Structural and thermal investigation of gadolinium gallium mixed oxides obtained by coprecipitation: Observation of a new metastable phase

Polycrystalline gadolinium gallium mixed oxides were prepared by coprecipitation and annealing at various temperatures below 1000 °C. The oxide materials appear to be X-ray amorphous after a heat treatment at 500 °C for 30 h, but after 30 h at 800 and 900 °C a major, unreported, hexagonal phase, isostructural with TAlO₃ compounds (where T=Y, Eu, Gd, Tb, Dy, Ho, Er) appears to crystallize. On the other hand, a highly energetic mechanical treatment of the amorphous powder previously annealed at 500 °C changes considerably the shape and position of exothermal events occurring in the range from 700 up to 900 °C. Subsequent annealing at 900 °C of the mechanically treated powder gives rise to the complete formation of the Gd₃Ga₅O₁₂ garnet structure at the expense of the hexagonal phase and of the minor Gd₄Ga₂O₉ oxide phase. However, a 7.0 wt% contamination is found to be due to tetragonal zirconia coming from vials and balls colliding media. The garnet phase may have strong deviations from the nominal stoichiometry of the garnet, as suggested by the refined lattice parameter obtained from the powder diffraction patterns and by the remarkable absence of intensity relative to the (220) Bragg peak position.     

Differentiation of the geographical origin of durum wheat semolina samples on the basis of isotopic composition

The identification of the geographical origin of foods is attracting great interest from consumers and producers since it may be used as a criterion for guaranteeing quality and authenticity. Stable isotope techniques can provide useful information on the origin of food products. The natural abundance isotopic ratios of carbon, oxygen and nitrogen were determined and a comparison made between the isotopic content of samples originating from Italy and those of samples originating from other countries (Canada, Turkey, Australia). A correlation was found between the isotopic composition and geographical origin of the samples. The relationship between isotopic content and the latitude has been confirmed.     

Solubility of Propene in Water and in a Mineral Medium for the Cultivation of a XanthobacterStrain

Solubility measurements of propene in water at atmospheric pressure and at thirteen temperatures in the range 293.15–323.15 K were carried out with a precision of 0.3%, using an apparatus based on the saturation method. The mole fractions, at a gas partial pressure of 101.325 kPa, were fitted to a Clarke–Glew–Weiss equation, which was used to calculate the standard molar Gibbs energy, enthalpy, and entropy changes for the process of transferring the propene molecules from the gaseous to the water phase. Solubility measurements of propene in the growth medium ofXanthobacter Py2 at 298.15, 303.15, and 308.15 K were also carried out at atmospheric pressure. These experimental data, expressed in Ostwald coefficients at the total pressure of 101.325 kPa, were about 2% lower than their water counterparts.     

A novel general route for the synthesis of C-glycosyl tyrosine analogues

A general method for the synthesis of C-glycosyl amino acids is described here. The stereoisomerically pure tyrosine analogues alpha-L-4 and beta-L-6 are prepared in reasonable overall yields from allyl derivatives 10 and 11. The key step is a benzannulation procedure which is employed in the creation of the aromatic ring that bears the amino acid function.     

Toward the Total Synthesis of Scleritodermin A: Preparation of the C(1)-N(15) Fragment

The synthesis of the C₁-N₁₅ fragment of the marine natural product Scleritodermin A has been accomplished through a short and stereocontrolled sequence. Highlights of this route include the synthesis of the novel ACT fragment and the formation of the α-keto amide linkage by the use of a highly activated α,β-ketonitrile.     

Optimal, quasi-optimal and superlinear band-Toeplitz preconditioners for asymptotically ill-conditioned positive definite Toeplitz systems

In this paper we are concerned with the solution of Hermitian Toeplitz systems with nonnegative generating functions . The preconditioned conjugate gradient (PCG) method with the well-known circulant preconditioners fails in the case where has zeros. In this paper we consider as preconditioners band-Toeplitz matrices generated by trigonometric polynomials of fixed degree . We use different strategies of approximation of to devise a polynomial which has some analytical properties of , is easily computable and is such that the corresponding preconditioned system has a condition number bounded by a constant independent of . For each strategy we analyze the cost per iteration and the number of iterations required for the convergence within a preassigned accuracy. We obtain different estimates of for which the total cost of the proposed PCG methods is optimal and the related rates of convergence are superlinear. Finally, for the most economical strategy, we perform various numerical experiments which fully confirm the effectiveness of approximation theory tools in the solution of this kind of linear algebra problems.