Formation Of Mixed Crystals Research Articles

Preparation of Metal Sulfide Composite Ultrafine Particles in Reverse Micellar Systems and Their Photocatalytic Property.

CdS, ZnS and their composite ultrafine particles were prepared using nanometer-sized waterpools in AOT/isooctane reverse micellar solutions as reaction media. The size of the ultrafine particles was controlled by changing the water content Wo (=[H2O]/[AOT]) of the reverse micellar solution and this was almost independent of the feed concentrations of Cd2+, Zn2+ and S2–. Stable ultrafine particles, which do not cause excess aggregation and the band gap of which continues to be larger than that of the bulk semiconductor for a long time after formation, could be prepared at a value of Wo less than 8. Mixing a micellar solution containing both Cd2+ and Zn2+ with a solution of S2– gave coprecipitated semiconductor ultrarine particles. Gradual precipitation of ZnS in a micellar solution which had contained CdS particles gave ZnS-coated CdS ultrafine particles. The use of an excess of S2– for precipitation was effective in making the composition of the resulting particles close to that of the initial solution, and also avoided the formation of mixed crystals. The composite ultrafine particles thus prepared could be directly applied to the photocatalytic reduction of water and were found to be improved in their photocatalytic activity compared to CdS particles. The photocatalytic properties of the coprecipitated particles varied according to the particle band gap.

Structure, energy and charge transport in two-dimensional crystals of cyanine dyes

In order to develop systems with controlled energy or charge transfer across molecular dimensions it is highly desirable to build two-dimensional crystals of functional dye molecules: they can be addressed from the third dimension and the lateral intermolecular interactions can be well characterized due to a fixed and defined geometry. In an effort to study these possibilities we prepared crystals of different, negatively charged cyanine dyes, formed after adsorption from a water subphase to a positively charged monolayer. These crystals are one monolayer thick, of uniform dimensions between 10 and 100 μm (depending on nucleation conditions) and of rectangular shape. Single crystals are studied by transmission electron diffraction and by polarized absorption and emission spectroscopy. We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicular polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centred rectangular one of the dyes. It is incommensurate and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice. The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands. Replacing the positively charged lipid molecule with saturated bonds by one with a diacetylene group in the aliphatic region, one can sensitize the photopolymerization of diacetylenes. In accordance with the established mechanism for photopolymerization this indicates efficient charge transfer from the dye crystal to the linear polymer.

Two-dimensional dye crystals with controllable optical properties

The structure, molecular ordering and optical properties of single crystals of cyanine dyes grown by adsorption from a water subphase to a positively charged lipid monolayer are discussed. These crystals are one monolayer thick, of uniform dimensions between 10 and 100μm (depending on nucleation conditions) and of rectangular shape. Single crystals were studied by transmission electron diffraction and by polarized absorption and emission spectroscopy. We show that the crystals consist of two rows of densely stacked molecules with two different orientations of the long molecular axes. This leads to two perpendicularly polarized absorption bands. The measured splitting is in accordance with results of extended dipole calculations. The latter were performed for crystals of three slightly different molecules where the angles between the long axes varied between 70° and 100°. The aliphatic tails form a lattice which is epitaxially related to the centered rectangular one of the dyes. It is incommensurate, and the tails are tilted with tilt azimuth either parallel to the a or to the b axis (depending on the type of dye) of the dye lattice. The procedure allows formation of mixed crystals with structural parameters, depending on composition, between those of the pure compounds. Hence also the optical spectra can be tuned via composition maintaining sharp band edges and emission bands.

Formation of a Topochemically Photoreactive Mixed Crystal by Grinding and Its Mechanistic Interpretation

Abstract A mixed crystal of ethyl and propyl α-cyano-4-[2-(4-pyridyl)ethenyl]cinnamate was formed in the solid state by simple grinding. The mixed crystal formation was ascertained by the change in the X-ray diffraction pattern upon grinding and by 1H NMR and HPLC analyses of the photoproducts. According to several attempts, it was concluded that a mechanical treatment was necessary for the formation of the mixed crystal phase. The kinetic data concerning the analysis of the topochemical photoproducts revealed that the following growth of the mixed crystal phase was triggered by each of the mixed crystal nuclei and that the growth of the mixed crystal phase was accelerated by either thermal annealing or exposure in humidity. During the early stage of mixed crystal growth, the growth rate was nearly proportional to the surface area of the nuclei of the mixed crystal.

Formation of mixed crystals in microbial conversion of sterols and steroids

The phenomenon of formation of mixed crystals during microbial conversions involving water-insoluble substrate and product was examined through the investigation of two biotransformations: (a) dehydrogenation of hydrocortisone by Arthrobacter simplex and (b) oxidation of cholesterol by Rhodococcus erythropolis. Evidence for the intrinsic ability of hydrocortisone and prednisolone molecules to form mixed crystals from both artificial solutions and microbial conversion systems was provided by x-ray powder diffractometry and differential scanning calorimetry. Mechanical agitation was shown to further promote the bioconversion of mixed crystals from an initial value of 83% prednisolone to an ultimate value of 95%. The solids in the bioconversion system of cholesterol were shown by their x-ray diffractions to be a mixture of single substrate and product crystals. A scheme of the various crystal structures likely to be formed in bioconversions—mixed single crystals, mixed crystals, and coated crystals—was presented. A solubility-based classification of the sterols and steroids with regard to their likelihood to form mixed crystals in bioconversion systems was suggested.

Melt‐polymerized aliphatic‐aromatic copolyamides. I. Melting points of nylon 66 copolymerized with aromatic diamines and terephthalic acid

AbstractMeasurements of melting points were made for melt‐polymerized aliphatic‐aromatic copolyamides; (i) N66PT, (ii) N66CT, and (iii) N66ST, i.e., nylon 66 [poly(hexamethylene adpamide)] copolymerized with three kinds of aromatic diamines, p‐phenylenediamine (P), bis (p‐aminophenyl) methane (C), and bis (p‐aminophenyl) sulfone (S), and terephthalic acid (T). As the mole fraction of copolymerized component n1, increases in the case of (ii) or (iii), the melting point decreases uniformly according to the relationship of Flory. In the case of (i), the melting point first decreases and then begins to increase after passing a minimum at n1 ≃ 0.2. A preliminary WAXD (wide‐angle X‐ray diffraction) study on N66PT crystals shows that the basal plane of the crystal lattice shifts from that of nylon 66 toward hexagonal packing with the increase of n1. It was suggested that the formation of mixed crystals for a series of N66PT will be attributed to their melting point behavior.

Growth and structure of lamellar mixed crystals of octacalcium phosphate and apatite in a model system of enamel formation

Lamellar mixed crystals of octacalcium phosphate (OCP) and apatite were synthesized in a model system of enamel formation in the presence of 1 ppm F − at 37°C and at pH 6.5. The crystal has long and thin plate-like morphology and contained a distinct OCP lamella in the center of the apatite matrix. The thickness of the OCP lamella in the a -axis direction is one to several unit cells. Some apatite crystals embed a central layer instead of the distinct OCP lamella. The OCP lamella and the central layer are parallel to the (100) plane of the apatite, while the c -axis of the OCP is parallel to the c -axis of the apatite. Analysis suggests that (1) F − causes the growth of apatite on OCP and regulates the formation of the lamellar mixed crystals of OCP and apatite, (2) the OCP lamella acts as a template for the subsequent epitaxial growth of apatite, and (3) the lamellar mixed crystals grow mainly in the c-axis direction of both the OCP and apatite. These results strongly support the idea that enamel crystals take a thin and long ribbon-like morphology when the initially formed OCP acts as a template for the subsequent growth of apatite in the enamel formation.

Crystallization and mesomorphic disordering of di-n-hexylsilylene/di-n-pentylsilylene copolymers

A series of di-n-hexyl/di-n-pentylsilylene copolymers with monomodal molecular weight distribution have been prepared by Wurtz-coupling of dichlorosilanes using isooctane as a co-solvent. Crystallization and mesomorphic disordering were studied with regard to thermochromic effects and the silicon-chain conformation by DSC, MAS29Si-NMR, and UV-spectroscopy. The results show that the formation of a columnar mesomorphic phase is little affected by copolymerization of DHS and DPS units. Due to the preordering in the mesophase, three-dimensional ordering in the crystal is not hindered by topological constraints and isodimorphic mixed crystal formation was observed over the whole copolymer composition range. The predominantly all-trans structure, which is observed for crystalline poly(di-n-hexylsilylene), is maintained for copolymers containing up to 20%n-pentyl substituents.

Mixed-perovskite substrates for high-Tc superconductors

Mixed perovskites of high quality have been grown from multicomponent melts using the Czochralski method. The formation of mixed crystals is accomplished by coupled substitution. Rare-earth aluminium perovskites are used as host lattices. The cations of the host lattice are partially replaced by the guest ions Sr2+, Ca2+, Ta5+ and Nb5+. The distribution coefficients of the cations are adjusted to unity by appropriate choice of the melt compositions. Single crystals up to 30 mm in diameter and 80 mm in length have been obtained. Depending on the composition the crystallographic structure of the mixed perovskites is cubic, tetragonal or orthorhombic. The cubic crystals are twin free. The lattice constants can be varied in the range 0.378 ≤ a0 ≤ 0.388 nm. The dielectric constants (13 ≤ ϵ ≤ 21) and the dielectric losses (tan δ ≤ 1 × 10-3 are reasonably low. The thermal expansion coefficient α of the cubic perovskites is about 1 × 10-5ΔC-1 in the temperature range between 100 and 900ΔC.

Untersuchungen an elektronenleitenden Oxidsystemen xv. Mischkristallbildung und Leitfähigkeit im system Mg 2TiO 4MgTi 2O 4

The formation of mixed crystals has been studied in the spinel system Mg(Mg 1− x Ti 1− x IVTi 2x III)O 4. Vegard's rule appears to be approximately obeyed. MgTi 2O 4 cannot be prepared by thermal synthesis because of the partial evaporation of magnesium. The electrical conductivity is based on polaron hopping. In comparison with solid solutions Mg 1− x Ti 1− x IVTi IIIO 3 in the spinel series, the non-linear increase of the conductivity with increasing x caused by screening of the Coulomb attraction of the d-electrons is less effective. Potential fluctuations resulting from random occupation of the B-sites by Mg II, Ti IV and Ti III atoms contribute to the high activation energies exhibited in the spinel series.

Applicability of reversed-phase liquid chromatography for the speciation of vanadyl and nickel metalloporphyrins in oil extracts

Gradient reversed-phase liquid chromatography was examined with a view to using it as a separation technique for the speciation of vanadyl and nickel porphyrins in oil extracts. Poor separations were obtained when precipitation of the sample constituents occurred in the starting mobile phase. The reasons for the precipitation phenomena were found to be highly complex. Mixed crystal formation, slow dissolution kinetics and saturation may influence the elution behaviour of species present in the oil extracts. When precipitation was avoided the separation of the vanadyl porphyrins was significantly improved. Thus far, no satisfactory separations have been obtained for the nickel porphyrinic species.

Hydrogenation of palm stearine: Changes in chemical composition and thermal properties

AbstractThe progress of hydrogenation of palm stearine using atrans promoting catalyst has been followed by monitoring the chemical composition and the thermal properties by NMR and DSC techniques. The results show that hydrogenation occurs in 2 stages. In the first stage, the formation of the triglyceride, PEP from the lower melting PLP, allows an enhanced mixed crystal formation with the high melting PPP. Although this promotes a greater rate of crystallization in the α form and transformation to the β′ form in tempering, further transformation to the β form is inhibited. Tm is thereby reduced in the early stages even though both the rate of crystallization and the solid fat content at temperatures <40°C are increased. In the later stages of hydrogenation, the conversion of the triglyceride POO to the higher melting POS increases the proportion of the sample which can crystallize in the high melting fraction to at least 80%. Although this still cannot be transformed into the β crystal form, Tm for the overall sample and the solids content at all temperatures are now greater than those of the original palm stearine sample.

Notizen: Austausch von Nitrit- gegen Bromidanionen in Na3NO3 / Replacement of Nitrite by Bromide Anions in Na3NO3

Na3O(NO2) and Na3OBr are isostructural and form solid solutions with a miscibility gap χ = 0.28 to 0.62 as referred to Na3OBrx(NO2)1-x, at room temperature. Above 593 Κ formation of mixed crystals was observed for all compositions. In contrast to pure Na3O(NO2), the solid solutions do not undergo any phase transition in the investigated temperature range from 293 Κ to 130 K. These findings reveal that collective interactions between the dynamically disordered NO2 --groups are involved in the phase transitions of Na3NO3.

Hydrothermal synthesis of (Zr0.33 Ti0.67) O2–srilankite in the system ZrO2TiO2H2OMF; (M = Na, K)

AbstractThe hydrothermal synthesis in the system ZrO2TiO2H2OMF; (M = Na, K) is discussed for the temperature range 573 K–773 K and the pressure of 100 MPa to 150 MPa. Starting with Zr‐ and Ti‐ butoxides their co‐precipitated hydrolysates which had been annealed in different ways were used as initial products. The formation of mixed crystals (Zr, Ti)O2 analogous to the rare mineral srilankite was dependent on the crystallinity and homogeneity of the initial products. 573 K and 100 MPa proved to be the most favourable conditions for synthesis. Higher temperatures (773 K) produced multiphase crystallizates of (Zr, Ti)O2, baddeleyite, anatas, and rutile.

Dielectric relaxation in a mixture of a side chain liquid-crystalline polymer and a low molecular liquid crystal

Abstract Dielectric measurements on two mixtures of a low and high molecular liquid crystal and the pure components have been carried out in the frequency range from 1 Hz to 100 kHz. In the SA and supercooled SA phase two absorption ranges were observed. The low frequency absorption belongs to the low molecular weight liquid crystal. The shift in the relaxation frequency of both compounds indicates complete miscibility. The strong difference of the absorption intensity of the polymer on heating and cooling can be associated with the formation of mixed crystals at smaller concentrations of the low molecular weight compound.

Dielectric properties of condensed fluoromethanes and fluoromethane mixtures

Dielectric measurements were performed on liquid and solid (CF4)1−x (CMF3)x with M=H, Cl, and Br. The dielectric behavior of the polar molecules in the liquid state is well described by an Onsager equation. At the melting point, pure hydrogenated and brominated fluoromethanes condense into dipolar rigid phases, while pure tetrafluoromethane forms a plastic crystal. CClF3 exhibits dipolar relaxational phenomena in the solid state. The data indicate that near the melting point this compound is close to a transition into a plastic phase. Mixtures of CF4 with CHF3 and CBrF3 exhibit monotectic phase diagrams with a limited solubility in the liquid state and complete immiscibility in the solid state. However, formation of mixed crystals is found in (CF4)1−x (CClF3)x which exhibits a eutectic phase diagram.

ChemInform Abstract: Molecular Discrimination in the Formation of Mixed Crystals of Some Substituted Chlorocinnamic Acids.

AbstractIt is shown that the cinnamic acids (Ia) and (Ib) form a distinct 2:1 complex (II) which is neither a solid solution nor a mixture of (Ia) and (Ib).

Molecular discrimination in the formation of mixed crystals of some substituted chlorocinnamic acids

Molecular discrimination in the formation of mixed crystals of some substituted chlorocinnamic acids

Ordnugszustände in binären Systemen aus Poly(vinylidenfluorid) und Poly(vinylfluorid)

AbstractBinary systems of poly(vinylidene fluoride) (PVDF) and poly(vinyl fluoride) (PVF) have been studied by differential scanning calorimetry and X‐ray diffraction. The polymers were prepared by 60Co‐γ‐inducted bulk polymerization at 273 K. The increase of the glass transition temperature of PVDF in binary systems with low concentrations of PVF can be explained by a partial miscibility of the components in the non‐crystalline state. In the same way the higher thermal stability of PVF in the binary systems up to temperatures of 30 K above the melting point of PVF can be explained. PVDF and PVF crystallize separately. There is no indication of the formation of mixed crystals of the two polymers. On the other hand, the crystallization of PVDF is influenced by the presence of PVF.

Roentgenographic and thermodynamic study of the existence of mixed crystals in the LaC2-YC2 system

The reaction heats for the mixed crystal formation in the LaC2-YC2 system were obtained from the equilibrium gas pressures measured during the carboreduction. The low positive values of the reaction heats are consistent with the X-ray diffraction study of the carbothermal reduction products of the oxides, where the formation of the (La, Y)C2 dicarbide mixed crystals was only observed in the two side regions up to 25 mol.% of each of the minority components. The composition of the gaseous hydrolysis products of the (La, Y)C2 mixed crystals corresponds to the hydrolysis products of the dicarbide. No methane was detected for samples with low yttrium contents.