The crystallisation of alkaline-earth metal tungstates from metal chloride melts: solubility – temperature phase diagram studies

The solubility vs temperature phase diagrams, for magnesium, calcium, strontium and barium tungstate solutions in lithium, sodium, potassium chloride melts and in the alkalineearth metal chloride melts, have been analysed. The solution in the alkali metal chloride melts are pseudo-binary reciprocal ternary mixtures; the more soluble barium tungstate systems show small deviations from ideality, while the less solube magnesium tungstate systems show marked deviations from ideality, especially in sodium and potassium chloride melts. These deviations are related to electrostatic effects in solution, that depend on some function of reciprocal solute and solvent cation radii. The solutions in the alkaline-earth metal chlorids melts are binary mixtures with no solvate formation; these systems show some from ideality at low mole fractions (x = 0.1 to 0.25) but they are practically ideal at mole fractions from 0.25 to 0.6.     

Effect of the defect structure character in calcite single crystals on X-ray diffraction

The paper presents experimental results for the dependence of the integral intensity of anomalous transmission T_i and Laue reflection R_i of X-rays on the structural quality of calcite single crystals as obtained by a two-crystal spectrometer. The relation between X-ray dynamic and kinetic scattering as a function of the sample thickness, densities of perfect dislocations in perfect CaCO₃ crystals and of atom-vacancy complexes in calcite crystals with „background”︁ has been found. Distortions due to perfect grown-in dislocations in calcite are shown to exceed those produced in crystals with „background”︁. The latter appear to be more X-ray transparent. A comparison with results of twin layer thickening experiments on CaCO₃ crystals of different qualities again points to the different nature of these distortions. In CaCO₃ crystals with the dislocation density of up to 2 × × 10³ cm⁻², the X-ray scattering, in our experiments, was consistent with the dynamic diffraction theory, while at higher dislocation densities, the X-ray scattering markedly differs from it.     

Kinetic investigations within the transition region from polyhedral to skeletal growth mode (I). Growth kinetics of the basal face of zinc single crystals under diffusion conditions

The effect of diffusion in the vapour phase on the kinetics of growth of zinc single crystals in the presence of argon has been studied. The shift of the basal face in normal direction as a function of the time has been measured at constant temperature and supersaturation and argon pressures varying within the range 5–250 Torr as well as in vacuum 1 × 10⁻⁶ Torr. It has been established that the crystal size R changes with the time t following a linear and a parabolic law in the kinetic and the diffusion regimes, respectively. The kinetic critical size R_k for the transition from a kinetic to a diffusion regime has been experimentally obtained. A relatively good agreement is found between the experimentally established and theoretically calculated values of the critical size. The problems related to the transition of the basal faces of zinc single crystals from a kinetic to a diffusion growth mode is discussed.     

On the origin of supersaturation barriers during the growth of single crystals from aqueous solutions containing impurities – a review

A generalised treatment of the appearance of supersaturation barriers σ_d, σ* and σ** during the growth of single crystals is outlined from the standpoint of well‐defined critical values of relative step velocities on a face. The final theoretical expressions are based on the premise that: (1) there are critical values of the relative step velocities associated with different average distances between adsorbed impurity particles during instantaneous, time‐dependent and time‐independent adsorption of the impurity on the growing surface, (2) the growth rate of a face is proporptional to velocity of steps on the growing face, and (3) Freundlich and Langmuir adsorption isotherms apply for different impurities. The theoretical expressions are then used to critically analyse the experimental data on supersaturation barriers observed during the growth of ammonium oxalate monohydrate and potassium dihydrogen phosphate single crystals from aqueous solutions containing different impurities. It was found that: (1) Langmuir adsorption isotherm is more practical for the analysis of the experimental data of the dependence of supersaturation barriers σ_d, σ* and σ** on the concentration c_i of an impurity, and (2) the ratios σ_d/σ* and σ*/σ** of successive supersaturation barriers for an impurity either increases or remains constant with an increase in impurity concentration c_i, and may be explained in terms of the mechanism of adsorption of impurity particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Part 1: Kinetics and mechanism of the crystallization process

The kinetics of spontaneous crystallization of sodium chloride from aqueous‐ethanol solutions were studied. During the crystallization the electrical conductance and optical transmission of the supersaturated solutions were measured automatically. For monitoring of the total surface of growing potassium chloride crystals at the crystallization the turbidimetric method was used. The growth rate and activation energy were determined. The crystal growth rate was proportional to supersaturation. When the volume fraction of ethanol in solution increased from 14.85 to 29.72%, the activation energy of the growth process did not change and was about 50 kJ· mol^‐1. Aggregation of the crystals was found. The aggregation kinetics of the crystals may be described approximately by the famous Smoluchowski equation for coagulation of colloidal particles. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

In situ study of growth and dissolution kinetics of ammonium oxalate monohydrate single crystals from aqueous solutions containing cationic impurities

The results of an in situ investigation of the effect of four different bi‐ and trivalent cations (Fe(III), Cu(II), Mn(II) and Cr(III)) on the displacement velocity of individual growth steps on the (110) face of ammonium oxalate monohydrate crystals as a function of supersaturation are described and discussed. It was observed that: (1) at a particular temperature of pure solutions and solutions containing impurities, the velocity v of movement of the [110] growth steps is always greater than that of the [111] steps, (2) fluctuations in the velocity of individual growth steps occur in all solutions containing similar concentrations of different impurities, (3) the value of kinetic coefficient β for growth steps decreases with an increase in the concentration c_i of Cu(II) impurity, but that for dissolution steps does not depend on c_i; moreover, the value of kinetic coefficient β for growth steps is higher than that of dissolution steps, and (4) in the presence of Mn(II) and Cr(III) impurities, the kinetic coefficient β for dissolution steps is several times greater than that for growth steps. The results are explained from the standpoint of Kubota‐Mullin model of adsorption of impurities at kinks in the steps and the stability of dominating complexes present in solutions. Analysis of the results revealed that: (1) the effectiveness of different impurities in inhibiting growth increases in the order: Fe(III), Cu(II), Mn(II), and Cr(III), and this behavior is directly connected with the stability and chemical constitution of dominating complexes in saturated solutions, (2) fluctuations in the velocity of growth steps is associated with the effectiveness of an impurity for adsorption; the stronger the adsorption of an impurity, the higher is the fluctuation in step velocity v, and (3) depending on the nature of the impurity, the kinetic coefficient for the dissolution steps can remain unchanged or can be higher than that of the growth steps. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Rietveld refinement of Y2O3 using the Pearson VII profile shape function

Very good agreement between the observed and the calculated intensities was obtained using the Pearson VII profile shape function. The variable exponent m could be refined only in the damped iterations involving exclusively the profile parameters. Simultaneous refinement of the m with the positional and/or thermal parameters has led to large oscillations of the shifts of almost all parameters. Precision of the refined atomic coordinates is better than those from previously published Rietveld refinement and is comparable with single crystal results. The degree of serial correlation in residuals has been determined by the use of the Durbin-Watson d statistic. Normal probability plot based on δR_i has been almost linear with the slope greater than one. The weighting scheme with w_i = 1/y_oi has proved to be good first approximation.     

Growth and thermal studies on pure ADP,KDP and mixed K1‐x(NH4)xH2PO4 crystals

The investigations on the formation of mixed crystals of ammonium dihydrogen orthophosphate (ADP) and potassium dihydrogen orthophosphate (KDP) i.e. potassium ammonium dihydrogen phosphate, K_1‐x(NH₄)_xH₂PO₄ have been presented in this paper. Pure and mixed crystals of ADP and KDP have been grown by slow evaporation technique from the supersaturated solution at an ambient temperature 26±1 °C for ammonium concentration x in the range 0.0 ≤ x ≤ 1.0 in the case of mixed crystals. Crystal compositions were determined by flame atomic absorption spectroscopy and chemical analysis. The results of the X‐ray analysis of the grown crystals are also reported. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used to study the kinetic process of dehydration and the high temperature phase behaviour. DTA showed the distinct thermal events attributed to dehydration of ADP, KDP and K_1‐x(NH₄)_xH₂PO₄. The results of thermal analysis and chemical analysis are consistent with each other. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Morphological lattice complexes

Binary and pseudobinary compounds AB like NaCl and CaCO₃, with the center of charge at carbon position, are characterized by self‐coordination numbers of nearest, second and third neighbors T ₁ T ₂ T ₃ of A and B positions. The maximum Madelung factor of compounds with identical T _i values of A and B atoms is obtained for a maximum of B positions at the border of the A–A Dirichlet domain. The A and B positions form a packing with different T _i values, if all B positions are on the border of the Dirichlet domain. This packing like the primitive cubic packing of Na and Cl atoms with T _i values 6 12 8 or the rhombohedrally distorted packing 6 8 6 of CaCO₃ as morphological lattice complexes can be related to cubic NaCl crystals or the cleaved rhombohedron of calcite. A different habit of crystals is expected for sphere, rod or layered packings. Many well‐defined nanoparticles can be obtained from microemulsions at variation of temperature, water content or water partial pressure. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Lead Determination in Alkali Halides by Spectroscopic Method

The octahedral complex [PbX₆]^4- (X=Cl, Br) in natrium/potassium chloride or bromide monocrystals as well as in the concentrated solutions of these alkali halides, impurified by lead, shows a characteristic absorption in the region 250-300 nm. Using the height of this absorption band, a quantitative method for determination of lead in the range 8x10¹⁴ - 3.2x10¹⁶ atoms/cm³ of halide monocrystals or 0.5 - 20 mg/ml of solutions respectively, was elaborated. Standard graphs of the peak extinction of [PbX₆]^4- in natrium/potassium chloride and bromide as function of the lead concentration have been obtained.     

On crystal growth of ammonium and potassium dichromate. A study of the shubnikov effect

Growth and equilibrium forms of ammonium (A2/a) and potassium dichromate (P 1 ) have been calculated by means of the Fourier transform method of crystal morphology. Twin laws of potassium dichromate and heteroepitaxial intergrowth of both substances can be explained by partitions of space. According to Shubnikov the (001) planes of potassium dichromate crystals grown in aqueous solutions are always smooth, the parallel opposite planes, however, roughened. Wagner observed that this effect only occurs at temperatures shortly above or below 25 °C. The present investigation has shown that roughening of (001 ) appears at 25 °C after addition of potassium chromate to the potassium dichromate solutions. This means that the one-sided intergrowth can be explained by a stereospecific adsorption of polychromates. Solution additions like potassium permanganate and Chromotrope 2B generate roughenings on both {001} faces. X-Ray diffraction investigations can be interpreted in such a way that the intergrown plates are mirror symmetric oriented to two planes being normal to the a- and b-axes. This is in accordance with repeated formation of intergrown plates.     

Kinetic investigations within the transition region from polyhedral to skeletal growth mode (III). Growth kinetics of the basal face of zinc single crystals and its edge under diffusion conditions

The growth kinetics of the basal face and its edge of zinc single crystals grown from zinc vapours in the presence of argon at pressures 5 – 250 Torr has been investigated simultaneously. The alterations of the crystal size R and the length of the basal edge a with time t have been measured at constant temperature and supersaturation. It is shown that the R²(t) and a²(t) dependences change their initial shape into linear at one and the same moment. It is established that the transition from a kinetic to a diffusion regime can be determined both by the kinetic critical size R_k and by the kinetic critical size a_k. It is found also that in the kinetic regime the lateral growth rate of the basal edge is higher than the normal growth rate of the {0001} face. During the growth in a diffusion regime the R²(t) and a²(t) curves of a given single crystal become parallel, due to the isotropic character of the transport processes of the material. The rate determining role of the diffusion in the parent phase on the growth of the basal face and its edge is demonstrated.     

Optische Eigenschaften von Wismut–Antimon-Aufdampfschichten

The change in position and in magnitude of absorption peaks due to interband transitions E_i (i = 1, 2, 3) for Bi_xSb_1–x evaporated layers have been studied for different compositions in the range 0 ≦ (1 − x) ≦ 0.30 in the visible part of the spectrum. It has been founded nonlinear shiftings for all absorption peaks in function of layer composition. The joint density of states due to transition E₂ is more sensitive as the two other transitions with modification of alloy composition.     

The crystallisation of alkaline-earth metal oxides from metal chloride melts: Solubility-temperature phase diagram analyses and preliminary experimental studies

The solubility v temperature phase diagrams for magnesium, calcium, strontium and barium oxide solutions, in lithium, sodium and potassium chloride melts and in the alkaline earth metal chloride melts, have been analysed. The solutions in the alkali metal chlorides are pseudo-binary reciprocal ternary mixtures; the more soluble barium and strontium oxides showed small deviations, calcium oxide showed larger deviations while the extremely sparingly-soluble magnesium oxide showed extensive deviations from ideality. These deviations were related to electrostatic interactions in solution, that depended in turn on some function of reciprocal solute and solvent caiton radii. The solutions in the alkalineearth metal chlorides are binary mixtures with MO · 4 MCl₂ solvate formation. — Some preliminary crystallisation experiments were carried out, by slow cooling of saturated solutions in the metal chloride melts: calcium and strontium oxide crystallisation from lithium chloride melts and barium oxide crystallisation from all three alkali metal chloride melts would be worth further investigation.     

Effective kinetic coefficients for some binary metallic alloys within the small and finite supercooling region

The normal crystallization kineties concerning the base of In, Rb, Ga some binary metallic melts either within the small supercoolings (Δ T ≦ 1 K) or finite ones (ΔT ≦ 20 ÷ 60 K) has been considered. When crystallizing the binary alloys: In + 2.3 at.% Pb; Rb + 10 at.% K; Ga + 2 at.% Sn; Ga + (2; 4; 8) at.% In; Ga + (0.5; 3.5) at.% Zn; Ga + (5; 8; 10) at.% Zn very dilute binary solid solutions are supposed to be formed. The solid solutions have the B-component distribution coefficients which are either much less or less compared with unit, i.e. k_B ≦ 1, k_B < 1 in all the layers of diphase transition region separating the melt from the crystalline phase. By virtue of fluctuation theory of normal crystal growth within the small and finite supercoolings' region for the above-mentioned binary metallic melt some effective kinetic coefficients have been calculated. A good agreement between theoretical effective kinetic coefficients and available experimental ones in the case of given binary systems has been established.     

A Unusual Bridging Mode of Diphenylhydantoin in Pentaaquabis[(5,5-Diphenylhydantoinato)Potassium]

Abstract  A new complex of diphenylhydantoin, pentaaquabis[(5,5-diphenylhydantoinato)potassium], has been synthesized and the structure has been solved by X-ray diffraction. The crystals are monoclinic, space group C2/c, with a = 34.959(5) ?, b = 8.273(4) ?, c = 11.692(2) ?, β = 109.069(13)°, M _r = 670.80, V = 3196.0(17) ?³, Z = 4 and R = 0.050. The potassium cations are assembled in dimers bridged through the carbonyl O atoms of the anions. The metal atom is in a distorted octahedral coordination environment. Index Abstract  In pentaaquabis[(5,5-diphenylhydantoinato)potassium] the potassium cations are coupled in dimers through the carbonyl O atoms of the anions.      

Optical Sign Determinations on Cholesteric Lyotropic Mesophases

Optical signs of type I and II micellar cholesteric lyotropic mesophases were for the first time determined. Lyomesophases were prepared from the following amphiphiles: potassium l-N-lauroyl-serinate, potassium laurate, sodium decylsulfate, cesium decylsulfate and decyl-ammonium chloride. Cholesterol was added to all mesophases in order to obtain the chevron pattern for magnetically oriented samples. The method involves the direct observation of the textures at the polarizing microscope. Using a quartz-wedge retardation plate the directions of the slow and fast light vibration components are determined. This method is simpler than the usual observation, under conoscopic illumination, of the basal interference figure which is hardly obtained in micellar lyotropic cholesteric mesophases. All type I cholesteric lyotropic mesophases here investigated exhibited a positive optical sign, whereas all type II cholesteric lyomesophases showed a negative birefringence.     

Untersuchungen zur Wechselwirkung von Ar-, O2- und H2-Niederdruckplasmen mit Metallschichten

By the interaction of the positiv column of a glow discharge (plasma gases: Ar, O₂, H₂) with the surface of metal films (Au, Ni, Sn) sputtering processes are observed. The sputter yields are larger than will be assumed from known yields of low energetic ions which are dominant in the investigated surface layer. Impulse interaction increases the sputter yields. This effect is reduced to a destruction of metal lattice caused by reconstruction of the surface layer and the inertion of gases during the intervals between the impulses. The sputter yields strongly depend on the plasma gases and the current density directed to the wall (i_W ∼10⁻⁵ A. cm⁻²) it is possible by decreasing the sputter yield to observer reactive processes and gas insertion in consequence of the plasma-wall interaction. Sustained upon electron diffraction of Ni- and Au-films and kinetic analysis of sputter yields a model is developed assuming a critical surface layer of 30–50 Å thickness during the plasma influence in “equilibrium” with a metal vapour-plasma gas phase above it.     

Structure types – a review

Structures of binary inorganic A_xB_y compounds with A and B on two Wyckoff positions are characterized by the self‐coordination numbers T_i, i = 1–3, of A and B atoms. The T₁ = 0–12 values of 122 structure types can be correlated with different topologies, A–A interactions and densities. High T₁ values like T₁ = 12 are obtained for fcc or hcp sphere packing with maximum density of A atoms and interstitial B atoms. Hexagonal or tetragonal layered compounds are recognized by the T_i values 6 6 6 or 4 4 4. The T₁ = 2 values are found for chains, T₁= 1 for ordered molecules like N₂ or Br₂. The A and B atoms of few compounds like NaCl or CsCl form a pc or bcc sphere packing. The T₁ = 0 values of Na or Cs atoms indicate repulsive Na–Na or Cs–Cs interactions. Distorted structures of a structure type are recognized, if atoms with slightly different A–A or B–B distances are included in the proper coordination shells. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)