crystals. In the temperature range 255–270 K, the DSC signal was found to exhibit sharp Λ-shaped peaks with a thermal hysteresis of 8.5 K, which indicates the occurrence of a first-order phase transition. The experimental data are used to determine the thermodynamic parameters of the phase transition.     

Thermodynamics of the surface of <Superscript>4</Superscript>He crystals

The conditions for applicability of the mean field theory for describing the thermodynamics of the surface of ⁴He crystals are investigated based on analysis of experimental data. It is found that although the faceting phase transition itself is a Berezinsky–Kosterlitz–Thouless phase transition, the thermodynamic potential outside a narrow neighborhood of the transition temperature can be expanded into a series in terms of the Landau theory of second-order phase transitions.     

On the universal regularities of the lattice dynamics in ferroelectrics

The main regularities revealed for the dynamic response in recent Raman spectroscopic investigations of ferroelectric crystals of the displacive type (LiNbO₃, LiTaO₃) and the order-disorder type (KDP, DKDP) are discussed. The characteristic feature of the dynamic response for all the crystals (KDP, DKDP, LiNbO₃, LiTaO₃) under investigation in the vicinity of the phase transition temperature is an intense central peak that follows from the theoretical predictions for the order-disorder phase transition. The possible factors responsible for the central peak, the character of the behavior of this peak outside the aforementioned temperature range, and the specific features of the behavior of the width and intensity of the central peak over a wide range of temperatures for order-disorder crystals are analyzed.     

Computer simulation of the deformation and fracture of crystals

Methods for computer simulation of strength testing of crystals are proposed. The methods employed are similar to usual static methods, and they are used to investigate deformation and fracture of perfect fcc crystals having different orientations with respect to the tensile force. A strain-induced phase transition from the fcc to the hcp structure is detected, and the formation and displacement of crystal twins are observed. Plastoelastic deformation and fracture of crystals are investigated.     

拉曼光谱研究不同风化过程中七水硫酸镁晶体相转变行为

通过不同的相对湿度(relative humidity,RH)调节方法,获得了 MgSO4·7H2O 晶体不同风化过程中的原位共聚焦拉曼光谱,实现了不同阶段相转变细节的观测.在快速和慢速风化过程中,MgSO4·7H2O 晶体在 RH 3%时均转变生成 MgSO4·3H2O 晶体.而在潮解过程中,不同风化过程形成的 Mg...     

Raman spectroscopic study of the phase transitions induced by hydrostatic pressure in a Rb2KScF6 crystal

The Raman spectra of Rb₂KScF₆ elpasolite crystals are studied in the pressure range up to 7 GPa. A phase transition is revealed at a pressure of approximately 1 GPa. Analysis of the changes in the spectral parameters shows that the phase transition is accompanied by a doubling of the volume of the primitive cell in the initial cubic phase. Judging from the character of the variations in the pressure dependences of the frequency of the observed vibrations, there can exist another transition to the phase with a lower symmetry at a pressure of approximately 2.1 GPa.     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15 K 到室温范围的介电特性及热电特性的研究. 分析其介电特性和热电特性与极化电场的关系, 用x 射线粉末衍射进行佐证. 证实在50 一70 K 的范围内, SBN , PBN , KNSBN 三类铁电钨青铜铌酸盐晶体均存在着一个新的相变. 相变是由点群4mm铁电相到点群mm2 铁电相的转变. 铁电自发极化方向由四方晶胞的c轴方向转变到正交晶胞的b 轴方向.介电特性的高频及低频测量表明该相变具有扩散(或称弥散)型特征. 比热的实验结果证明相变是属于高于一阶相变的高阶相变. 对相变前后的晶胞结构提出了一个模型解释. 关键词：     

Effect of charge carriers on the magnetic properties of the CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript> ferromagnetic semiconductor

The results of an investigation into how the composition of annealing mixtures influences the Curie temperature of single crystals of the CdCr₂Se₄ ferromagnetic semiconductor are reported. The mechanism of doping of single crystals is analyzed, and the role played by each of the components of the annealing batch is established. It is concluded that the indirect exchange involving n-type carriers near impurity gallium ions leads to a sharp increase in the phase transition temperature (from 130 to 172 K) of the samples. The dependences of the magnetization of samples with different phase transition temperatures on the temperature and magnetic field are compared.     

On magnetic transition of the plane spiral in a commensurate intermediate state

The temperature dependence of the wave vector of a plane spiral in rhombohedral crystals is studied. The conditions are shown under which there occurs temperature-induced transition of the spiral structure into intermediate commensurate phase. The instability of paramagnetic phase with respect to the formation of magnetic spiral near a Lifshitz point is investigated.     

Probing the material properties and phase transitions of ferroelectric liquid crystals by determination of the Landau potential

The full Landau potential of several, widely varying ferroelectric liquid-crystalline materials has been experimentally determined. Tilt angle and polarisation data is analysed across the SmA to SmC transition for varying applied electric-field amplitudes, allowing the determination of all the coefficients of the generalised Landau model of ferroelectric liquid crystals. The materials investigated encompass different materials, including low-polarisation mixtures to high-polarisation single-component materials. The materials also possess a variation in the order of the SmA to SmC phase transition from strongly first order to strongly second order. The effects of both the polarisation and order of phase transition of the system are discussed with respect to the various terms of the generalised Landau model. Further, the mechanisms behind the difference between a first- and second-order phase transition are discussed with respect to the Landau potential and the second Landau coefficient b .     

The structural and optical properties of ZnO bulk and nanocrystals under high pressure

The elastic properties of high-quality ZnO crystals and nanopowder of grain size of about 65 nm are studied for both wurtzite (low pressure) and rock-salt high pressure phases. The measured values of bulk moduli for wurtzite and rock-salt phases of bulk ZnO crystals are equal to 156±13 and 187±20 GPa, respectively, and considerably larger for ZnO nanocrystals. The phase transition begins at a pressure of about 9 GPa and it is completed at a pressure of about 13.8 GPa for bulk crystals, whereas the values of pressure at which the phase transition occurs are lower for nanocrystals. A carefull Rietveld analysis of the obtained data does not exhibit the presence of any intermediate phases between low pressure wurtzite and high pressure rock-salt phases of ZnO. The phase transition is accompanied by a strong decrease in the near-band-gap photoluminescence intensity. In addition, the pressure coefficient of the near-band-gap luminescence in ZnO nanocrystals exhibits strong deviation from the linearity observed in bulk crystals. An analysis of the results shows that defects present in the nanopowdered sample are responsible for the observed effects.