Structural characterisation of the perovskite series SrxCa1−x−yNdyMnO3: Influence of the Jahn-Teller effect

The crystal structures of the perovskite manganites Sr_xCa_1−x−yNd_yMnO₃ with y=0.1 or 0.2 have been investigated using synchrotron X-ray powder diffraction. At room temperature the structures change from depending on the cation distribution, the different structures exhibiting different tilts of the MnO₆ octahedra. High temperature diffraction measurements demonstrate the presence of, an apparently continuous, isosymmetric I4/mcm to I4/mcm phase transition associated with the removal of long range orbital ordering. Heating the manganites to still higher temperatures results in a continuous transition to the cubic structure. A feature of such transitions is the continuous evolution of the octahedral tilt angle through the I4/mcm to I4/mcm phase transition. The orthorhombic structures do not exhibit orbital ordering and although a first order transition to the tetragonal structure is observed in Sr_0.4Ca_0.5Nd_0.1MnO₃, this high temperature tetragonal structure does not exhibit orbital ordering.     

A facile synthetic route for the preparation of the Au/Mox/Al2O3 catalysts towards CO oxidation

Summary A new facile single-step synthetic route is reported for the preparation of Au/MO_x/Al₂O₃ catalysts. The preparation method has the merit of facility but leads to not only the simultaneous load of both gold source and MO_x precursor on Al₂O₃ support, but also the formation of Au/MO_x/Al₂O₃ with high gold loading ratio, high dispersion and high activity for CO oxidation.     

An investigation of the promotional effect of propylene or propane traces on the activity of sulfatedPt/g-Al2O3andPt-Sn/g-Al2O3inCH4combustion

Summary A strong promoting effect of the presence of C₃H₈or C₃H₆was determined for the combustion of CH₄in excess oxygen, over pre-sulfated 1%Pt/g-Al₂O₃and pre-sulfated 1%Pt-2%Sn/g-Al₂O₃catalysts.</o:p>     

Effects of the A-site cation number on the properties of Ln5/8M3/8MnO3 manganites

The properties of manganites can be tuned by changing the doping level x in Ln_1−xM_xMnO₃. A second mechanism allows tuning of magnetic and electronic properties, for fixed x values, by varying the average A-cation radius, 〈r_A〉. Moreover, for fixed x and 〈r_A〉 values, the changes in the A-cation size variance, σ², also modify the ferromagnetic and metal-insulator transition temperatures. Here, we investigate the influence of the number of A-site cations on Ln_5/8M_3/8MnO₃ manganites, where x, 〈r_A〉 and σ² values are kept constant, and in the absence of phase separation phenomena. We have found that the number of cation species at the A site (N_A) has a strong influence on the width of the ferromagnetic and metal-insulator transitions, and a small influence on the average transition temperature. This behavior is opposite to that observed for increasing values of the variance σ² in manganites, with the same x and 〈r_A〉 values, where average transition temperatures are strongly reduced.     

Ni_(x)Cu_(y)-B_(24)N_(28)催化CO_(2)氧化C_(3)H_(8)速控步骤的反应机理研究北大核心CSCD

采用密度泛函理论(DFT)研究了C_(3)H_(8)和CO_(2)在Ni_(x)Cu_(y)-B_(24)N_(28)(x+y=4,x=1、2、3、4)表面吸附及速控步骤反应机理.计算了C_(3)H_(8)、CO_(2)和相应中间体在Ni_(x)Cu_(y)-B_(24)N_(28)表面的吸附能以及6条可能路径下的反应热和活化能.计算结果表明,C_(3)H_(8)和CO_(2)在Ni_(x)Cu_(y)-B_(24)N_(28)表面是物理吸附,C_(3)H_(8)+CO_(2)→CH_(3)CHCH_(3)+OCOH是最有利的路径,其在不同催化剂表面的活化能顺序是NiCu_(3)-B_(24)N_(28)(1.42 eV)、Ni_(2)Cu_(2)-B_(24)N_(28)(1.57 eV)、Ni_(3)Cu-B_(24)N_(28)(1.62 eV)、Ni_(4)-B_(24)N_(28)(1.75 eV).由此可知,在Ni_(x)Cu_(y)-B_(24)N_(28)催化CO_(2)氧化C_(3)H_(8)的体系中,Cu含量直接影响其催化活性,即NiCu_(3)-B_(24)N_(28)用于催化CO_(2)氧化C_(3)H_(8)有一定优势.     

Adsorption isotherms for the system n-heptane/ethanol/HDPE

The equilibrium isotherms for the adsorption of n-heptane and ethanol on high density polyethylene (HDPE) are reported at 27–110°C and at 20–100 mmHg.

The experimental apparatus and procedures are described for three methods, namely flow, static and chromatographic.

The obtained results are discussed on the basis of the Henry law for adsorbates at low coverage on solids.     

Analysis of nonvalent interactions in the crystals of conformational polymorphs of the composition CaHbNcOdSe by means of molecular voronoi-dirichlet polyhedra

Nonvalent interactions in crystals of compounds of the composition C _a H _b N _c O _d S _e that exhibit conformational polymorphism and have no fewer than four structurally characterized modifications are analyzed using molecular Voronoi-Dirichlet polyhedrals. A unique combination of the types of occurring intra- and intermolecular nonvalent contacts is shown to correspond to each conformational polymorph. Nonvalent interactions involving hydrogen atoms are found to occur most often in crystal structures, and least often those involving S, N, and O atoms.     

Textural and catalytic properties of the FexOy/Fe-KClO4 system

Selected properties of commercial iron powders, standardised in the atmosphere of hydrogen, have been studied. The reactivity of iron oxides in the thermal decomposition of KClO₄ in the solid-state mechanical mixture of Fe and KClO₄ containing 9, 13, 17, 21 and 25 wt.% of KClO₄, respectively, has been tested by the differential thermal analysis (DTA) and thermogravimetric analysis (TG). It has been established that the Fe₃O₄ phase on the surface of the iron powder act as an effective catalysts in the thermal decomposition of KClO₄.     

Study of the fluorescent dysprosium/EDTA/Tiron/surfactant system

A dysprosium/EDTA/Tiron/surfactant system at pH 11–12 is used for the spectrofluorimetric determination of dysprosium. The fluorescence intensity is increased 22-fold on addition of cetyltrimethylammonium bromide. There is no interference from at least a 100-fold molar excess of other rare earth ions, except for terbium.     

Synthesis and characterization of the La1−xSrxFeO3−δ system and the fluorinated phases La1−xSrxFeO3−xFx

The oxyfluorides La_1−xSr_xFeO_3−xF_x have been prepared by fluorination of the precursor oxides La_1−xSr_xFeO_3−δ via a low temperature route using poly(vinylidene fluoride) (PVDF). The structures of the oxides and oxyfluorides were investigated in detail by the Rietveld analysis of powder diffraction data. The oxyfluorides crystallize in the space group Pnma for 0<x≤0.9 (SrFeO₂F itself is cubic, space group Pm-3m) and show a sort of two-step structural distortion for decreasing x. Furthermore, a structural comparison of the oxyfluorides with the oxides is given, revealing an increase of the volume per La_1−xSr_xFeX₃ unit during fluorination, of which the magnitude highly depends on the value of x.     

Lanthanum pyrochlores and the effect of yttrium addition in the systems La2−xYxZr2O7 and La2−xYxHf2O7

The crystal structures of the compounds La_2−xY_xZr₂O₇ and La_2−xY_xHf₂O₇ with x=0.0, 0.4, 0.8, 1.2, 1.6, and 2.0 have been studied using neutron powder diffraction and electron microscopy to determine the stability fields of the pyrochlore and fluorite solid solutions. The limits of pyrochlore stability in these solid solutions are found to be close to La_0.8Y_1.2Zr₂O₇ and La_0.4Y_1.6Hf₂O₇, respectively. In both systems the unit cell parameter is found to vary linearly with Y content across those compositions where the pyrochlore phase is stable, as does the x-coordinate of the oxygen atoms on the 48f (x,,) sites. In both systems, linear extrapolations of the pyrochlore data suggest that the disordering is accompanied by a small decrease in the lattice parameter of approximately 0.4%. After the pyrochlore solid solution limit is reached, a sharp change is observed from x∼0.41 to 0.375 as the disordered defect fluorite structure is favoured. Electron diffraction patterns illustrate that some short-range order remains in the disordered defect fluorite phases.     

Effects of Rh on the thermoelectric performance of the p-type Zr0.5Hf0.5Co1−xRhxSb0.99Sn0.01 half-Heusler alloys

We show that Rh substitution at the Co site in Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 (0≤x≤1) half-Heusler alloys strongly reduces the thermal conductivity with a simultaneous, significant improvement of the power factor of the materials. Thermoelectric properties of hot-pressed pellets of several compositions with various Rh concentrations were investigated in the temperature range from 300 to 775 K. The Rh “free” composition shows n-type conduction, while Rh substitution at the Co site drives the system to p-type semiconducting behavior. The lattice thermal conductivity of Zr_0.5Hf_0.5Co_1−xRh_xSb_0.99Sn_0.01 alloys rapidly decreased with increasing Rh concentration and lattice thermal conductivity as low as 3.7 W/m*K was obtained at 300 K for Zr_0.5Hf_0.5RhSb_0.99Sn_0.01. The drastic reduction of the lattice thermal conductivity is attributed to mass fluctuation induced by the Rh substitution at the Co site, as well as enhanced phonon scattering at grain boundaries due to the small grain size of the synthesized materials.     

Phase behavior of the lecithin/water/isooctane and lecithin/water/decane systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.     

Temperature Effect on the Phase Behavior of the Systems Water/TritonX-100,114,405/n-Butanol/n-Hexane

We have investigated the effect of temperature on the phase behavior and the partition coefficients of nonionic surfactants in water/n-hexane systems. We found that the composition of the conjugate single and two phase solution in the pseudoternary diagram is continuously transformed from one state to other by increasing the temperature. We also demonstrated that the partition coefficient of a nonionic surfactant is a measure of the relative hydrophilic-lipophilic nature of the surfactant, if the measurement is performed at low concentration.     

Tmperature programmed techniques study on the nature of the oxidant in/on V2O5catalyst ?

Summary The nature of the oxidising species in/on a vanadium pentoxide (V₂O₅) catalyst has been studied using a combination of transient techniques: (i) Temperature programmed desorption (TPD), (ii) Temperature programmed reduction (TPR), (iii) Temperature programmed oxidation (TPO) and (iv) Temperature programmed reaction (TPRn). Chemisorbed oxygen was found not to exist on a fully oxidised V₂O₅ catalyst by TPD. The TPR in CO over V₂O₅ catalyst gave three peak maxima at 930, 982 and 1043 K, indicating that three types of kinetically different oxygen states exist in/on the catalyst. Reoxidation of the CO reduced V₂O₅ catalyst by N₂O resulted in the quantitative replacement of the lattice oxygen. A further reduction of the N₂O reoxidised catalyst gave a significantly different TPR profile compared to the original material suggesting that a less crystalline material had formed. The presence of phosphorus in (VO)₂P₂O₇ was found to labilise the lattice oxygen.     

铪掺杂对锆钛酸钡钙陶瓷电学性能的调控

用固相反应法制备了B位Hf掺杂的Ba_0.95Ca_0.05 Zr_0.05Ti_0.95-xHf_x (BCZ_0.05T_(0.95-x)-xH) (x=0、0.02、0.04、0.06和0.08)陶瓷。研究B位同时含有Hf、Zr、Ti三种元素时,BCZ_0.05T_(0.95-x)-xH陶瓷的相结构变化与电学性能的差异。XRD测试结果表明：Hf₄₊^融入晶格,造成晶格膨胀。BCZ_0.05T_(0.95-x)-xH(x=0、0.02)陶瓷的XRD测试结果表明,该体系中存在明显的四方相与正交相共存的多晶型相转变(PPT),但正交相逐渐转变为主晶相;BCZ_0.05T_(0.95-x)-xH (x=0.04、0.06)陶瓷的XRD结果表明,四方相进一步减少,三方相却逐渐增加;当x继续增大到0.08时,XRD测试发现该体系只存在正交相和三方相结构。SEM测试发现,随着Hf的掺杂量增加,晶粒尺寸逐渐减小。剩余极化随Hf掺杂量的增加先增大再减小,在x为0.02时取得最大值,Hf掺杂显著降低矫顽场(Ec)。压电测试结果表明BCZ_0.05T_0.96-0.04H陶瓷具有最大压电系数(d₃₃=259 pC/N)。     

Variations of the FeGa3 Structure Type in the Systems CoIn3−xZnx and CoGa3−xZnx

We present an investigation of the quasibinary systems CoIn_3−xZn_x and CoGa_3−xZn_x which were structurally characterized by X-ray diffraction experiments and, in the case of CoGa_3−xZn_x, additionally by neutron powder diffraction experiments. The limiting compositions were found to be x=0.81(2) and x=0.73(2) for CoIn_3−xZn_x and CoGa_3−xZn_x, respectively. The isotypic binary compounds CoIn₃ and CoGa₃ crystallize with the FeGa₃ structure type (tetragonal, space group P4₂/mnm, Z=4) in which the p-block atoms form an array of columns of centered cubes defined by two different crystallographic sites. The substitution of In or Ga by Zn takes place in an ordered fashion and produces “colored” variants of the FeGa₃ parent structure: In both systems Zn enters exclusively the position corresponding to the cube centers. Additionally, in CoIn_3−xZn_x this position is substituted in such a way that for a composition CoIn_2.5Zn_0.5, columns of Zn- and In-filled In₈ cubes along the c axis alternate. The latter substitution pattern is accompanied by a symmetry lowering of the parent FeGa₃ structure: The structure of CoIn_3−xZn_x is described by the space group P4₂/m in which the cube center position is split into two separate sites. By performing first-principles electronic structure calculations we investigated the general bonding situation of the compounds CoIn₃ and CoGa₃ and the particular electronic effect when incorporating Zn. With respect to the density of states of the binary compounds the exchange of Ga or In by Zn virtually affects only the electronic states just below the Fermi level. On increasing Zn concentration a dip is created in the density of states which approximately coincides with the location of the Fermi level for an electron count corresponding to limiting composition of the two systems.     

Water-containing derivative phases of the Srn+1FenO3n+1 series

The n=1, 2, 3 and ∞ members of the homologous series Sr_n+1Fe_nO_3n+1 of layered iron oxides are investigated for their tendency to accept additional layers of water in their crystals. The phases possess a Ruddlesden-Popper-type SrO-(SrO-FeO₂)_n crystal structure, where the n=∞ limit is nothing but the perovskite structure. It is revealed that the n=1, 2 and 3 phases readily accommodate one or two layers of water between adjacent SrO layers, whereas the n=∞ member which lacks the SrO-SrO double-layer unit remains intact in the presence of water. The speed of the water intercalation process is found to decrease with increasing n. Among the layered water derivatives, the n=2 phase with two water molecules per formula unit, i.e. Sr₃Fe₂O₇·2H₂O, was found to be most stable.     

Synthesis and characterization of the novel Nb3O5F5 niobium oxyfluoride: the term n=3 of the NbnO2n−1Fn+2 series

The solid-state synthesis of the oxyfluoride Nb₃O₅F₅, its crystal structure determined from X-ray powder diffraction data as well as some physical characterizations, are reported. Nb₃O₅F₅ constitutes the term n=3 of the Nb_nO_2n−1F_n+2 series related to the Dion-Jacobson phases. It crystallizes, at room temperature, in the tetragonal system (space group I4/mmm (no. 139); Z=4; a=3.9135(1) Å, c=24.2111(2) Å, and V=370.80(3) Å³). The crystal structure appears to be an in-between of the three-dimensional network of NbO₂F and the two-dimensional packing of NbOF₃ (term n=1 of the Nb_nO_2n−1F_n+2 series). This layered structure consists of slabs made of three Nb(O,F)₆ corner-linked octahedra in thickness (n=3) shifted one from another by a ()/translation. Oxygen and fluorine atoms are randomly distributed over all the ligand sites.