Raman,infrared and force field studies of K2(12)C2O4 x H2O and K2(13)C2O4 x H2O in the solid state and in aqueous solution,and of (NH4)2(12)C2O4 x H2O and (NH4)2(13)C2O4 x H2O in the solid state

The Raman and infrared spectra of solid K2(12)C2O4 x H2O are reported together with, for the first time, the corresponding Raman and infrared spectra of solid K2(13)C2O4 x H2O. Raman spectra of aqueous solutions of both isotopomers are also reported. In the solid state the oxalate anion is planar with D2h symmetry in this salt, whereas in aqueous solution the Raman spectra of the anion are best interpreted on the basis of D2d symmetry. The Raman spectra of solid (NH4)2(12)C2O4 x H2O and (NH4)2(13)C2O4 x H2O, in which the oxalate anion is twisted from planarity by 28 degrees about the CC bond, have also been recorded. Several reassignments have been made. The harmonic force field for the oxalate anion in the D2h, D2 and D2d geometries has been determined in part, and approximate values of key valence force constants determined. All the observed band wavenumbers and 12C/13C isotopic shifts are well reproduced by the force fields. The potential energy distribution of the totally symmetric normal modes of planar oxalate indicates that each mode consists of extensively mixed symmetry corrdinates and that the labels previously used for the bands seen here at 475 and 879 cm(-1) would better be described as v(CC) and deltaS(CO2), respectively, putting them in the same wavenumber order as v(NN) and deltaS(NO2) for the isoelectronic and isostructural molecule N2O4. The stretching force constants of N2O4 and planar C2O4(2-) are established to be in the order f(NN) < f(CC) and f(NO) > f(CO), consistent with the known relative bond lengths.     

Topotactic formation and exchange reactions of hydrated layered tin sulfides A x (H2O) y SnS2

Reduction of tin disulfide SnS₂ leads in a reversible topotactic reaction to the formation of layered hydrated phases A _x ⁺ (H₂O)_y[SnS₂]^x–. The latter exhibit polyelectrolyte character, i. e. ion exchange and solvent exchange reactions as a consequence of the high mobility of the interlayer species. Structure and properties of the tin sulfide hydrates are closely related to those of transition metal dichalcogenide hydrates.

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Topotaktische Bildung und Austauschreaktionen von hydratisierten Zinnsulfiden A_x(H₂O)_ySnS₂ mit Schichtstruktur

Zusammenfassung Die Reduktion von Zinndisulfid führt in einer reversiblen topotaktischen Reaktion zur Ausbildung von hydratisierten Phasen A _x ⁺ (H₂O)_y[SnS₂]^x– mit Schichtengitterstruktur. Diese weisen Polyelektrolytcharakter auf, d.h. sie zeigen Ionenaustausch- und Lösungsmittelaustausch-Reaktionen als eine Folge der großen Beweglichkeit der Moleküle zwischen den Schichten. Struktur und Eigenschaften der Zinnsulfid-Hydrate stehen denen der Übergangsmetalldichalkogenid-Hydrate sehr nahe.

     

Photoelectron imaging of Ag(-)(H2O)x and AgOH(-)(H2O)y (x=1,2, y=0-4)

The photoelectron images of Ag(-)(H(2)O)(x) (x=1,2) and AgOH(-)(H(2)O)(y) (y=0-4) are reported. The Ag(-)(H(2)O)(1,2) anionic complexes have similar characteristics to the other two coinage metal-water complexes that can be characterized as metal atomic anion solvated by water molecules with the electron mainly localized on the metal. The vibrationally well-resolved photoelectron spectrum allows the adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of AgOH(-) to be determined as 1.18(2) and 1.24(2) eV, respectively. The AgOH(-) anion interacts more strongly with water molecules than the Ag(-) anion. The photoelectron spectra of Ag(-)(H(2)O)(x) and AgOH(-)(H(2)O)(y) show a gradual increase in ADE and VDE with increasing x and y due to the solvent stabilization.     

Sm(2-x)Dy(x)Zr2O7 pyrochlores: probing order-disorder dynamics and multifunctionality

The present work involves the synthesis of a series of Sm(2-x)Dy(x)Zr(2)O(7) compounds (0.0 ≤ x ≤ 2.0) by a controlled gel combustion process. The powders were thoroughly analyzed by powder X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, and diffuse-reflectance UV-visible spectroscopy. The powder XRD studies revealed the system to be single-phasic throughout with retention of pyrochlore-type ordering until 40 mol % of Dy(3+), beyond which the pyrochlore lattice gives way to the defect fluorite structure. Interestingly, Raman spectroscopic studies (as against XRD studies) showed retention of pyrochlore-type ordering throughout the homogeneity range of the compositions studied. This is perhaps the first study that reports retention of a weak pyrochlore-type superstructure in the Dy(2)Zr(2)O(7) system, which was otherwise known to crystallize in the defect fluorite system. The ionic conductivity measurements showed an increase in the activation energy (E(a)) with an increase in the mole percent of Dy(3+) owing to the decreased mobility with an increase in the degree of disorder. The system possesses a tunable band gap with varying amounts of Dy(3+). First-principles calculations were performed to support a decrease in the band gap of the doped system with an increase in the Dy(3+) content. The potential as photocatalysts of some of these compositions was explored, and they exhibited high photocatalytic activity for degradation of xylenol orange, with t(1/2) increasing from pure Sm(2)Zr(2)O(7) to pure Dy(2)Zr(2)O(7).     

Pt@Pd(x)Cu(y)/C core-shell electrocatalysts for oxygen reduction reaction in fuel cells

A series of carbon-supported core-shell nanoparticles with Pd(x)Cu(y)-rich cores and Pt-rich shells (Pt@Pd(x)Cu(y)/C) has been synthesized by a polyol reduction of the precursors followed by heat treatment to obtain the Pd(x)Cu(y)/C (1 ≤ x ≤ 3 and 0 ≤ y ≤ 5) cores and the galvanic displacement of Pd(x)Cu(y) with [PtCl(4)](2-) to form the Pt shell. The nanoparticles have also been investigated with respect to the oxygen reduction reaction (ORR) in proton-exchange-membrane fuel cells (PEMFCs). X-ray diffraction (XRD) analysis suggests that the cores are highly alloyed and that the galvanic displacement results in a certain amount of alloying between Pt and the underlying Pd(x)Cu(y) alloy core. Transmission electron microscopy (TEM) images show that the Pt@Pd(x)Cu(y)/C catalysts (where y > 0) have mean particle sizes of <8 nm. Compositional analysis by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) clearly shows Pt enrichment in the near-surface region of the nanoparticles. Cyclic voltammograms show a positive shift of as much as 40 mV for the onset of Pt-OH formation in the Pt@Pd(x)Cu(y)/C electrocatalysts compared to that in Pt/C. Rotating disk electrode (RDE) measurements of Pt@PdCu(5)/C show an increase in the Pt mass activity by 3.5-fold and noble metal activity by 2.5-fold compared to that of Pt/C. The activity enhancements in RDE and PEMFC measurements are believed to be a result of the delay in the onset of Pt-OH formation.     

Ca(1)(-)(x)Na(2)(x)Al(2)B(2)O(7): a structure with tunable density of Na(+) vacancies

A series of samples with the composition Ca(1)(-)(x)Na(2)(x)Al(2)B(2)O(7) (0 < x < or = 1) was investigated and a hexagonal structure with unusually large range of homogeneity (at least from x = 0.01 to 0.95) was revealed. The hexagonal phase consists of [Al(2)B(2)O(7)](infinity)(2)(-) lamellae stacked along the c axis, as in CaAl(2)B(2)O(7) and Na(2)Al(2)B(2)O(7). Nevertheless, the configuration and stacking sequence of the [Al(2)B(2)O(7)](infinity)(2)(-) lamellae are different in these three structures. In the hexagonal structure of Ca(1)(-)(x)()Na(2)(x)()Al(2)B(2)O(7), Ca and half Na cations (Na1) statistically occupy the same crystallographic site which is located between the [Al(2)B(2)O(7)](infinity)(2)(-) lamellae, the other half Na cations (Na2) distribute in the planes bisecting the [Al(2)B(2)O(7)](infinity)(2)(-) lamellae. Depending on the composition, the site occupation factor of Na2 site can vary in the same range as x, leading to a tunable density of Na(+) vacancies in the structure. The AlO(4) tetrahedra and BO(3) triangles in the structure tilt in appropriate ways to improve the bond valence sum of Na2 cations which are not sufficiently bonded to the anions.     

Exchange between sub-surface and surface oxygen vacancies on CeO2(111): a new surface diffusion mechanism

Using first principles calculations for O vacancy diffusion on CeO(2)(111), we locate a surface diffusion mechanism, the two-step O vacancy exchange one, which is more favored than the most common hopping mechanism. By analyzing the results, we identify quantitatively the physical origin of why the two-step exchange mechanism is preferred.     

Crystal-chemical analysis of the structures of oxalic acid and its salts M x (C2O4) y ·nH2O (n=0−3)

Crystal-chemical analysis of the structures of oxalic acid and its salts M_x(C₂O₄)y·nH₂O (n=0?3) is carried out. It is shown that the dimensions of oxalate ions play a critical role for closest packing in the structures of the acid and anhydrous and aqueous metal salts. In most compounds, the centers of masses of oxalate ions lie in the planes and form trigonal loops with average bond lengths and angles of 5–6 Å and 60°, respectively. The dimensions and distortions of the nets depend on the proximity of the cations and/or water molecules to the planes. For the structures of one cation, the influence of the quantity of water on the arrangement of oxalate ions is shown. The relationship between the polymorphism and variations of orientations and packings of oxalate ions is discussed. The analysis gives grounds for predicting the structural arrangement of ionic or molecular compounds provided that the structure-forming units have a pseudospherical form.     

Direct conversion of bio-ethanol to isobutene on nanosized Zn(x)Zr(y)O(z) mixed oxides with balanced acid-base sites

We report the design and synthesis of nanosized Zn(x)Zr(y)O(z) mixed oxides for direct and high-yield conversion of bio-ethanol to isobutene (~83%). ZnO is addded to ZrO(2) to selectively passivate zirconia's strong Lewis acidic sites and weaken Br?nsted acidic sites, while simultaneously introducing basicity. As a result, the undesired reactions of bio-ethanol dehydration and acetone polymerization/coking are suppressed. Instead, a surface basic site-catalyzed ethanol dehydrogenation to acetaldehyde, acetaldehyde to acetone conversion via a complex pathway including aldol-condensation/dehydrogenation, and a Br?nsted acidic site-catalyzed acetone-to-isobutene reaction pathway dominates on the nanosized Zn(x)Zr(y)O(z) mixed oxide catalyst, leading to a highly selective process for direct conversion of bio-ethanol to isobutene.     

Highly ordered Nb2O5 nanochannel film with rich oxygen vacancies for electrocatalytic N2 reduction: Inactivation and regeneration of electrode

We report the fabrication of highly ordered Nb₂O₅ nanochannel film (Nb₂O₅-NCF) onto niobium foil by an anodization method. After thermal treatment, the obtained Nb₂O₅-NCF with rich oxygen vacancies exhibits electrochemical N₂ reduction reaction (NRR) activity with an NH₃ yield rate of 2.52 × 10⁻¹⁰ mol cm^-2 s^-1 and a faradaic efficiency of 9.81% at −0.4 V (vs. RHE) in 0.1 mol/L Na₂SO₄ electrolyte (pH 3.2). During electrocatalytic NRR, the Nb₂O₅-NCF takes place electrochromism (EC), along with a crystalline phase transformation from pseudo hexagonal phase to hexagonal phase owing to H⁺ insertion. This results in the reduced NRR activity due to the decrease of oxygen vacancies of hexagonal phase Nb₂O₅, which can be readily regenerated by low-temperature thermal treatment or applying an anodic potential, showing superior recycling reproducibility.     

Role of Zr4+ cations in NO2 adsorption on Ce(1-x)Zr(x)O2 mixed oxides at ambient conditions

Mixed oxides Ce(1-x)Zr(x)O(2) prepared by slow coprecipitation in NaOH were tested for NO(2) adsorption in dynamic conditions at room temperature. The samples were characterized before and after exposure to NO(2) by XRD, N(2)-adsorption, thermal analysis, potentiometric titration, and FT-IR. Mixed oxides show a better NO(2) adsorption capacity than the parent materials (CeO(2) and Zr(OH)(4)). This effect is linked to the presence of reduced cerium and oxygen vacancies induced by the addition of Zr(4+) cations to the structure. The results indicate that NO(2) reacts with Ce(3+) to form nitrite and nitrate species on the surface. The NO retention increases with an increase in the Zr(OH)(4) content. A decrease in the density of -OH groups on the surface after the exposure to NO(2), suggests their involvement in reactive adsorption of NO and/or NO(2). From the structural point of view, no real difference was observed on the Ce(1-x)Zr(x)O(2) materials before and after exposure to NO(2).     

Pu在Gd2Zr2O7基质中的模拟固化: (Gd1-xCex)2Zr2O7+x的热物理性能研究

采用高温固相反应,以NaF作助熔剂,在1000 ℃的温度下合成了锕系元素Pu的模拟固化体(Gd_1-xCe_x)₂Zr₂O_7+x (0 ≤ x ≤ 0.7).研究了模拟固化体的物相、热膨胀系数(TEC)、热导率(TC)随温度及组成的变化规律.粉末X射线衍射(XRD)测试结果表明: Gd₂Zr₂O₇基质本身呈弱有序烧绿石结构,而用Ce⁴⁺取代Gd³⁺的模拟固化体都呈缺陷萤石结构. (Gd_1-xCe_x)₂Zr₂O_7+x的Ce(3d) X射线光电子能谱(XPS)有六个峰,结合能分别位于881.7, 888.1, 897.8, 900.4, 907.1, 916.1 eV处,与CeO2的XPS图谱非常相似,说明Ce为四价.随着温度的升高,所有样品的热膨胀系数总体上呈增大趋势.在室温至750 ℃附近,大部分样品的热导率随温度的升高而降低,之后热导率又呈小幅上升.在相同温度下,固化体(Gd_1-xCe_x)₂Zr₂O_7+x (0 ≤ x ≤ 0.7)的热膨胀系数及热导率随组成变化呈相同趋势:在0 ≤ x ≤ 0.1范围内随x的增大而增大,随后在x = 0.1-0.7时逐渐减小.     

Systematic study of compositional and synthetic control of vacancy and magnetic ordering in oxygen-deficient perovskites Ca2Fe(2-x)Mn(x)O(5+y)and CaSrFe(2-x)Mn(x)O(5+y) (x = 1/2, 2/3, and 1; y = 0-1/2)

Ten compounds belonging to the series of oxygen-deficient perovskite oxides Ca(2)Fe(2-x)Mn(x)O(5) and CaSrFe(2-x)Mn(x)O(5+y), where x = 1/2, 2/3, and 1 and y ≈ 0-0.5, were synthesized and investigated with respect to the ordering of oxygen vacancies on both local and long-range length scales and the effect on crystal structure and magnetic properties. For the set with y ≈ 0 the oxygen vacancies always order in the long-range sense to form the brownmillerite structure containing alternating layers of octahedrally and tetrahedrally coordinated cations. However, there is a change in symmetry from Pnma to Icmm upon substitution of Sr for one Ca for all x, indicating local T(d) chain (vacancy) disorder. In the special case of CaSrFeMnO(5) the neutron diffraction peaks broaden, indicating only short-range structural order on a length scale of ~160 ?. This reveals a systematic progression from Ca(2)FeMnO(5) (Pnma, well-ordered tetrahedral chains) to CaSrFeMnO(5) (Icmm, disordered tetrahedral chains, overall short-range order) to Sr(2)FeMnO(5) (Pm3m, destruction of tetrahedral chains in a long-range sense). Systematic changes occur in the magnetic properties as well. While long-range antiferromagnetic order is preserved, the magnetic transition temperature, T(c), decreases for the same x when Sr substitutes for one Ca. A review of the changes in T(c) for the series Ca(2)Fe(2-x)M(x)O(5), taking into account the tetrahedral/octahedral site preferences for the various M(3+) ions, leads to a partial understanding of the origin of magnetic order in these materials in terms of a layered antiferromagnetic model. While in all cases the preferred magnetic moment direction is (010) at low temperatures, there is a cross over for x = 0.5 to (100) with increasing temperature for both the Ca(2)Fe(2-x)Mn(x)O(5) and the CaSrFe(2-x)Mn(x)O(5) series. For the y > 0 phases, while a brownmillerite ordering of oxygen vacancies is preserved for the Ca(2) phases, a disordered Pm3m cubic perovskite structure is always found when Sr is substituted for one Ca. Long-range magnetic order is also lost, giving way to spin glass or cluster-glass-like behavior below ~50 K. For the x = 0.5 phase, neutron pair distribution function (NPDF) studies show a local structure related to brownmillerite ordering of oxygen vacancies. Neutron diffraction data at 3.8 K show a broad magnetic feature, incommensurate with any multiple of the chemical lattice, and with a correlation length (magnetic domain) of 6.7(4) ?.     

Computation of the generalized Debye functionsδ(x,y) andD(x,y)

Automatic computer programs (BASIC-PLUS) are developed to calculate Debye functions also for non integer exponents. Functions of this type occur in the heat capacity analysis of polymer crystals, if simple continuum approximations are used. The heat capacity of completely crystalline polyethylene is calculated and compared with experimental data.     

脂肪酸铕配合物/PMMA的合成及发光性能

以一定比例正辛酸和月桂酸为第一配体,通过皂化法合成了脂肪酸铕配合物,并溶于甲基丙烯酸甲酯(MMA)单体通过本体聚合得到了含脂肪酸铕的PMMA光致发光聚合物材料AxByEu/PMMA(A为正辛酸根,B为月桂酸根,x、y分别表示正辛酸和月桂酸的摩尔比). 考察了不同的第二配体(咔唑、二甲基-联吡啶、邻菲罗啉、噻吩甲酰三氟丙酮HTTA)对脂肪酸铕聚合物发光性能的影响,选择出合适的第二配体HTTA,合成了A3Eu/HTTA/PMMA聚合物. 通过红外光谱、紫外光谱及荧光光谱测试技术对配合物及聚合物的结构和荧光性能进行表征. 结果表明,合成的含脂肪酸铕配合物及其聚合物均具有很好的光致发光性能,紫外激发能发射Eu3+离子的特征红光. 当HTTA的质量分数减小至MMA的0.02%时,仍能很好地促进体系的发光,且不影响聚合物本身的透明性.     

Low frequency Raman spectra of barium borate glasses in the system (BaCl2)y[(BaO)x(B2O3)1-y-x]1-y for various x and y

Low frequency Raman spectra of glasses of the types (BaO)_x·(B₂O₃)_1−x and (BaCl₂)_y·[(BaO)_x·(B₂O₃)₁-_y-_x]₁-_y have been reported. The temperature reduced Raman spectra show peaks at 67, 116 and 140 cm⁻¹ for the binary glass. The bands at 116 and 140 cm⁻¹ are ascribed to the librational motions of the borate groups and the 67 cm⁻¹ band arises because of the limited structural correlation range (SCR) of the glass network, causing a maximum of the frequency dependent Raman coupling coefficient. Due to addition of BaO in v-B₂O₃, the oxygen are mostly incorporated in the formation of BO₄ units; however large Ba²⁺ ions also enhance the number of non-bridging oxygen at higher concentrations of dopant. These barium ions as well as chlorine ions are accomodated in the interstitial vacancies of the glass network which leads to an expansion of the network structure.