Search

The paper presents the results of studies of longitudinal electric discharges in a supersonic air flow with a Mach number of M = 2 in the range of static pressures (2.94−4.9) × 104 Pa (∼220−367 Torr) and discharge currents of ∼1.5−1.8 A during the injection of propane, ethylene, and hydrogen, along with oxygen, into the discharge zone through the two types of upstream electrodes (anodes). The design of the electric-discharge modules was such that the discharge was carried downstream by the supersonic flow and closed, mainly to the cathode located downstream. Emission spectroscopy was used to obtain data on the composition of the emitting products that arose in the processes of converting a fuel-air mixture in a discharge and their spatial distribution. In particular, data on the distribution of the radiation intensity of CN, C2 molecules, and OH radical, as well as atomic hydrogen and oxygen in a number of sections of the discharge channel, are presented. The used methodology allowed us to determine the change in the transverse dimensions of the discharge channels along the length and the value of the rotational temperature of molecule C2 for discharges with the injection of propane and ethylene into the discharge zone. The relationship between its value and the conditions for the formation of the discharge and the coefficient of excess oxygen injected into the discharge is established.

A numerical simulation of an open direct-current arc in an external uniform axial magnetic field is performed. A circuit analog of fluctuations of the temperature of electrons is used for the numerical implementation of the open electric arc column of a helical shape. It randomly generates asymmetry in the temperature distribution of electrons and, consequently, other plasma characteristics. It is shown that the formation of the arc column of a helical shape is due to the asymmetric influence of the Ampere force, which shifts the cross sections of the arc relative to the central discharge axis. Furthermore, the cross sections of the arc rotate around the central axis at different speeds. The rotational speed at the initial part from the side of the cathode decreases downstream and increases upon approaching the anode. Such a character of the convective heat transfer causes the formation of a helical arc. If the direction of the external axial magnetic field coincides with the direction of the electric current, then the spatial orientation of the helical arc as a whole is right-handed. Otherwise, there is a left-handed structure of the arc column. The directions of rotation of the arc sections at the cathode and anode sides are opposite each other. A helical shape of the open arc is not constant in time and is periodically destroyed. Two modes of destruction were revealed: first, as a result of bridging the spiral of the helix and, second, as a result of the transformation of several spirals into one. Apparently, an open arc cannot maintain a stable and constant helical shape in an external axial magnetic field.

The characteristics of an electric arc of a direct current, burning in a cylindrical channel in a uniform external axial field, are numerically computed within a nonstationary three-dimensional mathematical model at partial local thermodynamic equilibrium. A method for the numeric modeling of the screw shape of an arc in this field is suggested. This approach is in addition to the “network” analogue of fluctuations for the temperature of electrons, which increases weak numeric asymmetry of electron temperature distribution that occurs randomly during computation. This asymmetry can be “picked up” by an external magnetic field and continue to increase up to a certain value, which is enough to form an arc column screw structure. If there are no fluctuations in the computation algorithm of fluctuations, the arc column in an external axial magnetic field maintains cylindrical axial symmetry and the arc screw shape is not observed.

Complexes of magnetic nanoparticles with antibodies are used for the selective and highly sensitive detection in medical diagnostics, environmental monitoring, and the product quality and safety control. Magnetic nanoparticles are applied as controllable carriers to concentrate samples, as a solid phase in assays, or as labels detected by virtue of their magnetic, optical, or other properties. This review covers the results of recent studies, including methods for the preparation of analytical reagents, new assays, and techniques for the detection of magnetic nanoparticles.

Specific stereochemical features of silicon in oxygen-containing compounds, including hybrid silicates with all oxygen atoms of SiOn groups (n = 4, 5, or 6) entering into the composition of organic anions or molecules, are described by characteristics of Voronoi—Dirichlet polyhedra. It is found that in rutile-like stishovite and post-stishovite phases with the structures similar to those of СаСl2, α-PbO2, or pyrite FeS2, the volume of Voronoi—Dirichlet polyhedra of silicon and oxygen atoms decreases linearly with pressure increasing to 268 GPa. Based on these results, the possibility of formation of new post-stishovite phases is shown, namely, the fluorite-like structure (transition predicted at ~400 GPa) and a body-centered cubic lattice with statistical arrangement of silicon and oxygen atoms (~900 GPa).

A numerical simulation was performed for an open direct current (dc) electric arc in a uniform external axial magnetic field. The calculation was carried out in the frameworks of a non-stationary three-dimensional mathematical model in approximation of partial local thermodynamic equilibrium of plasma. The model does not presuppose a helical shape of the arc, as is customary in theoretical studies based on linear perturbation theory. It was shown that the helical shape of an open arc is not constant in time and periodically collapses with subsequent restoration. In a certain range of external parameters of the arc (current strength, inter-electrode distance, and external axial magnetic field), a new mode of destruction of a regular structure was revealed – the arc section shunting between the spiral turns. Shunting can be single or double when the breakdown occurs simultaneously in two places. Apparently, it is caused by convective heat transfer by plasma flows as well as by the sliding of the arc thermal field. The appeared thermal field creates a zone of electric conductivity in which the breakdown occurs.

Strontianite (SrCO3)-aragonite (CaCO3) solid solutions have been simulated by the interatomic potential method. The composition dependences of the unit cell parameters, the unit cell volume, and bulk modulus have been constructed. It has been shown that the volume of the unit cell and bulk modulus show small negative deviations from additivity. The local structure of solid solutions has been analyzed. It has been established that the enthalpy of mixing is positive and, for the equimolar composition, reaches a maximum of 2.45 kJ/mol. Based on the composition dependences of the Gibbs free energy for the temperature range of 300–650 K, the solvus of the system has been constructed. According to the obtained data, the solubility of aragonite in strontianite under ambient conditions is 5.5 mol %, while that of strontianite in aragonite is 2.8 mol %. The miscibility gap of the system disappears at around 450 K. The calculated results have been compared with the experimental data.

The structures of chromium-containing cluster centers in forsterite laser crystals Mg2SiO4 : Cr and Mg2SiO4 : Cr : Li have been simulated using the interatomic potential method. A system of position-dependent parameters of interatomic interaction potentials in forsterite has been developed. In the ionic approximation, this system adequately describes the structure, properties, and defects of the crystal with correct representation of the preferred positional arrangement of chromium ions. The preferred mechanisms of chromium dissolution in forsterite crystals have been evaluated from a comparison of the energies of formation of chromium-containing clusters with different configurations. It has been demonstrated that the results of the simulation of interatomic interaction potentials are consistent with the experimental data obtained from electron paramagnetic resonance and optical spectroscopy studies.

The radiation resistance of the monazite LaPO4 and the compound YbPO4 (zircon structure type) has been investigated using the computer simulation. The number of Frenkel pairs, which are formed in the structure of these minerals after the passage of a primary knock-on thorium atom with an energy of 30 keV, has been calculated by the molecular dynamics method. The formation of Frenkel pairs and their recombination in the motion of recoil nuclei in the structure of the studied minerals have been discussed. It has been shown that the probability of the “survival” of Frenkel pairs in the LaPO4 monazite is significantly lower than in the YbPO4 compound. The tendency of these minerals toward amorphization under radiation damage has been described numerically. The obtained results have demonstrated that one of the main factors determining the radiation resistance of orthophosphates LnPO4 is the type of crystal structure, and the compounds with the monazite structure are more radiation resistant than the compounds with the zircon structure.

Results are presented from L-2M stellarator experiments on testing a possible method for detection of water microleakages in the cooling system of the first wall and vacuum chamber of ITER. The method consists in the spectroscopic detection of spectral lines of the OH hydroxyl, which forms via the dissociation of water molecules in plasma. Emission in the spectral band of 305–310 nm can be detected even at water leakage rates less than 10−4 Pa m3/s. Chemical reactions between water and boron compounds on the vacuum chamber wall delay the detection of leakages up to ∼2000 s. A similar phenomenon can be expected when a leakage will occur in ITER, where the materials suggested for the first wall (Be, Li) can also chemically react with water.

Structure and thermodynamic properties of zircon, coffinite, and zircon-coffinite solid solutions were subject to semiempirical atomistic simulation on the assumption of compositional disordering of mixed crystals: (Zr1 − x , U x )SiO4, where x = 0.02, 0.05, 0.08, 0.11, 0.14, 0.28, 0.50, 0.72, and 0.86. The solid solutions significantly depart from the Vegard and Retgers laws. The (Zr1 − x , U x )SiO4 mixed crystal structure is characterized by anisotropic expansion (largely in the a and b directions) under growth in x related to anisotropy of structural relaxation degree of the cation-oxygen interatomic spacing in the (Zr, U)O8 polyhedron. Increase in x parameters is accompanied by increase in average size of cation-oxygen polyhedrons and silica-oxygen tetrahedrons and by growth of interatomic spacing dispersion; maximum dispersion values are observed at x = 0.5–0.6. The distortions in local structures of ZrO8 and UO8 dodecahedrons and silica-oxygen tetrahedrons in the solid solution were analyzed on the basis of calculated functions of interatomic distance distribution. The obtained results demonstrate the possibility to assess numerically the structural (geometrical) disordering degree of the compositionally disordered solid solution depending on its composition. The calculated thermodynamic characteristics of solid solutions forecast the following solubility range limits: 2 mol % USiO4 in zircon and 5 mol % ZrSiO4 in coffinite under ∼1750 K.

Results of testing of a possible method for location of water microleakages in the cooling system of the first wall and vacuum chamber of ITER are presented. The method consists in spectroscopic detection of the emission lines of atoms and ions of the Xe additive dissolved in water. These lines are excited when the water with dissolved Xe contacts the plasma. The high electron cyclotron resonance heating (ECRH) power deposited in a relatively small plasma volume in the L-2M stellarator (P = 0.5 MW, V = 0.24 m3, and the specific heating power ∼2 MW/m3) makes it possible to achieve plasma parameters close to those in the edge plasma of ITER for different operating modes, including the H-mode with an edge transport barrier. In test experiments, several lines of Xe ions were revealed suitable for detection of xenon in plasma with parameters close to those in the edge plasma of ITER at leakage rates at a level of ∼10−6 Pa m3 s−1 and spatial resolution of ∼0.5 cm.

The structure of oxygen hole centers in forsterite crystals has been simulated using the interatomic potential method. The energies of isolated oxygen hole centers, as well as the energies of their clusters with intrinsic and extrinsic defects of the crystal, have been estimated for different arrangements of point defects in the structure. It has been shown that the most energetically favorable position for isolated oxygen hole centers is the O3 position, in which the gain in the formation energy is equal to 0.17 eV as compared to the O2 position and 1.66 eV as compared to the O1 position. The maximum energy gain due to the association energy can be achieved when the oxygen hole centers are located at the vertices of the tetrahedron with a silicon vacancy. The presence of chromium in the forsterite crystal can increase the probability of the formation of silicon vacancies. The obtained results have been discussed in terms of the experimental investigations of the color centers generated in the Mg2SiO4 and Mg2SiO4: Cr crystals under ionizing radiation.

The crystal structure of the new mineral magnesioneptunite (K0.8Na0.1−0.1)Na2Li(Ti0.39Mg0.34Fe0.27)2(Ti0.59Mg0.22Fe0.19)2[Si4O11]2(O,OH) from the xenolith of Verkhnechegemskaya caldera (Lakarga Mountain, North Caucasus) has been investigated by X-ray diffraction (XCalibur-S diffractometer, R = 0.0244): a = 16.3271(7) A, b = 12.4788(4) A, c = 9.9666(4) A β = 115.651(5), V = 1830.5(1) A3, sp. gr. C2/c, Z = 4, and ρcalcd = 3.152 g/cm3. The disordered distribution of Ti, Mg, and Fe atoms in the octahedra forming the basis of the cationic framework is established. It is shown that the isomorphic occupation of octahedral positions by cations of three types corresponds to the centrosymmetric crystal structure and is likely caused by the high-temperature crystallization of the mineral.

Chrysoberyl and alexandrite crystals have been grown from solutions in melts based on the Li2CO3-MoO3, Bi2O3-MoO3, PbO-V2O5, Na2B4O7, and K2MoO4-MoO3 systems using natural alexandrite and chrysoberyl debris as the initial BeAl2O4 compound. An analysis of the morphology and homogeneity of the crystals grown has revealed the Bi2O3-MoO3 solvent to be the most appropriate. The optimal color characteristics (“quality” of alexandrite effect) manifest themselves when adding about 5 mol % Cr2O3. The largest crystals (up to 10 mm in size) were obtained from a solution in melt based on PbO-V2O5 at a ratio of the crystal-forming component to the solvent of 9: 91 wt %; These characteristics, along with a relatively low operating temperature (970°C), give grounds to consider this type of solvent promising.

The radiation resistance of the mineral xenotime YPO4 has been investigated using computer simulation methods. The optimal criteria for the appropriate choice of the parameters of the interatomic potentials used for the simulation of α-decay in minerals have been proposed. The formation of a cascade of displaced atoms in the structure of xenotime after the passage of a primarily knocked thorium atom with an energy of 20 keV has been studied using the molecular dynamics method. The specific features of the formation and annealing of individual defects in the structure of xenotime have been considered using the Mott-Littleton method. The dependences of the energy of formation of Frenkel pairs and the probability of their annihilation during the annealing of a cascade of atomic displacements on the distance between the vacancy and the interstitial site have been analyzed within the framework of the supercell approximation.

An immunoanalytical system was developed for the determination of ochratoxin A with the use of a surface plasmon resonance (SPR) sensor amplified by the anti-species antibody–colloidal gold particle (CGP) conjugate. The use of the binding of immune complexes to the CGP–anti-species antibody conjugate leads to the SPR signal amplification by a factor of more than 10 and results in the 60 pg/mL limit of detection of ochratoxin A with an assay time of 30 min. These characteristics are superior to those obtained both in the conventional enzyme immunoassay with the use of the same reagents and the SPR assay with unmodified antibodies and specific antibodies conjugated to colloidal gold.

The thermodynamic and elastic properties of mixing have been calculated and the local structure of Be(Al,Cr,FeIII)2O4 solid solutions has been analyzed using a previously elaborated set of interatomic potentials for atomistic modeling of simple and complex beryllium oxides, which almost exactly reproduced their structural, elastic, and thermodynamic properties. Calculations have been performed for a (4a × 2b × 2c) supercell of the olivine structural type with the relieved nontranslational symmetry (space group P1), which has made it possible to specify the distribution of trivalent atoms over nonequivalent positions and to provide relaxation of the local structure. The mixing properties have been calculated over the entire range of compositions, the changes in the Gibbs free energy with variations in temperature and the regions of stability of the solid solutions have been estimated, and the critical values of the temperature and composition have been determined. The histograms of the distributions of M-M, M-O, and O-O interatomic distances, as well as MO6 octahedron volumes, have been constructed; the compliances of cation positions have been evaluated; and the groups of atoms that are most readily shiftable from their ideal positions have been established. The performed analysis has demonstrated that two different octahedral positions M1 and M2 in the olivine-type structure, namely, chrysoberyl, are extremely sensitive to their own atomic environment. This leads to a preferable incorporation of Al3+ cations into the M1 octahedral position, whereas larger-sized Cr3+ (Fe3+) cations preferentially occupy the M2 octahedral position.

Standard enthalpies of formation of GdBaCo2−xMxO5.5 (M = Fe, Mn; x = 0, 0.2) from constituent oxides, Δ o x H i ∘ , and elements, Δ f H i ∘ , at 298 K were calculated using dissolution enthalpies of corresponding double perovskites. Both Δ o x H i ∘ and Δ f H i ∘ were found to decrease in the sequence of GdBaCo2O5.5, GdBaCo1.8Fe0.2O5.5 and GdBaCo1.8Mn0.2O5.5 indicating growth of relative thermodynamic stability of GdBaCo2O5.5 caused by the substitution of Fe or Mn for Co.

The influence of the oxygen partial pressure P O 2 in the growth atmosphere on the coefficient of chromium distribution between the crystal and the melt of forsterite, the Cr3+ and Cr4+ ion contents in crystals, and the concentration of color centers induced by irradiation has been investigated. It has been established that the crystals grown at low oxygen partial pressures P O 2 (0.01–0.05 kPa) are characterized by low concentrations of Cr4+ ions and color centers. A change in the oxygen partial pressure to P O 2 ∼ 0.85 kPa leads to an increase in the Cr4+ center concentration by a factor of ∼10 and in the color center concentration by a factor of ∼5. A further increase in the oxygen partial pressure to P O 2 to 12 kPa remains the concentration of these centers almost unchanged. A model has been proposed according to which the intrinsic defects formed under conditions of a relative excess of oxygen leads to both the self-oxidation of chromium and the formation of color centers in the forsterite crystals under irradiation.

A set of partially covalent interatomic potentials has been developed with the aim of reproducing the experimentally known crystal structures and predicting the unknown crystal structures and the thermodynamic and elastic properties of bromellite BeO, chrysoberyl BeAl2O4, and its isostructural analogs BeCr2O4 and BeFe2O4. The calculated structural, elastic, and thermodynamic properties of these minerals are in good agreement with the available experimental data. This gives grounds to believe that the prediction of a number of unknown properties of these compounds and the model crystal structure of Fe chrysoberyl are sufficiently reliable. A theoretical analysis of the energy characteristics of the Schottky and Frenkel point defects is carried out for all the substances under investigation.

A detailed historical review is presented and the difficulties arising when defining such a fundamental concept as structural type in crystal chemistry are analyzed. Particular attention is paid to the genetic relationship between different structural types and to the criteria of choosing between them. Many other terms and concepts of modern crystal chemistry defined for completeness.

The dependences of average V-O distances in inorganic compounds of vanadium of different valence on the degree of distortion of coordination polyhedra have been obtained by careful statistical treatment of modern structural data banks. Values of bond lengths in undistorted (regular) polyhedra are recommended. Theoretical analysis of the statistical data made it possible to calculate the most likely values of the parameters of the bond valence model: the interatomic distance for the single (two-electron) bond, corresponding to the single valence, and the bond softness parameter. Calculations of the sums of bond valences for some complicated cases (different coordination numbers, mixed vanadium valence) confirmed reliability of the recommended parameters.

A technique for the computer modeling of disordered binary oxide solid solutions MO-M′O in a wide composition range has been developed. The method of atomistic pair potentials was used for 4 × 4 × 4 supercells. The parameters of the potentials are optimized using the structural and elastic properties of pure components MgO, CaO, SrO, and BaO. The temperature dependences of the heat capacity and entropy are calculated for pure components. The excess mixing properties (enthalpy, volume, bulk modulus, vibrational entropy) are found for different compositions of MgxCa(1 − x)O, CaxSr(1 − x)O, and SrxBa(1 − x)O solid solutions. Temperature and composition dependences of the excess Gibbs energy were constructed, which made it possible to approximately estimate the critical decomposition temperatures and limits of component miscibility. Statistical analysis of lattice distortions in the first and second coordination spheres reveals a detailed picture of the solid-solution local structure.

The structural, elastic, and thermodynamic properties of halite NaCl and sylvine KCl and the miscibility properties of the NaCl-KCl solid solution found by computer simulation are in good agreement with the experimental data. Analysis of the relaxation of the solid solution structure suggests that both anion and cation sublattices are distorted; however, the anion sublattice is distorted much more strongly. Calculations of the local bond valence at all types of ions in the solid solution show opposite deviations from the balance at cations, whereas the general balance is retained. The values of the electrostatic potential in the ion positions reflect weakening of bonding in the solid solution with respect to its pure components. In addition, with an increase in the average interatomic distance in the first coordination sphere around cations, the modulus of the electrostatic potential at cations decreases.

An original technique of computer modeling of substitutional solid solutions has been applied to Al2O3-Cr2O3, Al2O3-Fe2O3, and Fe2O3-Cr2O3 binary systems. The parameters of semiempirical interatomic potentials were optimized using the experimentally studied structural, elastic, and thermodynamic properties of pure components. Among point defects, the most energetically favorable ones for all three oxides are Schottky vacancy quintets. To model (Mx1M1 − x2)2O3 solid solutions, 4 × 4 × 1 disordered supercells with M1: M2 cation ratios of 1: 5, 1: 2, 1: 1, 2: 1, and 5: 1 have been constructed in the cation sublattice containing 192 atoms. The mixing enthalpy and volume, interaction parameters, bulk moduli, and vibrational entropy were found by minimizing the interatomic interaction energy in supercells with the symmetry P1. Calculations of the Gibbs energy made it possible to estimate the fields of stability of the Al2O3-Cr2O3 and Al2O3-Fe2O3 solid solutions; these estimates were compared with the experimental data. Histograms of M-M, M-O, and O-O interatomic distances were constructed and the local structure was analyzed for the Al1.0Cr1.0O3, Al1.0Fe1.0O3, and Fe1.0Cr1.0O3 compositions.

Different methods currently employed for choosing and analyzing model configurations of binary systems with isomorphous substitution in the sublattice were considered. A new more efficient algorithm was proposed for determining the most disordered atomic configuration of an arbitrary composition for a cell with any size. The algorithm was implemented in the form of a computer program which makes it possible to approach the most optimum topology of the arrangement of solid solution components over atomic positions. The program was tested using a number of binary systems and can be recommended for ab initio calculations and simulation with semiempirical methods. Moreover, this approach can be used to decompose complex experimental spectra of isomorphous mixtures which are difficult to interpret without recourse to model concepts regarding the local structure of multicomponent systems.

Knowledge of the high-pressure and high-temperature elasticity of Fe–Ni alloy with low (5–25%) Ni content is crucial for geosciences since it is probably the major component of the cores of the terrestrial planets and the Moon. Here we present a study of a FeNi alloy with 22 at.% of Ni to 72 GPa and 715 K, using inelastic X-ray scattering (IXS) and X-ray powder diffraction from polycrystalline material. The X-ray diffraction (XRD) study revealed stability of the face centred cubic (fcc) over the hexagonal close packed (hcp) phase in the whole investigated pressure–temperature range. The study presents first investigations of elasticity of fcc phase of iron–nickel Fe0.78Ni0.22 alloy. The isothermal equations of state were derived at room temperature and at 715 K (K300 = 162(1) GPa, K ′ 300 = 4.97 ( 1 ) , V300 = 6.89(1) cm3/mol; K715 = 160(1) GPa, K ′ 715 = 4.97 ( 2 ) , V715 = 6.96(1) cm3/mol). Inelastic X-ray measurements allow the determination of the longitudinal acoustic wave velocity VP, and provide, combined with the measured equations of state, the full isotropic elasticity of the material. We found that within experimental errors our data follow the Birch's law. We did not observe any significant deviations for fcc Fe0.78Ni0.22 from elastic properties of pure ɛ-iron.

This paper reports on the results of a structural investigation of an important group of synthetic compounds with derivative structures of the mineral murataite, a potential matrix for immobilization of radioactive wastes. A model describing the structure of the entire group of compounds is proposed on the basis of analyzing the results obtained using chemical (electron microprobe) analysis methods, high-resolution transmission electron microscopy (high-resolution electron images and microdiffraction patterns), and X-ray diffractometry of a large array of artificial murataite ceramics. In essence, the proposed structural model is as follows: in the structure, layers of two (pyrochlore and murataite) packing types alternate along the [111] direction of the cubic supercell, so that the resulting structures can undergo substitutional and displacive modulations. Experiments on irradiation of different murataite modifications with heavy ions demonstrate that these modifications have close values of the radiation resistance, which is higher than that of the pyrochlore coexisting with them.

The synthetic mineral urusovite $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ was prepared through the wet chemistry route and characterized over a wide temperature range in terms of studies of magnetization, specific heat, and X-band electron spin resonance. The basic structural units of the compound are distorted square pyramids $\mathrm{Cu}{\mathrm{O}}_{5}$ assembled into corrugated honeycomb layers separated by $\mathrm{As}{\mathrm{O}}_{4}$ and $\mathrm{Al}{\mathrm{O}}_{4}$ tetrahedrons. Both thermodynamic and resonant measurements indicate that $\mathrm{CuAl}(\mathrm{As}{\mathrm{O}}_{4})\mathrm{O}$ is a spin gap system with a gap of $\ensuremath{\sim}350\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. The electronic structure calculations performed within the framework of density functional theory suggest a weakly interacting dimer model with antiferromagnetic signs for both intradimer and interdimer superexchange interactions. This establishes the valence bond solid as the quantum ground state of the title compound. The pronounced discrepancy between experimental data and calculations within the weakly interacting dimer model at elevated temperatures is ascribed in part to the steep increase of the intradimer exchange interaction parameter driven by the thermal expansion effects.

Mossbauer study of synthesized ferrite-garnet samples containing Zr, Th, Ce and Gd of the following composition: 1C — Ca2, 5 Ce0, 5 Zr2 Fe3 O12, 2C — Ca1, 5 GdCe0, 5 ZrFeFe3 O12, 1T — Ca2, 5 Th0,5Zr2 Fe3 O12 and 2T — Ca1, 5 GdTh0, 5 ZrFeFe3 O12 are carried out. As a result of 57Fe Mossbauer study it is found that iron atoms in all investigated samples of garnets are in a trivalent state. The analysis of experimental Mossbauer spectra definitely specifies a various structural state of iron atoms in two investigated groups of samples: 1T, 1C and 2T, 2C. X-ray study have shown that 1T and 1C garnet samples crystallize in tetragonal space group I41/acd, but 2T and 2C samples crystallize in cubic space group Ia3d.

The crystal structure of a synthetic analog of the mineral lipscombite (Fe2.32+Fe4.73+)[PO4]4O2.7(OH)1.3 obtained under hydrothermal conditions in the LiF-Fe2O3-(NH4)2HPO4-H2O system is resolved (R = 0.040) by X-ray diffraction analysis (Bruker Smart diffractometer with a highly sensitive CCD detector, MoKα radiation): a = 14.776(3) A, b = 14.959(3) A, c = 7.394(1) A, β = 119.188(4)°, sp. gr. C2/c, Z = 4, ρexp = 3.8 g/cm3, ρcalcd = 3.9 g/cm3. Fe2+ and Fe3+ cations are statistically distributed in each of four crystallographically independent positions, while occupying the corresponding octahedra with probabilities of 60, 90, 100, and 91%. The ratio Fe2+/Fe3+ in the composition of the crystals was established by Mossbauer spectroscopy. Lipscombite is interpreted as a mineral of variable composition described by the formula (Fex2+Fen−x3+)[PO4]4Oy(OH)4−y. The field of stability is determined as a function of the iron content and the ratio Fe2+/Fe3+. It is shown that at n = 6 iron cations are ordered in octahedra and barbosalite structure is formed. An interpretation of genetically and structurally related members of the lipscombite family within a unified polysomatic series is proposed.

The effect of the cathode geometry on the characteristics of an electric arc is treated. It is found that the characteristics of plasma in discharges with cathodes of different geometry (cone, sphere, cylinder) are almost the same on condition of equality of the areas of current-conducting surfaces. It is assumed that a ring-shaped arc discharge may be realized in the case of axisymmetric arc spot on the external side surface of a cylindrical cathode.

Some features are treated which are characteristic of a flow of arc plasma in the vicinity of a profiled cathode surface. Comparison of theoretical and experimental results leads one to assume that the formation of a plasma circulation zone in the vicinity of a profiled cathode surface contributes to reducing the erosion of the cathode material.

The crystal structure of mineral sakhaite Ca48[Mg13.2(Fe, Mn)2.8](CO3)16{ Al[SiO3.75(OH)0.25]4}(BO3)28[(H2O)3.3(HCl)3.3] from the Solongo deposit, Zabaykalye, is established (R 0.047) by X-ray diffraction analysis (Bruker Smart CCD diffractometer, 2θ: θ scan, λMoK α radiation, graphite monochromator): a = 14.679(2) A, sp. gr. Fd3m, Z = 1, ρcalcd = 2.99 g/cm3. It is shown that some part of BO3 triangles in the structure of sakhaite from Solongo is replaced with five-member complexes {Al[SiO3(O,OH)]4}, established previously in the harkerite structure. Three schemes of isomorphism, which are characteristic of the sakhaite-harkerite mineral series, are selected and the general formula is proposed: Ca48(Mg,Fe,Mn)16(CO3)16{Al[SiO3(O, OH)]4}y(B3)32−y · n(H2O,HCl) (Z = 1), y max = 8, n max = 16 − y. The structural relationship of sakhaite-harkerite minerals with borate-sulfates of the tychite family Na6(Mg,Fe,Mn)2(BO3)4(SO4) is revealed. The correlation between the number of harkerite fragments forming the mineral structure, the structural symmetry, and the degree of imperfection is ascertained.

A hierarchical approach to the choice of the most probable structural models of a compound with a specified chemical composition and to the subsequent refinement of these models with the use of different methods is analyzed. Most attention is given to one of the first stages of the choice of the starting model (on the basis of the principles of total and local crystal electroneutrality) and to the solution of the problem of construction and determination of the bond graphs and connectivity matrices. Simple criteria for the most stable configurations of chemical bonds are introduced reasoning from the principles of minimum dissymmetrization and maximum informational entropy.

The possibility is examined of using the method of fictitious regions to calculate the characteristics of an electric arc in a multiply connected region. Analysis of the results points to the legitimacy of using this method.

The distortion theorem is analyzed by the use of the Bond Valence Model for the case of an isolated coordination polyhedron with bonds of different lengths. It is shown that a correlation should exist between the average bond length increase and the mean-square deviation of individual bond lengths from the average one, at least for nonstrained crystal structures. It is proved that the correlation parameters are strongly dependent on a mode of distortion (symmetry of coordination environment). These conclusions are in general and sometimes in quantitative agreement with available experimental data, although in most cases the distortion of coordination polyhedra results from a variety of factors of chemical, geometric and electronic origin. It is assumed that an observed correlation between the average bond distance and the distortion degree may be taken as a measure of relative role of intrinsic bond strengths and influence of crystalline environment.

A steady-state three-dimensional mathematical model is used to calculate an electric arc in an external longitudinal magnetic field. A helical shape of arc is obtained in the case of a certain correlation between the external parameters being regulated. The possibility of using the mathematical model for numerical analysis of helical arcs is demonstrated.

The structure and some physical properties of malayaite CaSnOSiO4 have been studied by the precision X-ray diffraction, Mossbauer spectroscopy, and computer simulation of the structure. The unit-cell parameters a = 7.152(2) A, b = 8.888(2) A, c = 6.667(2) A, β = 113.37(2)°, V = 389.0(3) A3, and μr = 0.68 are refined on a synthetic impurity-free sample. The distribution of the deformation electron density is analyzed in the basic fragments of the crystal structure forming an anionic framework. The constructed potentials of pair and three-particle interaction reproduced quite well the elastic, dielectric, and energy characteristics and allowed us to predict their numerical values, which are in good agreement with the limited available experimental data.

This paper reports on the results of crystal chemical analysis and computer simulation of the defect structure of potassium dihydrogen phosphate (KDP) containing impurities of bivalent and trivalent metals. It is shown that these impurities can form defect centers of different types: isolated centers formed by M 3+ and Ni2+ ions and, in part, by Co2+ ions at interstitial sites, chains composed of M 2+ impurity ions with radii from ≈0.65 to ≈1.1 A, and centers created through the substitution of large-sized bivalent cations for potassium ions either with the formation of additional potassium vacancies or through the heterovalent isomorphism mechanism. The calculations are performed using different-type interatomic interaction potentials, and a comparative analysis of the results obtained is carried out.

Different types of defect sites generated by the impurities of divalent (M 2+) and trivalent (M 3+) metals in the structure of potassium dihydrophosphate KH2PO4 (KDP) were revealed by crystal-chemical analysis and computer simulation. These sites cause different deformations of the crystal matrix by generating different local strains, which enhance the inhibiting effect of impurity atoms adsorbed on the surface. This fact accounts for the different influence of di-and trivalent cations on the growth kinetics and face morphology of KDP crystals. The effect of the M 3+ ions is associated primarily with their adsorption on the face surfaces, whereas the influence of the M 2+ ions results from their insertion into the surface layer of the crystal.

The results of numerical investigations of an electric arc in an external transverse magnetic field are given. The effect of external controlled discharge parameters and of the magnetic field of the electrodes on the characteristics of a flow of arc-discharge plasma is treated. Some singularities of the flow formed by an electric arc in an external transverse magnetic field are revealed.

It is proposed to develop a three-dimensional mathematical model of electric-arc plasma with due regard for its deviation from thermal and ionization equilibrium. The combined solution of the equations of electric-arc column and of the thermal problem in electrodes enables one to estimate the effect of the magnetic field of the electrodes on the characteristics of the arc column. The model is tested by a comparison of the calculation data with the experimental results, as well as with the results of calculation using two-dimensional models. It is shown that the proposed model enables one to obtain physically reliable results.

The structure of KDP crystals doped with trivalent (Al3+, Fe3+, Mn3+, V3+, and La3+) and divalent (Ni2+, Co2+, Fe2+, Mn2+, Ca2+, Sr2+, and Ba2+) cations was simulated by minimizing the energy of atomic interactions. Three types of defects were revealed: isolated defect centers formed by M3+ and Ni2+ ions, cluster chain centers formed by M2+ ions with ionic radii exceeding 0.9 A, and complex centers formed via the replacement of potassium ions by large Ba2+ dopants with the simultaneous replacement of some of the phosphorus atoms by silicon ones. The corresponding energies of defect formation are calculated. The surface morphology of the crystal faces is studied. The changes in morphology in the presence of M3+ dopants are caused by their adsorption, whereas for M2+ dopants, these changes are caused mainly by their incorporation into the crystal structure.

The dependence of detection limits of ion-selective electrodes (ISEs) based on ionophores with phosphoryl-containing terminal groups on the nature and concentration of lipophilic anionic compounds was investigated. It was shown that by including superstoichiometric amounts of lipophilic anionic compounds into the membrane, the lower detection limit can be decreased significantly. A model describing the mechanism of the dependence of the lower detection limits on both the amount and nature of the lipophilic anionic additives used is presented.