Synthesis of Novel Organosiliconsulfur-Containing Tetrasubstituted Imidazoles Sonocatalyzed by LaxSr1−xFeyCo1−yO3 Nanoperovskites

Abstract

The one-pot synthesis of tetrasubstituted imidazoles by use of a series of La_xSr_1 ? xFe_yCo_1 ? yO₃ perovskites as catalysts is described. The La_0.8Sr_0.2Fe_0.34Co_0.66O₃ nanocatalyst had the greatest activity in the heterogeneous cyclocondensation of an aldehyde, benzil, ammonium acetate, and a primary aromatic amine in water under ultrasonic irradiation. Some of the derivatives generated during this work were utilized as substrates for the synthesis in good yields of novel multifunctional tetrasubstituted imidazoles with Me₃Si, C=S, and SH groups, via nucleophilic attack of tris(trimethylsilyl)methyllithium (TsiLi) at the carbon of carbon disulphide.     

Mössbauer spectroscopic analysis of Bi1−xSrxFeO3−δ perovskites

Bi_1?xSr_xFeO_3?δ perovskites synthesised by solid-state reaction in ambient atmosphere or in a closed vessel were analysed by X-ray diffraction and Mössbauer spectroscopy. The evolution of the valence state of iron with both Bi/Sr ratio and oxygen content has been more particularly discussed. The samples are single phase and homogeneous for x ≤ 0.5 (Bi-rich) and x > 0.8 (Sr-rich). For intermediate Sr contents, the samples are less homogeneous and tend to contain both Bi-rich and Sr-rich phases. The appearance of Sr-rich phases for x > 0.5 corresponds to the appearance of Fe⁴⁺, to compensate for the lack of positive charges due to the replacement of Bi³⁺ by Sr²⁺.     

A Mössbauer spectroscopy investigation of SrFe1−xScxO3−δ perovskites

Polycrystalline samples of oxygen deficient perovskites SrFe_1?xSc_xO_3?δ (0 ≤ x ≤ 0.5) have been synthesized by direct solid state reactions. Each compound has been stabilized with two different oxygen stoichiometries. The structural study shows, firstly, the good cationic homogeneity of the samples and, secondly, that the Sc and Fe atoms are randomly distributed over the same crystallographic site, whatever the scandium content. This implies that no anionic order is possible. A detailed Mossbauer spectroscopy study clearly shows that the substitution of scandium for iron involves an oxygen content decrease which decreases the tetravalent iron content until its total disappearance for x = 0.5. The evolutions of the isomer shift, the quadrupolar splitting and the relative intensity versus the Sc³⁺ content are depicted in the present paper.     

Behavior of Mn3+ ions in four-layered hexagonal (Sr1−x−yLaxBay)MnO3

Stoichiometric four-layered hexagonal (4H) (Sr_1?x?yBa_xLa_y)MnO₃ was synthesized using a standard ceramic technique. Rietveld analysis at room temperature indicated that the Mn–O(1) distance increased and the Mn–O(2) distance decreased with the increase in x. The samples were n-type semiconductors and exhibited hopping conductivity in a small-polaron model below 533 K. The Mn³⁺ ion acted as a donor and the electron transfer became active through the Mn³⁺–O–Mn⁴⁺ path. The samples were antiferromagnetic and the Néel temperature (T_N) was constant regardless of y when x was fixed to 0.3, whereas T_N shifted to a high temperature when y was fixed to 0.02. The face-sharing Mn³⁺–O(2)–Mn⁴⁺ interaction strengthened as the Mn–O(2) distance decreased, and T_N shifted to a high temperature as a result.     

Structure,nonstoichiometry and magnetic properties of the perovskites Sr1−xCaxMnO3−δ

The structural, thermal and magnetic properties of the perovskite-type alkaline-earth manganites of the series Sr_1−xCa_xMnO_3−δ (0⩽x⩽1) were investigated. SrMnO_3−δ forms a hexagonal perovskite lattice and shows a first-order transformation to a highly defective cubic high-temperature modification. By substituting Ca for Sr (x>0.25) the hexagonal perovskite is suppressed and a cubic (or orthorhombic) lattice becomes stabilized for all temperatures. For x=0.5 and 0.75 cubic perovskites with a large nonstoichiometry (e.g., δ=0.25 for x=0.5) are obtained at 1400 °C. The defective perovskites are prepared by either quenching from high temperature or by cooling in an inert atmosphere. The oxygen vacancies are easily filled by subsequent reoxidation at low temperature (400–600 °C) and stoichiometric samples are obtained. Orthorhombic perovskites are formed at T⩽1200 °C with the nonstoichiometry δ increasing with increasing temperature (e.g., δ=0.06 at 1000 °C and δ=0.14 at 1200 °C for x=0.5). Slow cooling in air results in almost complete reoxidation (δ=0). CaMnO_3−δ is an orthorhombic perovskite with a large range of nonstoichiometry (0⩽δ⩽0.30). The cubic to hexagonal phase transformation of the Sr-rich samples is accompanied by a large expansion of the lattice that is reduced by Ca substitution. The Ca/Sr-manganites are antiferromagnets with T_N of 170 K for x=0.5 and δ=0.02 and 120 K for x=1 and δ=0.05.     

Synthesis and properties of BaCe1−xYxO3−δ–BaWO4 composite protonic conductors

Barium cerate doped by trivalent rare earth metal ions is a potentially huge component of materials for electrochemical industry due to its high protonic conductivity. However, the poor chemical stability especially in the presence of CO₂, SO₂ or H₂O, resulting in decreasing the mechanical durability of obtained materials, limits their possible applications. The new approach towards stable ceramic protonic conductors with high electrical conductivity is presented. Thermal stability of yttrium doped (10 mol%) of BaCeO₃ was enhanced by forming the composite material BaCe_0.9Y_0.1O₃–BaWO₄ (10 mol% of BaWO₄). The synthesis was performed by solid-state reaction method. The detailed study of thermal decomposition of starting powders mixture was performed using thermogravimetry and differential thermal analysis (TG/DTA) techniques combined with Evolved Gas Analysis (EGA—mass spectrometry). Structure, phase composition and microstructure together with thermal stability of sintered materials were determined. The exposition tests were performed to characterise the stability of composites in carbon dioxide and water vapour-rich atmospheres. The samples were exposed to atmosphere containing CO₂/H₂O (7 % of CO₂ in air, 100 % RH) at temperature of 25 °C for 300 h. Thermal analysis supplied with mass spectrometry was applied to analyse the materials after the test. The results of this experiment showed better chemical resistance of composite material—BaCe_0.9Y_0.1O₃ with 10 mol% of BaWO₄ compared to single phase material.     

Defects in doped perovskite-like lanthanum cobaltite crystals La1−x SrxCo1−y MeyO3−δ (Me = Cu and Mn)

The paper presents a thermodynamic analysis of the formation of equilibrium defects in perovskitelike La_1?x Sr_xCo_1?y Me_yO_3?δ oxides, where Me = Cu or Mn, x = 0.0 or 0.3, and y = 0.0, 0.25, or 0.3, at high temperatures (873 K ≤ T ≤ 1373 K) depending on the composition and oxygen pressure (10^?8 atm ≤ $p_{O_2 } $ ≤ 1 atm). The results were used to study the nature of charge transfer. Small-radius polarons were shown to be responsible for the electric properties of the cobaltites under consideration; their concentrations and mobilities were calculated.     

Magnetoresistance in the ferromagnetic metallic perovskite SrFe1−xCoxO3

The investigation of the magnetic and transport properties of the oxygen deficient perovskites SrFe_1−xCo_xO_3−δ shows that these compounds exhibit both ferromagnetism and metallicity in a wide compositional range (0≤x≤0.70). Negative magnetoresistance is evidenced for the first time in these oxides, in contrast to SrCoO_3−δ. These properties are explained by superexchange interactions between cobalt and iron according to the scheme Fe³⁺OCo⁴⁺↔Fe⁴⁺OCo³⁺. This model is strongly supported by ⁵⁷Fe Mössbauer measurements which show the existence of two sites at room temperature, high spin localized Fe⁴⁺ and delocalized Fe^3+α sites, whereas magnetic disordering suggesting spin fluctuations is observed at 5 K as soon as cobalt is introduced into the SrFeO₃ structure.     

Electrical and electro-chemical characterisation of La0.99Fe1−x Ni x O3−δ perovskites

(LFN, 0<x<0.6) perovskites were synthesised by a solid-state route and were characterised by powder XRD, dilatometry, four-point DC conductivity measurements and electro-chemical impedance spectroscopy (EIS) on cone-shaped electrodes using a Ce_1.9Gd_0.1O_1.95 (CGO10) electrolyte. All the compounds were of single phase, and they belong to either the cubic or the hexagonal crystal system. The thermal expansion coefficient (TEC) was in the range 10.7*10⁻⁶ K⁻¹ to 13.4*10⁻⁶ K⁻¹, which continued to increase with increasing nickel content. The highest electronic conductivity was measured for the composition giving a value of 670 S/cm at 380 °C. The highest electro-chemical performance was measured for the composition giving an area specific resistance as low as 5.5 Ωcm² at 600 °C based on EIS measurements on a cone-shaped electrode. Composite cathodes made from and CGO10 revealed a rather low performance due to an un-optimised micro-structure.

---

Dual-Defects Adjusted Crystal-Field Splitting of LaCo1−xNixO3−δ Hollow Multishelled Structures for Efficient Oxygen Evolution

To boost the performance for various applications, a rational bottom-up design on materials is necessary. The defect engineering on nanoparticle at the atomic level can efficiently tune the electronic behavior, which offers great opportunities in enhancing the catalytic performance. In this paper, we optimized the surface oxygen vacancy concentration and created the lattice distortion in rare-earth-based perovskite oxide through gradient replacement of the B site with valence alternated element. The dual defects make the electron spin state transit from low spin state to high spin state, thus decreasing the charge transport resistance. Furthermore, assembly the modified nanoparticle subunits into the micro-sized hollow multishelled structures can provide porous shells, abundant interior space and effective contact, which enables an enhanced mass transfer and a shorter charge transport path. As a result, the systemic design in the electronic and nano-micro structures for catalyst has brought an excellent oxygen evolution performance.     

Synthesis,structure, and properties of randomly mixed and layer-ordered SrMn1−xGaxO3−δ perovskites

We report the synthesis of SrMn_1−xGa_xO_3−δ perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques, we have extended the solubility limit of Ga³⁺ in the cubic perovskite phase to x≈0.33. Higher Ga concentrations lead to mixed phases until a single-phase ordered double-perovskite structure is obtained at x=0.5, i.e., Sr₂MnGaO_6−δ. In the cubic perovskite phase the maximum oxygen content is 3−x/2, which corresponds to 100% Mn⁴⁺. All maximally oxygenated solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases. Reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn_0.7Ga_0.3O_2.5 below 30 K. The brownmillerite phase at low oxygen content, Sr₂MnGaO₅, is found to have Icmm crystallographic symmetry. At 12 K its magnetic structure is found to order in the Icm′m′ magnetic symmetry corresponding to a G-type antiferromagnetic structure of Mn³⁺ ions. At higher oxygen content, Sr₂MnGaO_5.5 is found to have Cmmm crystallographic symmetry with disordered oxygen vacancies. At 12 K two competing long-range magnetic structures are found for the Mn⁴⁺ sublattice having C_Im′m′m symmetry (G-type), and C_Pm′m′m symmetry (C-type), together with a G-type short-range magnetic correlations.     

Zur Kenntnis der Salpetersäure. X Methoden zur Gasanalyse im System N2−NO−NO2−N2O4−N2O3−HNO2−HNO3

Zusammenfassung Es werden vier Methoden zur Analyse einer Gasmischung N₂–NO–NO₂–N₂O₄–N₂O₃–HNO₂ besprochen und ihre Bedeutung für die Bestimmung der einzelnen Molekelarten kritisch auseinandergesetzt. Nach keiner der Methoden kann man den Gehalt an Salpetriger Säure und Salpetersäure neben den Oxyden des Stickstoffes auffinden. Aus diesem Grunde ist die Analyse der genannten Gasmischung nach den vier Methoden allgemein nicht genau durchführbar.Mit 1 Abbildung.Herrn Prof. Dr.A. Skrabal zum 75. Geburtstag gewidmet.     

Mixed valence states of Cr and Fe in La1−xSrxFe0.8Cr0.2O3−y

The La_1?xSr_xFe_0.8Cr_0.2O_3?y (x = 0.2, 0.4, 0.6 and 0.8) phases were studied by X-ray photoelectron spectroscopy at room temperature and ⁵⁷Fe Mössbauer spectroscopy at different temperatures. Mixed valence states were observed both for chromium and iron ions, justifying the complex magnetic behaviour exhibited by these compounds. The Mössbauer results indicate the simultaneous presence of Fe³⁺, Fe⁴⁺ and Fe⁵⁺ at 4.2 K and the co-existence of Fe³⁺ and Fe⁽³⁺ⁿ⁾⁺ at T = 293 K, with the latter fraction increasing with increasing strontium content. The presence of Cr^3+/4+ is interpreted as being mainly responsible for the incomplete charge disproportionation reaction of iron at low temperature, as deduced from the Mössbauer results.     

The Cl−NH3, Cl−H2O,F−NH3 and F−H2O clusters and their photoelectron spectra

The geometries of the Cl^–NH₃, Cl^–H₂O, F^–NH₃ and F^–H₂O clusters are optimized using the coupled cluster method. The four lowest ionization potentials are then calculated, leading to the ground and low excited states of the neutral species. The first three IPs describe ionization from the externalp state of the halogen atom, whereas the fourth corresponds to ionization from the NH₃ or H₂O moiety, leading to charge transfer complexes. These complexes were recently observed in the photoelectron spectrum of Cl^–NH₃, in full accord with our calculations.Supported in part by the U.S.-Israel Binational Science Foundation     

Raman spectroscopy investigation on (Pb1−xLax)(Zr0.90Ti0.10)1−x/4O3 ceramic system

A Raman spectroscopy study at room temperature was carried out on (Pb_1−xLa_x)(Zr_0.90Ti_0.10)_1−x/4O₃ ceramics (x = 2, 3, 4 at%). The results were analyzed considering the x-ray patterns at room temperature showing a mixture of two phases: a rombohedral-ferroelectric phase and an orthorhombic-antiferroelectric, increasing the% of the second one with the lanthanum concentration. For x = 3 at%, the analysis was also carried out in a wide temperature range. Two anomalies were evaluated, one around 363 K, which has been associated to a ferroelectric-antiferroelectric phase transition; the second one around 430 K, which has been associated to a transition from an incommensurable state to a ferroelectric phase.     

Das Ultrarotspektrum von isotaktischem Polypropylen zwischen 1500 cm−1 und 330 cm−1

Zusammenfassung Die Frequenzen der Skelettschwingungen von isotaktischem Polypropylen werden mit Hilfe des Valenzkraft-Modells berechnet und mit den im UR-Spektrum gemessenen Frequenzen verglichen. Die übereinstimmung ist so gut, da? eine Zuordnung der Banden zu den Molekelschwingungen gegeben werden kann. Dagegen k?nnen die Intensit?tsverh?ltnisse nicht gekl?rt werden. Herrn Professor Dr.Hellwege danke ich für Anregungen und Diskussionen zu dieser Arbeit. Ferner danke ich den Firmen Kalle, Wiesbaden, und Farbwerke Hoechst für die zur Verfügung gestellten Polypropylenproben, der Deutschen Forschungsgemeinschaft für die Bereitstellung des Rechenautomaten und HerrnG. Günther für die Hilfe beim Programmieren.     

Properties and performance of BaxSr1−xCo0.8Fe0.2O3−δ materials for oxygen transport membranes

The present paper discusses the oxygen transport properties, oxygen stoichiometry, phase stability, and chemical and mechanical stability of the perovskites (BSCF) and (SCF) for air separation applications. The low oxygen conductive brownmillerite phase in SCF is characterized using in-situ neutron diffraction, thermographic analysis and temperature programmed desorption but this phase is not present for BSCF under the conditions studied. Although both materials show oxygen fluxes well above 10 ml/cm²·min at T=1,273 K and pO₂=1 bar for self-supporting, 200 μm-thick membranes, BSCF is preferred as a membrane material due to its phase stability. However, BSCF’s long-term stable performance remains to be confirmed. The deviation from ideal oxygen stoichiometry for both materials is high: δ>0.6. The thermal expansion coefficients of BSCF and SCF are 24×10⁻⁶ and 30×10⁻⁶ K⁻¹, respectively, as determined from neutron diffraction data. The phenomenon of kinetic demixing has been observed at pO₂<10⁻⁵ bar, resulting in roughening of the surface and enrichment with alkaline earth metals. Stress–strain curves were determined and indicated creep behavior that induces undesired ductility at T=1,073 K for SCF. Remedies for mechanical and chemical instabilities are discussed.     

Chromatography of Anions on Alumina Thin Layers: Effect of Transition Metals on CI−-Br−-I− and No− 2-NO− 3 Separations

Abstract

Chromatographic behaviour of eighteen anions on thin layers of alumina and alumina mixed with silica gel (1:1, 1:2 and 2:1) has been studied using mixed acidic organic solvent systems containing formic acid. Though the addition of silica gel to alumina enhances the mobility and clarity of detection of anions, but it causes the increased tailing for Fe(CN)^3- ₆, Cr0₄ ^2- and Cr₂O₄ ^2-. Formic acid is responsible for the differential migration of anions. All the anions remained at the starting line (R_F = 0) in pure organic solvents. Formic acid-Ketone systems gave better results compared to formic acid-alcohol systems. Development time increases with the increase of viscosity/mol. wt. of organic solvents. The mutual separation of C1, Br^?, Br^?, I^? and NO^? ₂ and NO^? ₃ were achieved on pure alumina using formic acid-acetone solvent systems. The effect of transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) on C1^?-Br^?-I^? and NO^? ₂-NO^? ₃ separations has been studied.