Crystal growth,structure, electron paramagnetic resonance and magnetic properties of (NH4)2V3O8

A method to grow single crystals of ammonium vanadate (IV, V) (NH₄)₂V₃O₈ has been devised. The crystal structure is tetragonal P4bm; residual factor is R = 0.030. Cell parameters are $\text{a = 8.891 ± 0.004}\text{A}\text{?}$ and $\text{c = 5.582 + 0.002}\text{A}\text{?}$. The V⁵⁺ atom lies at the center of a triangular pyramide (VO₄ tetrahedron) while the V⁴⁺ atom is on A 4-fold rotation axis at the center of a square-based pyramide VO₅ whose symmetry point group is almost C_4v with the short V = O bond lying along the 4-fold axis parallel to the c edge of the tetragonal cell. Crystals are thin platlets with (001) cleavage planes. The platlets have very often a square or rectangular shape limited by {100} or {110} planes. Each single crystal was not large enough to record a good e.p.r. spectrum, but by sticking on the same quartz plate a score of them it was possible to gather enough crystals so to record correct spectra and by orienting the plate to obtain resonance lines separately for g_∥ = 1.9263 et g_τ = 1.9755. Measurements at 283 K on powder samples gave times for spin-spin relaxation T₂ = 0.4 × 10^?7s and for spin-lattice relaxation T₁ = 1.6 × 10^?7s. The magnetic structure is characterized by an exchange narrowing ω_e = 3 × 10¹⁰rad/s which corresponds to a transition temperature of about 0.5 K. Static susceptibility measurements at high magnetic field show a paramagnetic behaviour with an antiferromagnetic interaction which is interpreted in the magnetic space group P_2c4bm as the interaction between V⁴⁺ ions from consecutive planes parallel to (001).     

Study by differential scanning calorimetry of the structure of aliphatic chains and water in Cs soaps-water systems

Summary Caesium stearate- and caesium palmitate-water systems are investigated by DSC measurements. Thermodynamical data of transition between crystallized form and mesomorphic state indicate a greater disorder of the chains for these soaps than for lecithin, in the same lamellar structure. The study of the water melting in an extended range of concentration allows us to distinguish different behaviors: the simplest two phases model for the two extremes ranges of concentration (y < 20% and > 60%) is proposed to explain the stability of the freezing temperature and enthalpy; the solubilization of conter-ions in water and the bonding of some water molecules could explain the depression of freezing temperature and the decrease of enthalpy, in the intermediate range of concentration.

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Résumé Nous avons étudié par DSC les transitions de phase dans les systemes stearate et palmitate de cesium-eau. Les parametres thermodynamiques de la fusion des chaines paraffiniques révèle que dans les phases mésomorphis le désordre des chaines est plus grand que dans les systemes lecithine-eau. L'étude de la fusion de l'eau, a l'intérieur d'une phase lamellaire bien caractérisée, nous permet de distinguer plusieurs comportements: le modele le plus simple ä deux phases permet d'interpreter la stabilité de l'enthalpie et de la température de fusion dans les deux domaines extremes de concentration en eau (y < 20% et > 60%); la solubilisation des contre ions dans l'eau et la fixation de quelques molecules d'eau pourrait expliquer l'abaissement de la température de fusion et la diminution d'enthalpie, dans le domaine intermédaire de concentration.

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With 4 figures and 1 table     

Photorefractive detection of antiparallel ferroelectric domains in BaTiO3 and BaTiO3:Co crystals

3 ). Rod-shaped domains with square cross sections are revealed. Received: 11 July 1997/Accepted: 19 November 1997     

Reduction—complexation des thiophosphinates par le nickelocene

2-Phenyl-1,3,2-dioxaphosphorinane rearranges at 250°C to afford 2-phenyl-2-oxo-l,2-oxaphospholane which is, in turn, converted into the 2-thio derivative by P₄S₁₀. This cyclic thiophosphinate reacts with nickelocene and allyl iodide to yield

(L = 2-phenyl-1,2-oxaphospholane) by reduction—complexation. The same scheme is successfully applied to the 1-oxa-2-phosphacyclohepta-4,6-diene nucleus. Thus, the P^IV=S reduction—complexation process works with POC compounds. This is not the case with PSC compounds (for example, with dithiophosphinates). Some chemistry of 2-phenyl-l,3,2-dithiaphosphorinane is also described: Arbuzov reaction with PhCH₂Br, thermal rearrangement and complexation with nickelocene and allyl iodide.     

Experimental study of the reaction vp → μ−pπ+: Gargamelle neutrino propane experiment

A sample of 357 events fitting the reaction ν + p → μ^? + p + π⁺ on free proton is obtained from the Gargamelle neutrino propane experiment at the CERN-PS. The average value of the cross section above 1 GeV is found to be (0.60 ± 0.07) 10^?38 cm². The reaction is dominated by the production of the Δ⁺⁺(1232) resonance. Results on the Δ⁺⁺ spin density matrix are given.     

La diphenyl(cyclopentadienyl-2 ethyl)phosphine: un nouveau type de coordinat chelatant phosphore(III)cyclopentadienyle pour les metaux de transition

Diphenyl(2-cyclopentadienylethyl)phosphne yields, by treatment of its cyclopentadienylstannyl derivative with BrMn(CO)₅, a substituted cymantrene. Under UV radiation this compound yields a cyclopentadienylphosphorus(III) chelate.