Analysis of chemical bonding in the α and β modifications of zinc diphosphide from X-ray diffraction data

With the aim of studying the characteristic features of chemical interactions between atoms in the α (red tetragonal) and β (black monoclinic) modifications of zinc diphosphide, the maps of deformation electron-density distribution at the P-P and P-Zn bonds were constructed from precision X-ray diffraction data. The P-P bonds were demonstrated to be of a pronounced covalent nature. The maxima in the Zn-P bonds are shifted to the electronegative phosphorus atom, which is indicative of a mixed ionic-covalent nature of the bond, with the ionicity being higher in the α than in the β modification. Based on the dependence of the contribution of the metallic component on the delocalized electron density, it was concluded that the contribution of the metallic component is more pronounced in the black than in the red modification. An increase in the bond ionicity and the absence of delocalized electrons result in an increase in the forbidden gap in the red modification compared to its increase in the black modification.     

Magnetostimulated changes of microhardness in potassium acid phthalate crystals

A decrease in microhardness along the (010) cleavage in potassium acid phthalate single crystals by 15–18% after the application of a permanent magnetic field is revealed for the first time. It is shown that the effect revealed is of the volume character. The role of interlayer water in the processes stimulated by a magnetic field is studied. Interlayer water does not cause the observed changes; it only plays the part of an indicator of these changes in potassium acid phthalate crystals in a magnetic field. It is established that microhardness in the (100) plane of the crystal in an applied a magnetic field first increases by 12–15% and then remains constant in time within the accuracy of the experiment. The possibility of varying the crystal structure of potassium acid phthalate crystals by applying magnetic fields inducing rearrangement in the system of hydrogen bonds or in the defect structure is discussed.     

Element incorporation in vapour grown III–V compounds

Some recent developments and ideas in the field of element incorporation in vapour grown III–V compounds are highlighted in this paper. The study is based mainly on GaAs and GaP. Specific areas of consideration are as follows. The role of equilibrium thermodynamics in predicting element incorporation. Here, theoretical developments are discussed and the difficulties in testing models in vapour growth in contrast to melt growth are noted. The residual impurity problem is considered, particularly the effects of vapour pressure parameters on the properties of epitaxial layers. The role of surfaces in relation to growth rate and the orientation-dependence of element incorporation are discussed. In connection with the field of defect chemistry, the effect of III–V ratio on element incorporation is debated. Also in the context of defects, the role of ionised As vacancies in GaAs is considered as a controlling parameter in element incorporation. This development allows one to account for the well-known insensitivity of doping to the intrinsic electron concentration.     

Growth Mechanism and Twinning of Sucrose Effected by Surface Adsorption Processes

Growth experiments with sucrose dependent on temperature and supersaturation of solutions were carried out to find the conditions for appearance of the three different growth types. Measurements of growth rates of {110} and {1 1 0} faces at 50°C and supersaturation c_s = 10% resulted in the known feature that in pure solutions a crystal grows faster in positive b-direction than in the negative one. Addition of raffinose stopped growth of {110}. In an electric field generated by 20 kV direct voltage (crystal orientation: negative pole of b-axis pointed towards the negative electric pole) the crystal grew in negative b-direction three times faster than in the positive one. In the presence of raffinose the growth rates in both directions become equal. If the crystal is turned by 180° in the solution (pure solution and in the presence of raffinose) the growth behaviour is the same as without an electric field. These results can be interpreted in such way that in pure solutions sucrose molecules adsorb to {1 1 0} (bonds exist between the fructose rings of molecules in liquid and solid phases) and that in presence of raffinose these molecules adsorb to {110} (bonds exist between glucose (solid) and galactose rings (liquid phases)). By the electric field the adsorption is hindered. The theory is in accordance with the fact that crystals situated in a suspension of the solution can intergrow with a host crystal in twin positions (in pure solutions on {1 1 0} and in the presence of raffinose on {110}).     

A new method of studying anisotropy of crystals

A new method is proposed of studying the anisotropy in crystal wafers cut out in respective orientations. For this purpose the TWYMAN effect, discovered in 1905 in the processing of optical glass, is used. It is shown that this effect, the physical nature of which is described in detail in another work of ours, is in force also with crystal wafers which bend to a strictly spherical surface in an isotropic state and to a characteristic ellipsoid surface in an anisotropic state. In the latter state a definite ratio exists of the large to the small axis of the ellipsoid, characterising the anisotropy of a given orientation.     

Interference pattern of X-ray waves inside the crystal and its image in vacuum in double-field approximation (II)

The energy flows of the X-ray waves, diffracted in thick crystals in double-field approximation, have been examined. It has been demonstrated that in a symmetric Laue case the energy flows in the upper layers are oriented in the direction of incidence and reflection, whereas in the lower layers they are oriented in the direction of the normal to the entrance surface of the crystal. The formation of pendular bands during diffraction of X-rays in thick parallelepiped and wedge-shaped crystals has been investigated. A method for obtaining pendular bands from such crystals is described.     

X-ray diffraction study of the complex of the enzyme SAICAR synthase with substrate analogues

The three-dimensional structure of the complex of the enzyme SAICAR synthase with analogues of natural substrates, namely, phosphoribosylaminoimidazolecarboxamide (AICAR) and succinic acid, was determined and refined by methods of protein crystallography. Two AICAR-binding sites were revealed in the protein molecule. One of these sites is located in the active center of the enzyme in the vicinity of the ATP-binding site, which has been found in the complex of SAICAR synthase with ATP that had been studied earlier. The second AICAR-binding site is located at the periphery of the protein molecule and coincides with the additional ATP-binding site present in the complexes studied earlier. The binding site of succinic acid was revealed in the active center of the enzyme in the vicinity of the AICAR molecule. The electron density distribution for the AICAR molecule in the active center is indicative of the possible lability of the atomic group of the adenine base.     

Point defects and dichroism in langasite and langatate crystals

The hypotheses about the nature of color centers in langasite family crystals that are discussed in the literature are analyzed. Optical transmission spectra in the wavelength range of 200–800 nm are recorded for langasite and langatate crystals grown in atmospheres of argon and argon mixed with oxygen in different concentrations, both in the initial state and after isothermal annealings at 1000°C in nitrogen, in air, or in vacuum. Dichroism is observed in langasite and langatate crystals, and spectral dependences of the degrees of dichroism are plotted.     

Ordering of carbon atoms in boron carbide structure

Boron carbide crystals have been obtained in the entire compositional range according to the phase diagram by self-propagating high-temperature synthesis (SHS). Based on the results of X-ray diffraction investigations, the samples were characterized by the unit-cell metric and reflection half-width in the entire range of carbon concentrations. A significant spread in the boron carbide unit-cell parameters for the same carbon content is found in the data in the literature; this spread contradicts the structural concepts for covalent compounds. The SHS samples have not revealed any significant spread in the unit-cell parameters. Structural analysis suggests that the spread of parameters in the literary data is related to the unique process of ordering of carbon atoms in the boron carbide structure.     

Evolution of twin-boundary geometry in Bismuth crystals with an increase of load

The effect of an increase in an external static load on the shape of wedgelike twins formed in the vicinity of stress concentrators in Bi crystals has been studied. A new method for investigating the intermediate stages of mechanical twinning in metals is suggested. It is shown that the applied load is not the determining factor in the development of plastic deformation via twinning in metal crystals.     

Nucleation in multidimensional Euclidean spaces

The problem of nucleation of a new phase in the bulk of the old one is considered for n-dimensional Euclidean spaces. Under the condition of a constant contact angle with the third phase, an increase in the space dimension results in a sharp decrease in the potential barrier of heterogeneous nucleation and, thus, in a decrease in the probability of homogeneous nucleation.     

Stacking interactions in the acridine dyes: spectrophotometric data and crystal structure of acridine orange hydroiodide and acridine orange hydrochloride monohydrate

The crystal structure of acridine orange hydroiodide and that of acridine orange hydrochloride monohydrate have been solved by X-ray diffraction analysis. The absorption spectra of both derivatives in the solid state have also been reported and compared with those of the acridinium ion in solution. The stacking interactions of the acridine orange molecule in both derivatives is discussed in the light of the visible absorption spectra in the solid state and in solution.     

Earth optimization of space experiment on growth of germanium by floating-zone technique with the use of rotating magnetic fields

The results are considered of the earth experiments on growth of high-purity and Ga-doped germanium single crystals 15 mm in diameter and 60 mm in length, which were performed in a Zona-4 “space furnace” under the technological regimes close to those existing in space orbits. It is shown that the use of a magnetohydrodynamic (MHD) factor [weak (0.15–0.2 mT) rotational (400 Hz) magnetic fields] during crystallization of semiconductors by the floating-zone technique is a very promising method for control of dopant distributions and electrophysical properties in a growing crystal. It is shown that in such magnetic fields, the effective coefficient of Ga distribution in Ge decreases by 10%. The shift of the donor-acceptor balance of the residual dopants in a compensated semiconductor during growth with the MHD-stirring of the melt was first established in growth of undoped germanium single crystals. It was also established that magnetic fields produce different effects on the resistivity microinhomogeneity in undoped and doped crystals. The mechanisms of the MHD effect on the properties of the grown crystals are discussed as well as the perspectives of performing analogous experiments aboard spacecrafts. It is predicted that, under the microgravitation conditions, the effects revealed in terrestrial experiments would be more pronounced.     

Nucleation in a gas‐solid state reaction

A nucleation in a reaction of a gas with a solid state where one of the products is solid (GSR‐S) has been discussed. It was pointed out that the phenomenon observed for nanopowders, i.e. nucleation in sequence of the crystallites sizes, should be valid in case of defected monocrystals. However in such a case the constituents of a monocrystal, i.e. mosaic domains separated by structural defects, should undergo phase transition in sequence of size or, in other words, the time needed for the initiation of a nucleus in the case of a small domain is shorter than in the case of big one. Finally the less defected the crystal is, the bigger domains are, and thus the longer nucleation time is needed.     

Treatment of data on metastable zone width leading to nucleation characteristics and measurements in the system potassium dichromate-water

Measurements of the width of metastable zone can be used for characterisation of the kinetics of nucleation. Methods of treatment these data discussed in literature are summarised, re-examined and arranged in order to enable smoothing of experimental data and simultaneous calculation of the values of certain characteristic nucleation parameters. An example (potassium dichromate in aqueous solution) shows that the values obtained in this way are within usual limits given in the literature. The results indicate that in unseeded solutions the nucleation was probably heterogeneous one, whereas in presence of a fixed seed an adsorbed layer mechanism of secondary nucleation followed by a fluid shear action seems to explain the results.     

Study of the growth mechanism in the ADP-type anion-doped crystals

A simultaneous simulation of the growth process in the A(K)DP-type anion-doped crystal and growing of this crystal from aqueous solution under controlled conditions has made it possible to analyse the mechanism of impurity influence on the growth rate of various crystal faces. The anisotropy of the face growth rate and the capture coefficient of various impurities by the faces have been determined by the difference in dissolution heats and in sizes of replacing ions. The increase in energy change, when the ion in a solution enters a crystal, causes an increase in the growth rate and the capture of impurities by the face with the largest change. Breaking of one of the joined faces and increased growth of another one resulted in forming of the new faces, i.e. in changing crystal morphology.     

Main dielectric relaxation of poly(methyl acrylate)–polystyrene interpenetrating polymer networks

Two series of poly(methyl acrylate)–polystyrene sequential interpenetrating polymer networks were prepared by free radical polymerization using ethyleneglycol dimethacrylate as cross-linking agent. In one of them the cross-linking density was very low and the IPNs presented phase separation. The main dielectric relaxation appears in the IPNs in the same position in the frequency axis and almost with the same shape as in the pure PMA network. Nevertheless the ratio of the relaxation strength to the weight fraction of PMA in the IPN decreases slightly with the increase of polystyrene content. The other series of IPNs were highly cross-linked, in these IPNs a dielectric relaxation still appears almost at the same temperature as in pure PMA network but it changes in its shape and an important decrease of the normalized relaxation strength is shown. The main relaxation ascribed to a polystyrene rich phase can also be observed due to the polarity of the cross-linking agent and a significant shift of this process towards low temperatures as the content of PMA in the IPN increases is observed. This feature was confirmed using dynamic-mechanical measurements.     

On DNA Motions under Action of Enzymes of Different Types. I

Different types of enzymes in rotational macromolecular motors of living cells are presented based on the data in the literature. Particular attention has been given to the restriction–modification enzymes and helicases, in which conformational changes in sites are physical reasons of linear motions and rotation of DNA molecules (caused by polar forces). It is shown that the structure and functioning of enzymes, similar to those in chiral smectic C-type liquid crystals, can be described within the formalism of generalized forces and currents. These forces include the membrane electrochemical gradient, the velocity of rotor mechanical motion, and the measure of change in free energy in the “fuel” adenosine triphosphate molecule.     

KDP晶体激光损伤机理研究

大口径KDP晶体是唯一可用作激光约束核聚变(ICF)中Pockels盒和倍频器件的晶体材料,但是低的抗激光损伤阈值使其应用受到了限制.本文从电子-空穴对的产生及稳定机制、光伤实体的本质等方面总结了多年来人们对KDP晶体激光损伤机理的研究进展,尤其从多光子电离、碰撞电离、激光加热三个方面定性阐述了电子-空穴对的产生机制, 而电子-空位对的稳定机制是探讨光损伤的关键步骤.另外从晶体生长过程及后处理两个方面初步讨论了提高光伤阈值和光学均匀性的途径.     

The Investigation of Recrystallization Processes of Ni Films

Electron diffraction and resistance methods are used to study the recrystallization process in nickel films. Recovery process, initial, and secondary recrystallization are observed in films in annealing. Unlike bulk deformed materials the distortions in films are produced in the process of their evaporation. The duration and sequence of stages of recrystallization are determined by the temperature of substrate in obtaining films and by the temperature of subsequent annealing. At temperatures of substrate above 150°C the reflexes due to recrystallized grains are observed just after evaporation of films. Temperature intervals of initial and secondary recrystallization of nickel films are calculated at temperatures of substrate equal to 130°C and 200°C. Correlation between the width of diffraction line, depending on the degree of lattice distortions, electrical resistance, measured at temperature of annealing and the number of recrystallized grains is noted. For reasons given it can be concluded that electron scattering in films takes place in lattice distortions in the crystallite and in their boundaries. The results obtained are of great interest in choosing optimum conditions for preparing films with given characteristics.