Cation-deficient lanthanum manganite oxides: Experimental and theoretical studies

Cation-deficient lanthanum manganite oxides with 0.8 ≤ La/Mn ≤ 1.25 were synthesized using a multi-step decomposition of gel precursors and investigated from experimental and theoretical point of view. The XRD (X-Ray Diffraction) analysis of the mixed LaMnO₃ oxide crystal concludes to a hexagonal structure, space group R-3c, excluding the presence of pure oxides such as La₂O₃, Mn₂O₃, or MnO₂ whatever the ratio La/Mn is. Oxides with nominal formulae La_1?xMnO_3+y and LaMn_1?x′O_3+y contain more than one defect structure involving valence defect (holes h), anionic vacancies as well as cationic vacancies in A and B sublattices of the perovskite structure. With the increase of La or Mn non-stoichiometry, the oxygen content y decreases more with La-deficient compositions than with Mn-deficient ones. The La/Mn ratio influences strongly the relationship between [h], [V_O], [V_La^?] and [V_Mn^?].The DFT-GGA (Density functional Theory, Generalized-Gradient Approximation) simulation of these compounds using VASP (Vienna Ab-initio Simulation Package) concludes that the electronic structure for the optimized stoichiometric La₆Mn₆O₁₈ is not optimal, relative to that expected considering Mn(III) ions with four alpha electrons each. The non-stoichiometry is the easiest way of reducing the Jahn–Teller instability by depopulating the half-filled e_g orbitals. A partial oxidation is then stabilizing. Creation of defects, either an O insertion or a cationic vacancy, allows finding an ideal count. In the case of pure cationic defects (missing a La(III) or a Mn(III) cation), the amount of vacancies is one missing cation upon twelve. The compact nature of lanthanum manganite oxide does not allow oxygen insertion within the bulk structure and oxidation can only be achieved at the surface without O penetration. The formation of antisites is endothermic. The creation of mixed vicinal vacancies, one oxygen and one cationic vacancy (La or Mn), is exothermic for a concentration of defects of one defect per twelve cations.     

Unicité des solutions des équations de Navier–Stokes dans les espaces de Morrey–Campanato

In a recent work [P.G. Lemarié-Rieusset, Uniqueness for the Navier–Stokes problem: Remarks on a theorem of Jean-Yves Chemin, Nonlinearity 20 (2007) 1475–1490], P.G. Lemarié-Rieusset proved the uniqueness of solution to the Navier–Stokes equations in the space provided that p>2 and q>d. In this paper, we prove a local version of this result which covers the limit case q=d. Precisely, we prove the uniqueness of solution to the Navier–Stokes equations in the space for every p>2 and r>2 where is the closure of the test functions in the Morrey–Campanato space Mr,d(Rd). The prove of our result relies on an extension of the Comparison Theorem of P.G. Lemarié-Rieusset (Theorem 21.1 in [P.G. Lemarié-Rieusset, Recent developments in the Navier–Stokes problem, Chapman & Hall/CRC, 2002]). Moreover, this extension allows us to prove the uniqueness of solution to the Navier–Stokes equations in a functional space closed to the critical space C([0,T],M2,d(Rd)).     

Thermal and spectroscopic properties of Tm3 + doped TZPPN transparent glass laser material

In this work a transparent bulk glass with the mol% composition 76TeO₂·10ZnO·9.0PbO·1.0PbF₂·3.0Na₂O doped with Tm^3 + has been synthesized. Results of differential thermal analysis (DTA) indicate a high thermal stability and low tendency to crystallization of this glass. The refractive indices at different wavelengths, the Urbach energy, the optical energy gap, the Sellmeier gap energy and the dispersion energy have been estimated. Spectroscopic quality factor of Tm^3 + was evaluated from optical absorption spectra. Electric and magnetic dipole transition probabilities, branching ratios, and radiative lifetimes of several excited states of Tm^3 + have been predicted using calculated intensity Judd–Ofelt parameters. The classical McCumber theory has been applied to evaluate the emission cross-sections for ³F₄ → ³H₆ transition around 1.8 μm. This study shows that TZPPN glass doped with Tm^3 + ions is a promising candidate for laser applications.     

Methyl dynamics flattens barrier to proton transfer in crystalline tetraacetylethane

We analyze the interplay between proton transfer in the hydrogen-bond bridge, O···H···O, and lattice dynamics in the model system tetraacetylethane (TAE) (CH(3)CO)(2)CH═CH(COCH(3))(2) using density functional theory. Lattice dynamics calculations and molecular dynamics simulations are validated against neutron scattering data. Hindrance to the cooperative reorientation of neighboring methyl groups at low temperatures gives a preferred O atom for the bridging proton. The amplitude of methyl torsions becomes larger with increasing temperature, so that the free-energy minimum for the proton becomes flat over 0.2 ?. For the isolated molecule, however, we show an almost temperature-independent symmetric double-well potential persists. This difference arises from the much higher barriers to methyl torsion in the crystal that make the region of torsional phase space that is most crucial for symmetrization poorly accessible. Consequently, the proton-transfer potential remains asymmetric though flat at the base, even at room temperature in the solid.     

Dynamic uniaxial extension of elastomers at constant true strain rate

Elastomers are widely used for damping components in various industrial contexts because of their remarkable dissipative properties: they can bear severe mechanical loading conditions, i.e., high strain rates and large strains. Depending on the strain rate, the mechanical response of these materials can vary from purely rubber-like to glassy. In the intermediate strain rate range (1-100/s), uniaxial extension experiments are classically conducted at constant nominal strain rate. We present here a new experimental methodology to investigate the mechanical response of soft materials at constant true strain rate in the intermediate strain rate range. For this purpose, the displacement imposed on the specimen by the tensile machine is an exponential function of time. A high speed servo-hydraulic machine is used to perform experiments at strain rates ranging from 0.01 to 100/s. A specific specimen is designed in order to achieve a uniform strain field (and thus a uniform stress field). Furthermore, an instrumented aluminium bar is used to measure the applied force; which overcomes the difficulties due to dynamic effects. Simultaneously, a high speed camera enables the measurement of strain in the sample using a point tracking technique. Finally, the method is applied to determine the stress-strain curve of an elastomer for both loading and unloading responses up to a stretch ratio λ = 2.5; the influence of the true strain rate on both stiffness and dissipation of the material is then discussed.     

Optical and crystal-field analysis of Er ion in Y2O3-P2O5 thin films

Luminescence spectra of erbium ions doped in Y₂O₃-P₂O₅ thin films, with different P₂O₅ content (from 3% to 47%), were analysed with crystal-field Hamiltonian model with D_2d symmetry including J-mixing effect. The empirical crystal-field parameters (CFPs) obtained for the best fit of calculated to experimental energy levels allows us to confirm the well-established YPO₄ phase for 47% of P₂O₅. The CFPs are compared to those calculated for Ce³⁺, Nd³⁺ and Dy³⁺ in the YPO₄ host. This work is a continuation of our previous results for erbium-doped Y₂O₃ thin films.     

Phytochemical analysis and in vitro antimicrobial and free-radical-scavenging activities of the essential oils from Euryops arabicus and Laggera decurrens

The essential oils of the aerial part of two Asteraceae species, namely Euryops arabicus Steud. and Laggera decurrens (Vahl.) Hepper and Wood, were studied by GC and GC/MS. In parallel the antimicrobial and antioxidant activities were evaluated. The investigation led to the identification of 48 and 44 compounds for both plants, respectively. The essential oil of E. arabicus was rich in oxygenated sesquiterpenes (39.9%). The oil also contained a high content of sesquiterpene hydrocarbons (24.1%). Compounds such as caryophyllene oxide (8.6%), T-cadinol (7.0%), spathulenol (5.2%), (E)-β-caryophyllene (6.0%) and 2-epi-(E)-β-caryophyllene (6.0%) were the main constituents of the oil. Oxygenated monoterpenes also predominated in L. decurrens (46.3%). The thymoquinone-derivative, 3-methoxy-2-methyl-5-(1-methylethyl)-2,5-cyclohexadiene-1,4-dione (28.1%), thymol (5.7%) and eudesma-11-en-4a-ol (7.0%) were the most abundant constituents. Both essential oils showed antimicrobial activity with MIC-values between 0.13-5.25 mg/mL. Furthermore, only the essential oil of L. decurrens exhibited a strong antioxidant activity (91%) at 500 μg/mL.     

Volumetric Properties of Ethylammonium Nitrate + <Emphasis Type="Italic">γ</Emphasis>-Butyrolactone Binary Systems: Solvation Phenomena from Density and Raman Spectroscopy

The densities of binary mixtures of ethylammonium nitrate (EAN) ionic liquid (IL) and γ-butyrolactone (BL) have been measured over the entire range of concentrations at 293.15, 298.15, 303.15, 308.15, 313.15 and 318.15 K and under ambient pressure. Experimental densities were used to calculate excess molar volumes V_m^EV_{m}^{\mathrm{E}}, isobaric and excess isobaric expansion coefficients α and α ^E. The excess molar volumes have both negative and positive values, while the excess isobaric expansion coefficients are negative over the entire composition range. The V_m^EV_{m}^{\mathrm{E}} values have been fitted to the Redlich-Kister polynomial equation, and other volumetric properties such as the partial molar volumes V _mi , the excess partial molar volume V^E_miV^{\mathrm{E}}_{mi} and the partial molar volumes at infinite dilution V^￥_miV^{\infty}_{mi} were calculated. The results have been interpreted in terms of dipole-dipole interactions, hydrogen bonds formation and structural factors of these mixtures. The FT-Raman spectroscopy study of the intensity variations of some characteristic bands such as the C=O stretching band at 1763 cm⁻¹, C–O symmetric stretching band at 932 cm⁻¹ and C–C stretching band at 872 cm⁻¹ of BL has been undertaken. The solvation phenomenon is evidenced by the modifications of these band intensities due to the presence of the IL ions. Moreover, the Raman spectroscopy corroborates the volumetric study. The average number of BL molecules in the primary solvation shell of the ethylammonium cation lies between 3 and 4 depending on the temperature.     

Cross-coupling reactions of alkenylsilanolates. Investigation of the mechanism and identification of key intermediates through kinetic analysis

The mechanism of the fluoride-free, palladium-catalyzed cross-coupling reaction of potassium (E)-heptenyldimethylsilanolate, K(+)(E)-1(-), with 2-iodothiophene has been investigated through kinetic analysis. The order of each component was determined by plotting the initial rates of the reaction against concentration. These data provided a mechanistic picture which involves a fast and irreversible oxidative insertion of palladium into the aryl iodide and a subsequent intramolecular transmetalation step from a complex containing a silicon-oxygen-palladium linkage. First-order behavior at low concentrations of silanolate with excess palladium(0) complex supports the formation of this complex as the turnover-limiting step. The change to zeroth-order dependence on silanolate at high concentrations is consistent with the intramolecular transmetalation becoming the turnover-limiting step.