Defect formation and transport in SrFe1-xAlxO3-δ

Perovskite-related phases derived from SrFeO_3-δ are among known mixed conductors with highest oxygen permeability and are thus of interest as the ceramic membrane materials for oxygen separation and partial oxidation of light hydrocarbons. Dense ceramics of SrFe_1-xAl_xO_3-δ (x=0.1–0.5) were prepared via the glycine-nitrate process. The cubic solid solution formation was found to occur in the concentration range x=0–0.35. Increasing aluminum content leads to decreasing thermal expansion coefficients (TECs), relative fraction of Fe⁴⁺ under oxidizing conditions, and also the total conductivity, predominantly p-type electronic at oxygen pressures close to atmospheric. The TECs vary in the range (13.5–16.4)×10^?6 K^?1 at 373–923 K and increase up to (18.6–31.9)×10^?6 K^?1 at 923–1273 K. The oxygen permeation fluxes decrease moderately with aluminum additions. The Mössbauer spectroscopy data and p(O₂) dependencies of electrical properties indicate a small-polaron mechanism of electronic transport in SrFe_1-xAl_xO_3-δ. Reducing oxygen partial pressure results in transition from dominant p- to n-type electronic conduction. The low-p(O₂) stability limit of SrFe_1-xAl_xO_3-δ perovskites lies between that of LaFeO_3-δ and Fe/Fe_1-γO boundary.     

Diffusion and correlation effects in iron-doped CoO

The diffusion of ⁵⁹Fe and ⁶⁰Co has been measured in pure CoO and dilute iron-doped CoO, (Co_1?cFe_cO, as a function of temperature (1000–1400°C) and oxygen partial pressure P_{o2), (10^?7 ≦ Po2 ≦ 0 21 atm) The enhancement factors for the diffusivities of iron and cobalt are nearly identical, which suggests that the primary cause of the enhancement is the increased concentration of charge-compensating cation vacancies with the addition of iron. The Fe ions dissolved in CoO appear to exist as a mixture of Fe²⁺ and Fe³⁺ ions, the fraction of iron ions in the three-plus state decreases with decreasing Po2 The simultaneous diffusion of ⁵²Fe and ⁵⁹Fe has been measured as a function of (itpo; at 1200°C The correlation factor for Fe impurity diffusion determined from the isotope-effect measurements is about the same as that for self-diffusion in CoO at high (itPo2} (2 × 10^?3 ≦ p_o2 ≦ 0 21 atm), but increases slightly with decreasing p_O2 Both the enhancement-effect and isotope-effect experiments suggest that the nearestneighbor interactions between Fe ions and vacancies is small, and that the dissolved Fe ions do not have strongly bound electron holes.     

Correlation and isotope effects for cation diffusion in magnetite

The simultaneous diffusion of ⁵²Fe and ⁵⁹Fe has been measured in Fe₃O₄ as a function of equilibrium oxygen partial pressure (10^?9 <p_o2 < 10^?4 atm) at 1200°C. The p_o2 dependence of D goes through a minimum near 10^?6 atm in agreement with earlier data of Dieckmann and Schmalzried. Comparison of the isotope effect results with correlation-factor calculations suggests that at p_o2 γ 10^?6, diffusion occurs predominantly by vacancy jumps between the normally occupied octahedral sites on the spinel lattice; jumps between tetrahedral sites probably play a lesser role. At p_o2< 10^?6 atm, diffusion occurs by an interstitialtype mechanism involving the simultaneous migration of two atoms. Five of the seven interstitialcy jumps considered in our correlation-factor calculations are consistent with the experimental results.     

Oxygen partial pressure dependence of in situ X-ray absorption spectroscopy at the Co and Fe K edges for (La0.6Sr0.4)(Co0.2Fe0.8)O3−δ

The oxygen partial pressure (P(O₂)) dependence of in situ X-ray absorption spectroscopy (XAS) at the Co and Fe K edges was measured simultaneously and continuously at 900 and 1000 K. These experiments, which were performed during reduction, changing P(O₂) from 1 to 10^?4 atm, were used to investigate each valence related to Co and Fe in (La_0.6Sr_0.4)(Co_0.2Fe_0.8)O_3?δ (LSCF). The absorption edge shift of the Co K edge was more than twice that of the Fe K edge at 1000 K during reduction. For quantitative analysis, X-ray absorption near-edge structure spectroscopy was carried out at the Co and Fe K edges; the results indicated that the Co valence decreased more easily than the Fe valence; that is, the oxygen preferentially left from the oxygen sites around Co.     

Ionic and electronic conductivity in CaTi0.9fe0.1o3-δ

Abstract

The electrical conductivity of CaTi_1?x Fe _x O_3-δ (x = 0.1) was measured by an alternating current van der Pauw technique versus oxygen partial pressure (10^?30-1 atm) and temperature (450–1200°C). The results were interpreted to reflect n-type, ionic and p-type conductivity at respectively low, intermediate and high oxygen partial pressures. The apparent activation enthalpy for the ionic conductivity, interpreted to reflect the mobility of oxygen vacancies, was 0.87 eV. The enthalpy of intrinsic formation of electronic defects (apparent band gap E _g) was 3eV. The results are compared with literature data for CaTi_0.8Fe_0.2O_3-δ and with Fe-substituted SrTiO₃ and discussed in terms of iron-oxygen vacancy association and ordering.     

Defect characterization in CeO2−x at elevated temperatures—I: X-Ray diffraction

The atomic defect structure of nonstoichiometric ceria was studied by means of X-ray diffraction. Polycrystalline samples of CeO_2?x (0?x?0.21) have been examined at 900 and 1000°C, with the stoichiometry controlled by adjusting the oxygen partial pressure between 1 and 10^?21 atm. It was observed that the lattice expands as a function of increasing defect concentration and exhibits only fluorite-like diffraction peaks. The integrated intensities of the Bragg reflections were analyzed for CeO₂ and CeO_1.91, at 900°C by difference electron-density techniques. It was concluded that the cation sublattice is essentially intact, and that the oxygen sublattice must be defective in nonstoichiometric ceria. Least-squares analyses on CeO_2?x (0?x? 0.21) at 900 and 1000°C supported the electron-density results and also showed that the temperature factors of both cations and anions increase with an increase in defect concentration, implying greater mean-square displacement of the atoms from their equilibrium positions.     

The structure of Ni(II)−Fe(III) hydroxy-chloride green rusts by Mössbauer spectroscopy and X-ray diffraction

The crystallographic structure of Ni(II)?Fe(III) hydroxy-chloride green rusts obtained by oxidation of a Fe_xNi_1?x(OH)₂ precipitate is isomorphous with that of the ferrousferric green rust one and independent of the value of P=Fe/Ni. Mössbauer spectra exhibit two quadrupole doublets after further oxidation of the compounds which correspond to a formula (3-x)Ni(OH)₂ · xFeOOH · Fe(OH)₂Cl. The Fe³⁺ ions are found to occupy preferentially the sites close to the Cl^? ions and the Ni²⁺ those far from them. However the ordering of the Fe³⁺ ions is not perfect.     

The relationship between magnetic behaviour and local structure around Fe ions in Fe-doped TiO2 rutile

The magnetic and structural characterization of Ti_1−xFe_xO₂ (x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO₂ and Fe₂O₃ as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO₂ structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe²⁺ fraction, i.e the number of oxygen near neighbours decreases when the Fe²⁺ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe²⁺ and/or the number of Fe near neighbour oxygen vacancies.     

Range of homogeneity and defect energetics in Co1−x S

Sulphur pressures (10^?6 to 150 atm) have been measured directly or indirectly (H₂S/H₂) inside and adjacent to the homogeneity range of Co_1?xS at temperatures ranging between 777 and 1368 K. The results are evaluated in terms of interacting cobalt vacancies. Additional defects near to the cobalt rich boundary of the homogeneity range were not detected (in contrast to Fe_1?xS). The phase diagram Co-S has been complemented.     

Cation distribution of the system Zn1−xCoxFeMnO4 by x-ray,electrical conductivity and Mössbauer studies

Structural, electrical and Mössbauer studies were carried out for the system Zn_1?xCo _x Fe MnO₄. It is observed that forx?0.6, the ionic configuration of the system is Zn _1?x ²⁺ Mn _x ²⁺ [Co _x ³⁺ Mn _1?x ³⁺ Fe³⁺]O ₄ ^2? and forx?0.8 Fe³⁺ ions occupy tetrahedral site also. On the basis of electrical and Mössbauer studies a probable valence distribution of CoMnFeO₄ has been suggested. All the compounds showed positive values of thermoelectric coefficient and electrical conduction takes place by a hopping mechanism. Activation energy and thermoelectric coefficient values decreased with decrease in concentration of Zn²⁺ ions. The compounds possess low mobility values varying between 10^?7 and 10^?9 cm²/V sec.     

High-Temperature Sulphidation of Fe-Cr Alloys

Scales resulted from a sulphidation of Fe _100?x Cr _x alloys (x ≤ 26) at 1073K in the atmosphere of H ₂/H ₂ S with the partial pressure of sulphur of 10^?8 atm were investigated by means of transmission and conversion electron Mössbauer spectroscopies, X-ray diffraction, electron microscopy and microprobe analysis. Although crystallographically single-phase, the scales were revealed to be composed of FeS (x ≤ 10) and Fe _1?x Cr _x S (x > 10).     

Diffusion of titanium in slightly reduced rutile

The self-diffusion of ⁴⁴Ti in slightly reduced rutile. TiO_2?δ, was measured along the c axis over the temperature range of 1000–1100°C between 0.2 and 1 × 10^?18atm. oxygen pressure. These measurements enabled the determination of the defect structure of TiO_2-δ for 0.02 ?gd ? 0.001. For oxygen pressures between 1 × 10^?13 and 1 × 10^?16atm. at 1058.4°C random tetravalent titanium atoms are the predominant defects evident from self-diffusion. The enthalpy of motion was determined as ΔH_m = 57.03 ± 4.9% kcal/mole. From the activation energy at 1.69 × 10^?16atm., the enthalpy of formation for tetravalent titanium interstitials was determined as ΔH_f = 276 ± 15.6% kcal/mole.For oxygen pressures less than 1 × 10^?16atm. at 1058.4°C, the tracer diffusion coefficient shows a continuous decline as the oxygen pressure is lowered. Comparisons with thermogravimetric studies and consideration of the similarity in structure between nonstoichiometric point defect phases and the first homologous series phase indicate that the order-disorder transition retains a considerable degree of short range order below the critical concentration in the form of Wadsley defects.     

Mössbauer and optical investigation of Co3−x Fe x O4 thin films grown by sol–gel process

Structural transformation and the related variation in magnetic and optical properties of Co_3?x Fe _x O₄ thin films grown by a sol–gel method have been investigated as the Fe composition varies up to x?=?2. The normal spinel phase is dominant below x?=?0.55 and the inverse spinel phase grows as x increases further. Conversion electron Mössbauer spectroscopy (CEMS) measurements indicate that the normal spinel phase have octahedral Fe³⁺ ions mostly while the inverse spinel phase contain octahedral Fe²⁺ and tetrahedral Fe³⁺ ions. For higher Fe composition (x?>?1.22), Co²⁺ ions are found to substitute the octahedral Fe²⁺ sites. The measured optical absorption spectra for the Co_3?x Fe _x O₄ films by spectroscopic ellipsometry support the CEMS interpretation.     

Hydrogen absorption in Zr(AlxB1−x)2 (BFe,Co) laves phase compounds

The hydrogen absorption capacity of the systems Zr(Al_xFe_1?x)₂ and Zr(Al_xCo_1?x)₂ (0? x ?1) was measured at hydrogen pressure of 70 atm and room temperature and at 40 atm and liquid nitrogen temperature. The two systems present very interesting and unexpected results.A dramatic rise in the hydrogen capacity occurs for small x values similar to previous results for the systems Zr(A_xB_1?x)₂ (A  V, Cr, Mn; B  Fe, Co; 0?x?1). The maximum hydrogen content in both systems is achieved for x ≈ $\text{2}\text{12}$ at 40 atm and 80 K. Further increase of the Al content leads, however, to a steep decrease in the hydrogen capacity. This general behaviour is well described by a phenomenological model, recently proposed by us, and thus supporting the importance of short-range neighbouring effects for the hydrogen absorption capacity. The influence of Al on the hydrogen sorption properties in different intermetallic compounds is discussed.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

Defect structure of lanthanum-doped calcium titanate

The defect structure of lanthanum-doped polycrystalline calcium titanate was investigated by measuring the oxygen partial pressure (10⁰–10^?18 atm.) dependence of the electrical conductivity at 1000° C and 1050°C. Two types of charge compensation were observed, namely electronic and ionic. For P₀₂ < 10^?15 atm. the carrier concentration was fixed by the amount of lanthanum (donor) added and the conductivity was found to be independent of oxygen partial pressure (electronic compensation). For higher oxygen partial pressure conditions (P₂₂ > 10^?13 atm.) the extra charge of the lanthanum was compensated by doubly ionized calcium vacancies (ionic compensation). In the ionic compensation region, a model involving a shear structure is discussed.     

Mass transport in cuprous oxide

The chemical diffusion coefficient of Cu₂O has been obtained for an oxygen partial pressure near 5 10^?4 atm as a function of the temperature in the range 700–900°C D? = 1 62 10^?4 exp(?5140 ± 600 cal mol ^?1)/RT cm²s^?1 This was easily achieved according to the electrochemical method used for the preparation of gaseous mixtures whose Po₂; is lower than 10^?5 atm The slight difference observed with the previously published results by Maluenda, and obtained for Po₂ values which increase with T between 10^?4 and 0.21 atm, may be due to an oxygen partial pressure effect already observed in the case of CoO. An ambipolar treatment of the chemical diffusion, in the case of p-type semiconductor M_aO_b, oxides, has allowed us to express the chemical diffusion coefficient as a function of the concentration of the prevailing defects and of their diffusion coefficient In the case where the prevailing defects are cationic vacancies α times ionized we have shown that the expression D? = (1 + α)D_vα can be generalized to the A₂O compounds This set of results has allowed us, according to the copper self diffusion data obtained recently by Peterson etal, to estimate the apparent enthalpy of formation of the catiomc vacancies ΔH_f 23 ± 0 8 kcal mol^?1.     

Initial oxidation stages on FeCr(100) and FeCr(110) surfaces

Simultaneous LEED and AES are used to follow early stages of oxidation of monocrystalline FeCr(100) and (110) between 700 and 900 K in the oxygen pressure range 10^?9–10^?6 Torr. A chromium-rich oxide region at the alloy/oxide interface is observed, which exhibits different surface structures on oxidized FeCr(100) and FeCr(110). The chromium concentration in this initially formed oxide film is found to be enhanced by low oxygen pressures or high temperatures. During further oxidation different behaviours are observed on FeCr(100) and FeCr(110), which are explained by assuming different ion permeabilities through the initial chromium rich oxide regions on the two surface planes. On FeCr(110) surfaces oxidation is initiated on chromium enriched (100) facets at 800 K or below. At 900 K a film consisting of rhombohedral Cr₂O₃ or (Fe, Cr)₂O₃ is epitaxially growing with its (001) plane parallel to the alloy (110) face. On FeCr(100) surfaces the chromium rich oxide region next to the substrate is of fcc type. As soon as the diffusion of iron from the alloy to the gas/oxide interface is observable, a spinel type oxide is formed and connected with the location of iron in tetrahedral lattice sites. Closer to the fcc lattice the spinel oxide consists of FeCr₂O₄ or a solid solution of FeCr₂O₄ and Fe₃O₄ whereas next to the gas phase the oxide is pure Fe₃O₄.     

Magneto-volume effect in invar-type amorphous Fe—B alloys

Amorphous Fe_100-xB_x(11.5 ≦ x ≦ 22) alloys having the invar characteristics were prepared by a single roller quenching method to investigate the magneto-volume effect. Forced volume magnetostriction, ?ω/?H, increased remarkably with decreasing boron content and the maximum value obtained was about 90 × 10^?10 Oe^?1 at 11.5 at.% boron. The estimated value of the pressure dependence of the Curie temperature, ?T_c/?P, was considerably large, being comparable to those of crystalline FeNi invar alloys. The ?T_c/?P curve plotted as a function of T_c approximately fitted Wohlfarth's expression.     

铁表面氧化的研究

用XPS,UPS和AES研究铁早期氧化过程,认为在温度不太高和氧分压不太大的条件下,多晶铁片与氧作用生成的化合物是Fe₃O₄而不是FeO和α-Fe₂O₃,也不是三种化合物的分层分布。在600℃以上高温下,氧分压近于1×10^-6Torr下,出现了铁表面上氧的热脱附过程而不是氧化过程。 关键词：