Correlated Huang scattering in hexagonal Jahn Teller compounds

This is a theoretical investigation on structural precursors in diffuse neutron scattering of disordered phases of cooperative Jahn-Teller compounds above their structural phase transitions. The theory is based on Huang scattering off elastic dipoles. Softening of shear modes in the smallq regime known from inelastic scattering predicts monoclinic and orthorhombic structures which are found in some of these compounds. For these smallq the Jahn-Teller effect looks dynamic and Huang scattering is not applicable. With larger wave vectors the dipoles look static and Huang theory works but one picks up neighbor correlations which have to be included. We believe to have identified the spiral correlation announcing the helical structure of CsCuCl₃ in its ordered phase. We also calculate diffuse scattering for RbCrCl₃ and the experimentally observed satellites announcing the cell doubling of the ordered phase.     

Specific features of phase transitions in molecular crystals of diols

A comparative analysis of the phase transitions in molecular crystals of diols with different chain lengths [1,15-pentadecanediol (CH₂)₁₄(OH)₂ and 1,16-hexadecanediol (CH₂)₁₅(OH)₂] has been performed using differential scanning calorimetry. A quantitative analysis of the temperature dependence of the heat capacity within the theory of diffuse (Λ-shaped) first-order phase transitions and the use of the temperature hysteresis have revealed a number of new effects associated with the specific features of the phase transitions in molecular crystals of diols.     

0.67 PbMg1/3Nb2/3O3-0.33 PbTiO3单晶中软模模式指认

应用显微拉曼技术对弛豫型铁电体0.67PbMg1/3Nb2/3O3-0.33PbTiO3(0.67PMN-0.33PT)的偏振拉曼光谱随温度变化的研究表明,在-196到600°C温度范围内0.67PMN-0.33PT存在两次相变:三方到四方(R-T)相变和四方到立方(T-C)相变。R-T相变的特征是106cm-1软模(-196°C时)随温度的升高而湮没到80cm-1"静态"模式中。依照群论分析,0.67PMN-0.33PT的80cm-1和106cm-1(-196℃时)两个模式分别为E(1TO)和A1(1TO)模。文章对三方相的对称结构(C3v点群)在背向散射配置下不同偏振方向的拉曼散射效率进行了计算。计算结果表明晶体中的A1(1TO)和E(1TO)模式能够通过使用不同的散射配置进行识别。理论计算和实验结果相比较得到了很好的吻合。     

Thermal diffuse scattering in sub-angstrom quantitative electron microscopy-phenomenon,effects and approaches

This paper reviews the recent progress in the following areas. (1) In quantitative high-resolution transmission electron microscopy, the theoretically calculated images usually give better contrast than the experimentally observed ones although all of the factors have been accounted for. This discrepancy is suggested due to thermal diffusely scattered (TDS) electrons, which were not included in the image calculation. The contribution from TDS electrons is especially important if the image resolution is approaching 0.1 nm and beyond with the introduction of Cs corrected microscopes. A more rigorous multislice theory has been developed to account for this effect. (2) We proved that the off-axis holography is an ideal energy filter that even filters away the contribution made by TDS electrons in the electron wave function, but conventional high-resolution microscopy do contain the contribution made by phonon scattered electrons. (3) In electron scattering, most of the existing dynamical theories have been developed under the first order diffuse scattering approximation, thus, they are restricted to cases where the lattice distortion is small. A formal dynamical theory is presented for calculating diffuse scattering with the inclusion of multiple diffuse scattering. By inclusion of a complex potential in dynamical calculation, a rigorous proof is given to show that the high order diffuse scattering are fully recovered in the calculations using the equation derived under the distorted wave Born approximation, and more importantly, the statistical time and structure averages over the distorted crystal lattices are evaluated analytically prior numerical calculation. This conclusion establishes the basis for expanding the applications of the existing theories. (4) The 'frozen lattice' model is a semi-classical approach for calculating electron diffuse scattering in crystals arisen from thermal vibration of crystal atoms. Based on a rigorous quantum mechanical phonon excitation theory, we have proved that the frozen lattice mode is an excellent approximation and no detectable error would be possible under normal experimental conditions.     

Influence of disorder in crystal structure on ferroelectric phase transitions

A new model of ferroelectric phase transitions in disordered crystals is developed. The model takes into account the nonequivalence of the structural environment of identical ions, which alters the balance of forces governing ferroelectric structural instability. In contrast with its predecessors, the new model can be used for qualitative and, in many cases, quantitative predictions of the temperature range of the diffuse ferroelectric phase transition and the value of the disorder-induced shift of the average transition temperature as a function of the degree of disorder and chemical composition. This conclusion is confirmed by a comparison of the theoretical results with an abundance of known experimental facts. Zh. éksp. Teor. Fiz. 111, 1817–1832 (May 1997)     

Phase transitions in molecular crystals of <Emphasis Type="Italic">n</Emphasis>-alkane alcohols

A comparative analysis of the phase transitions in molecular crystals of n-alkane alcohols with different lengths of the (C _n H_{2n + 2}OH) chain was performed using differential scanning calorimetry (DSC). Investigation of the temperature hysteresis made it possible to reveal a number of new effects associated with the specific features of the first-order phase transitions. A quantitative analysis of the temperature dependence of the heat capacity was carried out in the framework of the theory of diffuse (Λ-shaped) first-order phase transitions.     

Degree of structural perfection of icosahedral quasicrystalline grains investigated by synchrotron X-ray diffractometry and imaging techniques

A study of the structural perfection of icosahedral quasicrystalline grains of various alloys (Al-Pd-Mn, Zn-Mg-RE (RE L rare earth) and Al-Cu-Fe), grown by different slow solidification techniques (Czochralski, Bridgman, flux and annealing) was performed using high-resolution diffraction, including recording rocking curves combined with X-ray topography and phase contrast radiography, at a third-generation synchrotron radiation source (European Synchrotron Radiation Facility, Grenoble, France). For Al-Pd-Mn, additional coherent diffraction and diffuse scattering measurements were also carried out. After evaluating the potentialities of the techniques used, in the light of the criteria defined for crystals, it is shown that the structural perfection of icosahedral quasicrystals is quite comparable with that of metallic crystals but is considerably influenced by either uniform phason strains which can destroy the quasiperiodic long-range order, or by long-wavelength phason fluctuations leading to diffuse scattering. The structural perfection was also found to be extremely variable across the as-grown quasicrystalline grains and to be dependent on the presence and characteristics of inhomogeneities (pores and precipitates) often included in the quasicrystalline matrix. Regarding the grains that we used, it has been impossible to distinguish a clear influence of either the type of alloy or the growth method. It has, however, been noticed that Al-Pd-Mn and Al-Cu-Fe grains appeared less defective than Zn-Mg-RE grains and that the microstructure of these latter grains looks like that of crystals grown by the same technique. Annealing and mechanical polishing effects have also been analysed in the case of Al-Pd-Mn grains. It appeared that annealing improves the quasicrystalline lattice perfection by lowering phason strains insofar as no precipitates are nucleated. Mechanical polishing can introduce defects, located at the external surfaces, having the shape of bands.     

X-ray scattering and theα-α′ phase transition in coherent metal-hydrogen systems

Metallic singlet ground state systems have often anomalous elastic properties which result from the coupling of the phonons to the crystal field energy levels of the rare earth ions. It is shown that structural and magnetic phase transitions should occur in the same way in those systems. They are caused by a sufficiently strong electron-ion interaction. While the orbital part of this interaction is responsible for possible structural phase transitions, the spin dependent part is responsible for magnetic phase transitions. This is demonstrated in detail by considering as an example the aspherical Coulomb charge scattering and the isotropic exchange interaction. We calculate the sound velocity, the sound attenuation and the excitations in the presence of the aspherical Coulomb charge scattering. Furthermore we discuss the mutual coupling of structural and magnetic phase transitions. This includes a consideration of other types of coupling than aspherical Coulomb scattering and isotropic exchange.     

Lyotropic bicontinuous cubic phase single crystals investigated using high-resolved X-ray scattering

Single crystals of an Ia d bicontinuous direct cubic phase formed by a non-ionic surfactant in water are investigated using high-resolved X-ray diffraction. The shape of the Bragg peaks confirms the existence of a 3D long-range order inside the cubic phase. A weak diffuse scattered intensity signal is measured very near the Bragg peaks. We attribute this signal to thermal diffuse scattering (TDS) and we give an estimation of the contribution of elastic waves to this TDS. Received 4 May 2000