Interaction of H2S with α-Fe2O3(0 0 0 1) surface

The atomic-scale structural changes in an α-Fe₂O₃ (hematite) (0 0 0 1) surface induced by sulfidation and subsequent oxidation processes were studied by X-ray photoemission spectroscopy, LEED, and X-ray standing wave (XSW) measurements. Annealing the α-Fe₂O₃(0 0 0 1) with a H₂S partial pressure of 1 × 10⁻⁷ Torr produced iron sulfides on the surface as the sulfur atoms reacted with the substrate Fe ions. The oxidation state of the substrate Fe changed from 3+ to 2+ as a result of the sulfidation. The XSW measured distance of the sulfur atomic-layer from the unrelaxed substrate oxygen layer was 3.16 Å. The sulfide phase consisted of three surface domains identified by LEED. Formation of the two-dimensional FeS₂ phase with structural parameters consistent with an outermost layer of (1 1 1) pyrite has been proposed. Atomic oxygen exposure oxidized the surface sulfide to a sulfate () and regenerated the α-Fe₂O₃(0 0 0 1) substrate, which was indicated by a (1 × 1) LEED pattern and the re-oxidization of Fe to 3+.     

Surface and redox chemistry of Sm0.95Ce0.05Fe1 − xNixO3 − δ perovskites

A new series of perovskite materials with formula Sm_0.95Ce_0.05Fe_1 − xNi_xO_3 − δ (0 ≤ x ≤ 0.10) has been prepared by sol-gel combustion via a citrate precursor route. X-ray diffraction data showed that materials prepared by this method had a single orthorhombic phase belonging to the Pnma (62) space group. The study of powders sintered in air and in reducing atmospheres reveals that these materials do not show phase separation in air (up to 1350 °C) nor under 5% v/v H₂/N₂ (up to 700 °C), but a phase separation of Sm₂O₃ does occur at and above 800 °C under 5% v/v H₂/N₂ without deterioration of the perovskite phase. The surfaces of all the powders (fresh, in-situ reduced and ex-situ reduced) were Sm rich, and multiple oxidation states for Fe were observed. XP analysis of in-situ reduced samples (800 °C and above) shows that metallic Fe forms in all nickel doped materials except x = 0.07. The surface oxygen vacancies and percentages of lattice and adsorbed oxygen for this series of Ni doped materials were determined and the oxygen recapturing ability is explained in terms of the multiple oxidation states of Fe.     

Rovibrational spectroscopy of the Fermi-interacting ν4 = 1 and ν3 = ν6 = 1 levels of DCF3

The high-resolution infrared spectrum of deuterated fluoroform (DCF₃) was studied in the 700 and 1200 cm⁻¹ regions, with the aim of assigning and analyzing the ν₄ CF₃ asymmetric stretching vibration. The Fermi-type anharmonic coupling between the ν₄ = 1 and ν₃ = ν₆ = 1 rovibrational levels, already mentioned in an early work of Ruoff et al. [Spectrochimica Acta Part A 31A (1975) 1099-1100], was studied here for the first time under high resolution. Assignments in the ν₃ + ν₆/ν₄ band system were confirmed and extended by the identification of the ν₃ + ν₆ − ν₆ and ν₄-ν₆ bands in the 700 cm⁻¹ region, the latter being enhanced near the Fermi crossings of the studied levels. Data from both the hot and difference bands were included in the analysis. The close separation of the studied vibrational levels of about 14.8 cm⁻¹ produces a large variety of resonance crossings which involve levels with . Besides the Fermi () and Coriolis () resonances, they were accounted for by inclusion of additional higher-order ( and ) interaction terms between the vibrational states. The least-squares fit of more that 16,000 vibration-rotation transitions provides a quantitative reproduction of data in all bands.     

Investigations on the thermo-chemical stability and electrical conductivity of K-doped Ba3 − xKxCaNb2O9 − δ (x = 0.5, 0.75, 1, 1.25)

In this paper, we report the synthesis, crystal structure and electrical transport properties of new K-doped Ba₃CaNb₂O₉ (BCN) and investigate their chemical stability in H₂O and pure CO₂ at elevated temperature. The powder X-ray diffraction (PXRD) of Ba_2.5K_0.5CaNb₂O_9 − δ, Ba_2.25K_0.75CaNb₂O_9 − δ, Ba₂KCaNb₂O_9 − δ, and Ba_1.75K_1.25CaNb₂O_9 − δ showed the formation of a single-phase double perovskite (A₃BB^/₂O₉)-like cell with a lattice constant of a ∼ 2a_p (where a_p is a simple perovskite cell of ∼ 4 Å). Perovskite-like structure was found to be retained after treating with CO₂ at 700 °C and also after boiling H₂O for 120 h. The lattice constant of CO₂ and H₂O treated samples was found to be comparable to that of the corresponding as-prepared compound. The total electrical conductivity of all the investigated K-doped BCN increases with increasing K content in BCN in various atmospheres, including air, dry H₂, wet N₂ and wet H₂. The electrical conductivity in dry and wet H₂ atmospheres was found to be higher than that of air in the temperature range of 300-700 °C, while in wet N₂ a slightly lower value was observed. Among the compounds investigated in the present study Ba_1.75K_1.25CaNb₂O_9 − δ showed the highest total electrical conductivity of 1 × 10^− 3 S/cm in dry H₂ at 700 °C with an activation energy of 1.28 eV in the temperature range of 300-700 °C.     

The rotational spectra of the υ8 = υ9 = 1 and υ6 = υ7 = 1 interacting vibrational states of nitric acid (HNO3)

The analysis of the rotational spectrum of HNO₃ has been extended to include the υ₈ = υ₉ = 1 state at 1205.7 cm⁻¹ and the υ₆ = υ₇ = 1 state at 1223.4 cm⁻¹. Based on 78-519 GHz data, the assignments in the 8¹9¹ vibrational state have been significantly expanded from the previously reported microwave measurements [T.M. Goyette, F.C. De Lucia, J. Mol. Spectrosc. 139 (1990) 241-243]. A new microwave analysis is also reported for the 6¹7¹ vibrational state. A simultaneous analysis takes into account the localized ΔK_a = ±2 Fermi resonances between the vibrational states, describes the torsional splitting of 3.3 and 1.4 MHz for the 8¹9¹ and 6¹7¹ states respectively, and fits to experimental accuracy over 1500 rotational transition frequencies that extend up to J = 59. Infrared energy levels [A. Perrin, J.-M. Flaud, F. Keller, A. Goldman, R. D. Blatherwick, F. J. Murcray, C. P. Rinsland, J. Mol. Spectrosc. 194 (1999) 113-123] were also included in the analysis and fit to experimental accuracy. Measurement of strongly perturbed transitions in each vibrational state provide a determination of the band origin difference of 17.733184(17) cm⁻¹. The rotational constants agree well with those predicted by vibrational-rotational constants of the fundamental modes. Furthermore, the analysis will provide a very accurate simulation of the infrared spectrum of HNO₃ in the 8.3 μm region.     

Surface studies of 2,4-pentanedione on γ-Al2O3/NiAl (1 0 0) and NiAl (1 0 0)

The chemical compound 2,4-pentanedione (Hacac) has been shown to etch the oxidized metal surfaces metals such as copper and nickel, but not their unoxidized surfaces. Here it is shown that on the γ-Al₂O₃/NiAl (1 0 0) surface (oxidized NiAl (1 0 0)) etching of aluminum occurs at 170 K and 750 K. Reflection-absorption infrared spectroscopy (RAIRS) is used to show that Hacac binds to both the clean, metallic and oxidized surfaces, but decomposition and combustion products dominate on the metallic surface and no etching occurs. The binding process that involves a deprotonation reaction of the enol species was identified by redshift in the carbonyl peaks and the appearance of an Al-H peak observed in the IR spectrum. The implication of these results is that there is both an unusual low temperature and high temperature etching of the alumina by bound acac.     

Structural and optical properties of Ga2(1−x)In2xO3 films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Ga_2(1−x)In_2xO₃ thin films with different indium content x [In/(Ga + In) atomic ratio] were prepared on α-Al₂O₃ (0 0 0 1) substrates by the metal organic chemical vapor deposition (MOCVD). The structural and optical properties of the Ga_2(1−x)In_2xO₃ films were investigated in detail. Microstructure analysis revealed that the film deposited with composition x = 0.2 was polycrystalline structure and the sample prepared with x up to 0.8 exhibited single crystalline structure of In₂O₃. The optical band gap of the films varied with increasing Ga content from 3.72 to 4.58 eV. The average transmittance for the films in the visible range was over 90%.     

Oxidation mechanism and ferryl domain formation on the α-Fe2O3 (0 0 0 1) surface

Recent experimental evidence calls for a reinterpretation of the oxidized structure in chemically distinct domains of the hematite (0 0 0 1) surface as the ferryl (FeO) termination rather than the bulk terminated O₃-Fe-Fe-R structure. Although this interpretation is consistent with experimental data and ab initio thermodynamics calculations, it raises serious questions about how molecular oxygen can be dissociated on a surface where reactive iron centers are slightly more than 5 Å apart. Here, we propose a novel cooperative bimolecular mechanism that provides a reasonable pathway for the formation of the unusual ferryl surface termination and should be readily reversible, which is important for understanding the function of hematite surfaces as an oxidation catalyst.     

Performance and stability of niobium-substituted Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes

The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1 − xNb_xO_3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb⁵⁺ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800-900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO₂-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.     

Electrical Properties of Fe-doped Perovskite-like BaNb0.75-xFexNa0.25O3-δ (0.05 < x < 0.5)

In this paper, we report the electrical properties of Fe-doped perovskite-like compounds with a nominal chemical formula of BaNb_0.75-xNa_0.25Fe_xO_3-δ (0.05 < x < 0.5) (BNF). Various solid-state structural and electrical characterization techniques, including powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), alternating current (AC) impedance spectroscopy and direct current (DC) methods were used for characterization. PXRD patterns for BNF members show the formation of perovskite-like structure. The total electrical conductivity values were determined under ambient air and wet air in the temperature up to 700 °C. The Fe concentration was strongly correlated to the conductivity response, with the x = 0.5 member exhibiting the highest conductivity in air. A relationship between the humidity content and conductivity in air was also observed in low Fe concentration BNF members (x = 0.5, 0.15), suggesting the presence of potential proton conduction; while the conductivity of high Fe content samples (x ≥ 0.3) showed little dependence on the humidity. The chemical stability of BNF samples was investigated in boiling H₂O and in flowing 100% CO₂ at elevated temperatures and the results demonstrated that all members were structurally stable under both conditions, except the x = 0.5 member which decomposed into BaCO₃ in the presence of CO₂ at 800 °C.     

Structural and photoluminescence properties of SnO2:Ga films deposited on α-Al2O3 (0 0 0 1) by MOCVD

Gallium-doped tin oxide (SnO₂:Ga) films have been prepared on α-Al₂O₃ (0 0 0 1) substrates at 500 °C by the pulse mode metalorganic chemical vapor deposition (MOCVD) method. The relative amount of Ga (Ga/(Ga+Sn) atomic ratio) varied from 3% to 15%. Post-deposition annealing of the films was carried out at different temperatures for 1.5 h in ambient atmosphere . The structural, electrical, optical and photoluminescence (PL) properties of the films have been investigated as a function of annealing temperature. All the films have the rutile structure of pure SnO₂ with a strong (2 0 0) preferred orientation. A single ultraviolet (UV) PL peak near 337.83 nm was observed at room temperature for the 3% Ga-doped as-grown film and near 336 nm for the 15%-doped film, which can be ascribed to electron transition from the oxygen vacancy and interstitial Ga³⁺ donor levels to the acceptor level formed by the substitution of Ga³⁺ for the Sn site. After annealing, the luminescence spectra have changed a little bit, which is being discussed in detail.     

Highly thermal stable transparent conducting SnO2:Sb epitaxial films prepared on α-Al2O3 (0 0 0 1) by MOCVD

Antimony-doped tin oxide (SnO₂:Sb) single crystalline films have been prepared on α-Al₂O₃ (0 0 0 1) substrates by metal organic chemical vapor deposition (MOCVD). The antimony doping was varied from 2% to 7% (atomic ratio). Post-deposition annealing of the SnO₂:Sb films was carried out at 700-1100 °C for 30 min in atmosphere ambient. The effect of annealing on the structural, electrical and optical properties of the films was investigated in detail. All the SnO₂:Sb films had good thermal stability under 900 °C, and the 5% Sb-doped SnO₂ film exhibited the best opto-electrical properties. Annealed above 900 °C, the 7% Sb-doped SnO₂ film still kept high thermal stability and showed good electrical and optical properties even at 1100 °C.     

Characterization of YSr2Fe3O8 − δ as electrode materials for SOFC

YSr₂Fe₃O_8 − δ was prepared by traditional solid state reaction method and characterized by X-ray diffraction, ac impedance, dc conductivity, dilatometry and thermogravimetric analysis for possible use in solid oxide fuel cells (SOFCs). YSr₂Fe₃O_8 − δ crystallizes with tetragonal symmetry in the space group P4/mmm and found to be stable at high temperatures under H₂ and air. Four probe dc electrical conductivity measurements show that the conductivity increases up to 745 K and then decreases with temperature; the highest conductivity σ_745K = 43.5 S cm^− 1. The n-type conductivity at low oxygen partial pressure (pO₂) changes to p-type at high pO₂. Polarization behavior was investigated measuring the ac impedance response in symmetrical cell arrangements in air with YSZ and GDC electrolytes. Cathodic area specific resistance (ASR) varies with firing temperature. The lowest area specific resistance was observed with a GDC electrolyte fired at 1000 °C. In case of YSZ, ASR increases and in case of GDC, ASR decreases in air when electrode firing temperature decreases. At 800 °C ASRs are 0.20 Ω cm² and 0.65 Ω cm² with GDC and YSZ electrolytes, respectively, in air. Fuel cell measurements with symmetrical electrodes were performed using a thin YSZ electrolyte under H₂ at anode and air at cathode, show that the power density is about 0.035 W/cm² at 900 °C.     

Antiferroelectric phase transition in pyrochlore-like (Dy1− xCax)2Ti2O7 − δ (x = 0, 0.1) high temperature conductors

Low-temperature ordering transitions in polycrystalline high temperature conductors (Dy_1 − xCa_x)₂Ti₂O_7 − δ (x = 0, 0.1) prepared using co-precipitation, mechanical activation and solid-state reactions at 1400 or 1600 °C have been studied by impedance spectroscopy at low frequencies and thermal mechanical analysis (TMA). The dielectric permittivity and loss tangent of the ceramics obtained have been measured as a function of temperature at low frequencies (0.5−500 Hz). The results provide evidence for the relaxation of point defects, most likely oxygen vacancies, at 500−600 °C and an antiferroelectric low-temperature phase transition of the second order, associated with re-arrangement process in the oxygen sublattice of pyrochlore structure. The temperature of the antiferroelectric transition is 700 to 800 °C, depending on the synthesis procedure and ceramic composition. Calcium doping of Dy₂Ti₂O₇ leads to the formation of additional oxygen vacancies and, in the case of the samples prepared via co-precipitation, increases the peaks in permittivity due to the relaxation process and ordering transition by three or six times, respectively.     

High resolution infrared spectrum and global analysis of ν5, ν12, and ν12 + ν6 − ν6 in CH3SiH3

The vibration-torsion-rotation spectrum of CH₃SiH₃ has been measured from 470 to 725 cm⁻¹ at near-Doppler resolution. The full-width at half - maximum of the lines observed near 600 cm⁻¹ was 0.0011 cm⁻¹. The spectra were obtained using a Bruker IFS 125 HR Fourier transform spectrometer with the broadband source radiation being supplied from the synchrotron emission of the storage ring at the Canadian Light Source. Three vibrational bands were investigated: the lowest lying perpendicular fundamental ν₁₂ centred near 524 cm⁻¹, the lowest lying parallel fundamental ν₅ near 703 cm⁻¹, and the torsional hot band ν₁₂ + ν₆ − ν₆ near 534 cm⁻¹. For ν₁₂ and ν₅, the resolution and sensitivity are much improved over those in earlier studies, with many of the torsional multiplets now being resolved even in the cases where the upper levels are unperturbed. The primary motivation for the present work was the hot band, here reported for the first time, where the dependence of the silyl rock in ν₁₂ on the torsional motion is much more pronounced. In addition, for the vibrational ground state (gs), two “forbidden” high torsional overtones v₆ = 3 ← 0 and 5 ← 0 have been observed that become allowed through resonant mixing of the upper states with ν₁₂ and ν₅, respectively. In each case, two (K, σ) series have been measured where the mixing is largest. Here σ = 0, 1, −1 labels the torsional sub-levels. Using the Fourier transform waveguide spectrometer at E. T. H., the three σ-components of the (J = 1 ← 0) transition in ν₁₂ + ν₆ were observed, and a series of direct l-doubling transitions in ν₁₂ + ν₆ were measured for σ = 0. In a global fit, all the new data have been analysed along with the frequencies for other transitions obtained in earlier investigations. The analysis takes into account the relevant interactions among the torsional stacks of levels in the gs, ν₁₂, and ν₅. These include the previously known (gs, ν₁₂) Coriolis-like and (gs, ν₅) Fermi-like interactions along with a higher order (ν₁₂, ν₅) Coriolis-like coupling introduced here. This last is responsible for the strong perturbation of the ν₅ series with K = 10, 11, and 12, and of the corresponding hot band series. A good fit to 9282 frequencies including 7942 new measurements was obtained both with the Free Rotor model in which the torsion is classified as a rotation, and with the High Barrier model in which the torsion is classified as a vibration. The Hamiltonian is discussed with emphasis on the new terms required for treating ν₁₂ + ν₆ − ν₆.     

Si中S0/Se0,S0/Te0和Se0/Te0对的电子结构

利用紧束缚近似下的格林函数方法,讨论了Si中硫属元素混对杂质(即S⁰/Se⁰,S⁰/Te⁰和Se⁰/Te⁰)基态的电子结构。混对杂质在Si禁带中引入两个A₁能级,其中成键性的A₁能级位置在反键性的A₁能级之上。数值计算得到的混对杂质能级与实验符合得相当好。理论分析表明,在Si中测到的那些未定的比最近邻混对杂质能级更浅的能级(S⁰/Se⁰(X₁),S⁰/Te⁰(X₁),Se⁰/Te⁰(X₁)…)不是由非最近邻位型的混对杂质引入的。本文还指出了一个极性分子放入Si晶体中,两个不同原子间s波函数的转移方向与通常极性分子相反,并讨论其物理原因。 关键词：     

Cyclic voltammetry characterization of a La0.8Sr0.2Co0.2Fe0.8O3 − δ electrode interfaced to CGO/YSZ

The electrochemical characteristics of a La_0.8Sr_0.2Co_0.2Fe_0.8O_3 − δ cathode electrode interfaced to the CGO layer of a double layer CGO/YSZ electrolyte were studied using cyclic voltammetry, at temperatures of 600 to 850 °C and under oxygen partial pressures ranging from 0.07 to 21 kPa. The aim was to identify the electrochemical processes taking place under cathodic polarization on the basis of differences in the features of the cyclic voltammograms with changing conditions. Depending on temperature, sweep rate and oxygen partial pressure, current peaks appeared both in the forward and backward scans. Furthermore, reversed hysteresis was observed, i.e. higher currents in the backward scan than in the forward scan, with increasing oxygen partial pressure and decreasing temperature. The observed behavior was related to the electrochemical redox of B-sites and concomitant stoichiometry change as well as to the competing reaction of electrochemical oxygen redox, taking also into account the competitive action of chemical reactions occurring in the presence of gaseous oxygen.     

Non-universal dielectric relaxation in SrFeO3 − δ

SrFeO_3 − δ compound is prepared by the thermal decomposition method followed by ball milling. Analysis of Mössbauer spectrum and X-ray diffraction study proves the presence of multi-phase nature, i.e., Sr₈Fe₈O₂₃ and Sr₄Fe₄O₁₁ phases at room temperature. Furthermore, the Mössbauer spectrum at room temperature evidenced the presence of major Fe^3.5+ which is the resultant of equal contributions of Fe⁴⁺ and Fe³⁺. The Nyquist plot at all measured temperatures (80–230 K) suggests that the dielectric response is well associated with single relaxation time (exponential parameter, n∼1$n\sim 1$) i.e., the Debye-type. Modulus analysis exhibits the non-universal dielectric behaviour (stretched exponential parameter, β>1$\beta >1$) below 230 K and the Debye-type responds (β∼1$\beta \sim 1$) at and above 230 K. The Debye-type behaviour exhibited by SrFeO_2.81 at around room temperature in its defect state offers a new opening for this material for multifunctional applications.