Electron diffraction study of the structural features of kaolinites

The structural variations of kaolinites have been investigated based on oblique-texture electron diffraction patterns. The features of manifestation of the 2D lattice of kaolinite layers in the geometry of the 20l and 13l reflections have been revealed. The manifestation of violations in the regular alternation of layers in kaolinites in the diffraction patterns has been analyzed using a numerical simulation of the diffraction profiles along the first (02l, 11l) and second (20l, 13l) ellipses of oblique-texture electron diffraction patterns. The simulation was performed for finite sequences of ten layers using the statistical Markov model in the quasihomogeneous approximation. It is shown that oblique-texture electron diffraction patterns can be used to reveal the coexistence of two phases with different structural perfections and particle morphologies in kaolinite samples.     

Reliability of maximum likelihood-based figures of merit

Various schemes for determining the maximum likelihood-based figures of merit for phases of structure factors have been considered. It is shown that the use of the likelihood function of all the available structure factors provides the adequate estimates of the accuracy of phases calculated for the atomic models with independent errors in the coordinates, but, at the same time, systematically overestimates the figures of merit for models preliminarily refined in the reciprocal space. It is shown that the use of the marginal likelihood function calculated from the control set of reflections allows the elimination of the systematic bias estimates. A method for reducing the statistical dispersion of the estimates based on a small number of control reflections is suggested.     

Proton NMR studies of smectic-C phases

We present proton NMR second moments studies of some compounds exhibiting smectic-C phases. From the comparison of experimental and theoretical angular dependences of the second moments the tilt angles δ are derived. The ratio f of the second moments of samples with statistical distribution of domain orientations and of perfectly aligned samples is calculated and compared with experimental data. For certain substances with constant tilt here are significant deviations from the theoretically expected values suggesting that the underlying theoretical model has still to be modified.     

Parabolic Approximation to the Shape of Oriented Eutectic Interface

The calculation of interface shape for oriented eutectic growth has been presented for regular structure formation. The parblic approximation to the interface shape has been assume in the proposed model. The differences between the modes of both phases growth have been distinguished. The proposed calculation of the interface shape provides a justification for the existence of lead distance. The obtained equations contain the physical factors which affect the formation of concave-convex interface. The periodic character of interface shape shape has been taken into account. The proposed model has been related to the theory of completely coupled growth.     

Comparing investigation of the smectic C and A phases of the di-n-alkyl 4,4′-azoxycinnamates (II) X-ray studies

The paper continues the investigation of the smectic C and A phases of the title compounds which has been done in Part I by DSC calorimetry. Here the layer spacings in the S_C and S_A phases are presented which exhibit a remarkable different temperature dependence in the S_C phases within the homologous series. A tendency of a change from a second order S_C/S_A transition to a first order transition has been found. The layer spacings of the S_A phase show a chain length dependent difference to the lengths of the molecules which may be explained by an increasing amount of gauche conformations in the higher homologues.     

基于新元素和新多层结构的MAX相陶瓷材料研究进展

由于独特的层状结构和原子间特殊的化学键合,MAX相陶瓷材料(化学式为M_n+1AX_n)兼具金属和陶瓷材料的优异性能,在很多领域具有广泛的应用前景,自20世纪60年代问世以来就一直备受关注。至今已经发现了100多种MAX相陶瓷材料,其中包括80余种单相以及一系列固溶体。传统的MAX相局限于一定的元素范围和若干M₆X层与单A原子层交替堆垛的结构。最近含有Au、Ir、Cu、Zn等新元素的MAX相材料的成功合成大大丰富了MAX相家族,多A层和多MA层结构MAX相的发现也打开了新型MAX相研究的一扇大门。随着理论计算的发展和实验条件的进步,越来越多的新型MAX相陶瓷材料逐渐出现在人们的视野中。本文综述了基于新元素和新多层结构的MAX相的国内外实验合成和理论研究进展,并指出了后续研究需要克服的问题,最后对新型MAX相的研究方向和发展趋势进行了预测和展望。     

Fluctuation mechanism of normal crystal growth during solidification of binary metallic melts

A recently published theory on the solidification of a one-component melt has been extended to the more complex case of binary systems. The theory is based on the model of a two-phase transitional zone existing between the crystalline phase and the melt. The concentration of solid state atoms within each mono-atomic layer of the transitional zone are assumed to fluctuate due to thermal fluctuations. A crystal growth law has been derived expressing the crystallization velocity in terms of probability functions describing these concentrations fluctuations. When certain restricting conditions concerning the atomic interaction energies within the transitional zone and the distribution of the atoms among the solid and liquid phases at supercooling are fulfilled the crystal growth law attains a simple form predicting for small supercoolings a growth rate proportional to supercooling (linear growth law), roughly proportional to physical parameter θ_AA, and with a weak dependence on another parameter Δ.     

X-ray diffuse scattering from spherically symmetric clusters: The effect of defect size fluctuations

A theory of X-ray diffuse scattering from crystals with randomly distributed spherically symmetric clusters characterized by a statistical spread of sizes with respect to their mean has been developed. This consideration is performed within the model of defects whose strains, in contrast to the well-known Coulomb clusters, are spatially confined by a sphere of fixed radius. The effect of defect size fluctuations on the formation of reciprocal space maps is investigated.     

Study of the solid-state amorphization of (GaSb)<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> semiconductors by real-time neutron diffraction and electron microscopy

The spontaneous amorphization of high-pressure quenched phases of the GaSb-Ge system has been studied by neutron diffraction while slowly heating the phases at atmospheric pressure. The sequence of changes in the structural parameters of the initial crystalline phase and the final amorphous phase is established. The behavior of the phases and the correlation in the structural features of the phase transitions and anomalous thermal effects exhibit signs of the inhomogeneous model of solid-state amorphization.     

Diffusion induced segregation in the case of the ternary system sphalerite,chalcopyrite and cubanite

A mathematical model for describing natural and experimental diffusion induced segregation (DIS) in the case of a (Zn,Fe)S single crystal with three coexisting phases is derived. As main result, a new and quite general segregation principle for ternary systems is discovered where one phase has a flat free energy density and serves as catalyst for the segregation of the other two phases. The model includes also a stochastic noise term to represent fluctuations of the copper concentration. Numerical simulations in 2‐d underline the physical significance of the model and allow to make quantitative predictions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Peculiarities of liquid phase epitaxy in GaInPAs/InP lattice-matched heterostructures

The temperature phase stability of Ga_xIn_1−xP_yAs_1−y solid solution has been analyzed. A simple solution theory with the temperature-independent interaction parameters in solid and liquid phases has been used. The absence of miscibility gaps for all the compositions of the solid solution, lattice-matched to InP at a growth temperature of 640°C, has been demonstrated both theoretically and experimentally. The influence of the elastic deformations on the Ga_xIn_1−xP_yAs_1−y (λ_g = 1.4 μm) solid solution parameters has been observed assuming the model of the layer coherent conjugation in heterostructures.     

Solid phases in cycloheptanol: Dynamic disorder and glass transitions

The stable solid polymorphism of cycloheptanol (C₇H₁₄O, for short C₇–OH) is revised in the present paper. Four solid stable phases, denoted on cooling from the liquid as phases I, II, III and IV are identified from X-ray diffraction measurements. Lattice symmetry of phase III has been unambiguously stated as orthorhombic. Diffraction patterns corresponding to phase IV have been also indexed as orthorhombic, but only in a tentative way. A detailed broadband dielectric spectroscopy study devoted to these phases has been made. Such an experimental technique has no revealed differences between phases III and IV and from specific-heat measurements, the III–IV phase transition has not been observed yet. Dielectric results seem to evidence that the dynamic disorder of phase III and/or IV should be intra-molecular attributed to changes between the possible distinguishable molecular conformations along with possible hydroxyl group rotations. The possibility of C₇–OH as conformational glass-forming material is discussed.     

Domain structure in CsDSO4 crystals

A complex study of domain structure of CsDSO₄ crystals has been performed over a wide temperature range. The effect of internal and external stresses and the PO₄ impurity on the kinetics of domain growth is considered. The behavior of the domain structure of the CsDSO₄ crystals is compared with martensite phase transformations. A model of crystal-lattice transformations in various phases is suggested, including an intermediate phase with the unknown symmetry of CsDSO₄ crystals.     

Stacking Fault Energy of Silicon Carbide (SiC) Polytypes

A polytypic material grown under similar thermodynamic conditions occurs in several phases which are included as ordered and disordered ones. These phases occur with a minimum free energy difference ≅ kT. This relative free energy (stacking fault energy) has been evaluated here using different methods.     

Induced SA phases and reentrant nematic phases in binary systems of homologous terminal-nonpolar compounds

The isobaric phase diagrams of binary systems are presented in which one component is represented by members of the homologous series of swallow tailed compounds. The occurrence of induced S_A phases and reentrant nematic phases was studied in dependence on the length of the alkyl chain of the swallow tailed compound. In some selected binary systems X-ray investigations and calorimetric measurements were performed. An attempt has been made to interprete the results by a simple packing model.     

Structure and Packing in Smectic E and Smectic A Phases in the Series of 4-n-Alkyloxy-4′-alkanoylbiphenyls

By means of X-ray investigations the structure of the S_E and S_A-phases of the title compounds has been studied. The lattice parameters of the orthorhombic S_E structure are given. The thickness of the smectic layers is nearly the same in the S_E and S_A phases and agrees well with the length of the molecules in their most stretched form. That suggests a similar conformation of the alkyl chains in both phases. A model of the packing in the S_A phase is discussed which is based on a dense packing with antiparallel orientation of molecules with a non-symmetrical shape.     

The non-Hookian behavior of chalcogenide glasses under irradiation: A nanoindentation study

The combination of nanoindentation and band-gap irradiation has been used to study the photoplastic effect in vitreous semiconductors. It has been shown that a non-linear (non-Hookian) mechanism of the formation of the strain response is realized in the As–Se chalcogenide films subjected to the combined action of light and external mechanical loading, simultaneously with opposite changes in Young’s modulus and nanohardness. These observations have been considered in the frame of the two-phase model of glasses in which each of the phases has a time-dependent character of the mechanical response subjected to an external perturbation. We have confirmed this assumption by experiment with irradiation of the film surface by linearly polarized light and give a simple mechanical model for explanations of the obtained results.     

Electron microscope studies of phase transformations in thin films of Tin Diselenide

The present investigations explore the structural characteristics of the thin films of tin diselenide (SnSe₂). Unlike bulk, only sparse studies are available on the structural characteristics in the thin form. SnSe₂ is known to exhibit almost exclusively the 2H CdI₂ type structure in bulk (single crystal) form. Particular emphasis on phase formation and structural transformations obtained in the thin film form has been given. The results reveal that all as-deposited films are amorphous in nature and undergo an amorphous → crystalline transformation upon pulse annealing (∼200 °C), the crystalline phase exhibiting the 2H bulk structure. Further pulse annealing at higher temperatures (∼300 to 500 °C) results in thermal decomposition of this 2H phase, giving rise to several other phases, namely SnSe (orthorhombic), SnSe (cubic) and Sn₂Se₃ (tetragonal) phases, the SnSe phases exhibiting variations in lattice parameters. In addition, a new incommensurate phase of SnSe has been obtained. A model is proposed for the formation of this phase from the orthorhombie SnSe phase. The final product of decomposition is found to be tetragonal tin.