Derivation of the amplitude equation for a hydromagnetic convective problem

The amplitude equation for a convective system under a vertical magnetic field is derived. The coefficients in these equations have been numerically calculated for infinite Prandtl number fluids and for boundary conditions both free and rigid top and bottom. The results confirm that, for realistic parameter values only stationary convection can be present and the pattern is made by convective rolls.     

Thermal behaviour of the copper-substituted cobalt hydroxide fluoride series CuxCo1−x(OH)F

The basic salt cobalt(II) hydroxide fluoride was obtained by partial hydrolysis of a 0.2M solution CoF₂. Mixed Co(II)·Cu(II) hydroxide fluorides with an isomorphic substitution of Co(II) by Cu(II) ions up to 20% were also prepared. XRD data obtained on these compounds indicate that they are single-phase compounds, their X-ray powder patterns being almost identical to the Co(OH)_0.7F_1.3 diagram, with Zn(OH)F-type structure. However, their stoichiometries are very close to Co_1?xCu_x(OH)F (0≤x≤0.2). Thermal analysis studies carried out up to 1273 K in still air or under dynamic vacuum conditions show that in air CoO or a mixture of CoO and CuO is formed as final product. In vacuum, at high temperature, with a Ta heating surface, pure α-Co or a mixture of α-Co and Cu is formed at about 973 K.     

Determination of mercury in urine of Mexican dentists

Mercury levels in the urine of Mexican dentists were determined by instrumental neutron activation analysis. A control group with no suspicion of contamination by mercury was also studied. Acid digestion was used for samples treatment and the presence of ²⁰³Hg was determined by gamma spectroscopy. The ranges of mercury concentration found in urine were: for the dentists group from 0.19 to 11.56 mg Hg/l, with a mean value of 3.16 mg Hg/l and standard deviation of 2.74 mg Hg/l, and for the control group from 0.03 to 0.05 mg Hg/l with a mean value of 0.04 mg Hg/l and standard deviation of 0.01 mg Hg/l. The mercury levels found in urine of dentists are associated with several exposure factors which appear to be positively correlated to its concentration, such as years of dental practice, amalgamation techniques, etc.     

RATE CONSTANTS FOR INTERACTION OF 1O21Δg) WITH AZIDE ION IN WATER

Abstract— The rate constant for quenching of ¹O₂ by azide ion in water was determined to be (5.0 ± 0.4) × 10⁸ M ⁻¹ s⁻¹ using a variety of sensitizers (including humic acids) and ¹O₂ acceptors. The apparent second-order rate constant decreases with pH below pH 5.5 in accordance with the protonation of azide ion to form hydrazoic acid (p K _a= 4.6). Quenching by hydrazoic acid is at least 2 orders of magnitude slower than by azide ion. Greater than 99% of all interactions between ¹O₂ and azide ion involve physical quenching rather than chemical reaction. Humic acid triplets are not significantly quenched by azide ion at concentrations less than 2 m M , allowing azide ion quenching to be used as a diagnostic test for the intermediacy of ¹O₂ in photosensitized oxidations in natural surface waters.     

New approaches to the study of labile structures in poly(vinyl chloride) by phenolysis

Three samples of PVC having different tacticities were treated with phenol in cyclohexanone at 60° until there was no more reaction (96 hr). The incorporation of phenate into the chain was observed by u.v.-visible spectroscopy. The phenolysis extent was shown to be lower as the syndiotactic content of the polymer increased. On the other hand, the modified polymers showed enhanced thermal stability relative to the starting materials particularly for the more isotactic sample. The observed behaviour of either the starting materials in phenolysis or the treated polymers in thermal degradation appears to be very similar to that observed previously in the reaction with sodium thiophenate. On the basis of this similarity, it might be concluded that the phenolysis occurs with chlorine atoms at isotactic placements. That effect would confirm the thermal lability of some normal chlorine atoms as indicated by other results.     

Refined crystal structures of the scandium garnets Ca3Sc2Si3O12, Ca3Sc2Ge3O12, and Cd3Sc2Ge3O12

Journal of Structural Chemistry -     

Absolute structure of La<Subscript>3</Subscript>Ga<Subscript>5</Subscript>SiO<Subscript>14</Subscript> langasite crystals

The absolute structure of La₃Ga₅SiO₁₄ piezoelectric crystals (a = 8.1746(6) Å, c = 5.1022(4) Å, space group P321, Z = 1) with the positive sense of rotation of the plane of polarization is refined using X-ray diffraction analysis (R = 1.37%, R _w = 1.71%, 2413 unique reflections, max sinθ/λ = 1.15 Å^?1). The contributions from the anharmonicity of thermal vibrations of lanthanum atoms are calculated with the use of the components of the third-and fourth-rank tensors. It is demonstrated that these contributions can have a significant effect.     

Phase transitions in compounds with the Ca<Subscript>3</Subscript>Ga<Subscript>2</Subscript>Ge<Subscript>4</Subscript>O<Subscript>14</Subscript> structure

Possible structural changes described by the group-subgroup relationships in the Ca₃Ga₂Ge₄O₁₄-type structure (sp. gr. P321) are considered. The most probable phase transitions seem to be those accompanied by lowering of the symmetry to the maximal non-isomorphic subgroups P3 and C2. It is shown that only destructive phase transitions accompanied by symmetry rise up to the minimal non-isomorphic supergroups for the given structure type can take place. The change of the trigonal symmetry to monoclinic is revealed in La₃SbZn₃Ge₂O₁₄, whose crystal structure is refined as a derivative structure of the Ca₃Ga₂Ge₄O₁₄ structure type within the sp. gr. A2 (C2). At ～250°C, La₃SbZn₃Ge₂O₁₄ undergoes a reversible phase transition accompanied by symmetry rise, A2 ? P321. Similar phase transitions, P321 ? A2, are also observed in La₃Nb_0.5Ga_5.5O₁₄ and La₃Ta_0.5Ga_5.5O₁₄ under the hydrostatic pressures 12.4(3) and 11.7(3) GPa, respectively. The mechanisms of compression and phase transition are based on the anisotropic compressibility of a layer structure. With the attainment of the critical stress level in the structure, the elevated compressibility in the (ab) plane gives rise to a phase transition accompanied by the loss of the threefold axis. Attempts to reveal low-temperature phase transitions in a number of representatives of the langasite family have failed.     

Growth and structure of La<Subscript>3</Subscript>Zr<Subscript>0.5</Subscript>Ga<Subscript>5</Subscript>Si<Subscript>0.5</Subscript>O<Subscript>14</Subscript> crystals

The complete X-ray structure determination of Czochralski grown La₃Zr_0.5Ga₅Si_0.5O₁₄ single crystals with the Ca₃Ga₂Ge₄O₁₄ structure is performed (sp. gr. P321, a = 8.226(1) Å, c = 5.1374(6) Å, Z = 1, Mo K_α1 radiation, 1920 crystallographically independent reflections, R = 0.0166, R_w = 0.0192). The absolute structure is determined. It is shown that possible transition of some of La atoms (～1.2%) from the 3e to 6g position may give rise to the formation of structural defects.