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1.
A detailed investigation of the defect structure of the Co doped BIMEVOX solid electrolyte, Bi2V1 − xCoxO5.5 − 3x/2 ( x = 0.1 and x = 0.2), quenched from high temperature, has been carried out using X-ray and neutron powder diffraction data measured at room temperature. The structure is built up from alternating layers of [Bi 2O 2] n2n+ and [ V1 − xCoxO3.5 − 3x/2] n2n− with disorder limited to the vanadate layer. The ideal V/Co co-ordination is octahedral with corner sharing of equatorial oxygens. The refinements show that the true structure is distorted, with disorder in both apical and equatorial oxygens and oxygen vacancies concentrated in the equatorial positions. Detailed analysis of the oxygen site occupancies reveals two main types of V/Co co-ordination viz. distorted octahedral and distorted tetrahedral. The majority of the sites in both compositions are tetrahedral. 相似文献
2.
The maximum solid solubility of gallium in the perovskite-type La 1−xSr xFe 1−yGa yO 3−δ ( x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O 3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10 −6 K −1 at 300–800 K and (19.3–26.7)×10 −6 K −1 at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La 1−xSr xFe 1−yGa yO 3−δ ( x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La 1−xSr xFe 1−yGa yO 3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport. 相似文献
3.
Superconductivity and crystallographic properties of La 2 − xM xCuO 4 − δ (M = Na, K) are studied. In the La 2 − xM xCuO 4 − δ system, superconductivity is detected for x 0.2. Oxygen content analysis shows that the system has more oxygen vacancies than the La 2 − xSr xCuO 4 − δ system. These oxygen vacancies may reduce the hole concentration, and high Na-doping is needed to produce superconductivity. In the La 2 − xK xCuO 4 − δ system, superconductivity is observed for the first time. Resistivity and magnetic susceptibility measurements show that Tc(onset) is 40 K and the Meissner volume fraction is about 4% for x = 0.7. The system changes from orthorhombic to a tetragonal K 2NiF 4 structure at x ≈ 0.3 and only tetragonal samples show superconductivity. 相似文献
4.
New Scheelite-related solid solutions of the compositions Na x/2Bi 1−x/2Mo xV 1−xO 4 (0≤ x≤1) and Bi 1−x/3 Mo xV 1−xO 4(0≤ x≤0.2) have been synthesised by the substitution of Na and Mo at the A and B sites respectively of the ABO 4 type ferroelastic BiVO 4. The phases were characterised using chemical analysis, powder X-ray diffraction, scanning electron microscopy, EDAX, and Raman spectroscopy. While almost a continuous solid solution is obtained for the series Na x/2Bi 1−x/2Mo xV 1−xO 4, the absence of Na at the A-site results only in a narrow stability region for the other series, Bi 1−x/3 Mo xV 1−xO 4 where 0≤ x≤0.2. Raman spectra of selected samples at room temperature also suggest that vanadium and molybdenum atoms are disordered at the tetrahedral sites. 相似文献
5.
The electrical conductivity of the LaY 1−xIn xO 3 ( x=0.0–0.7) system has been studied from the viewpoint of crystal chemistry. The high temperature form of LaYO 3 ( x=0.0) was ascertained to be the Sm 2O 3-type (B-type rare earth) structure, not perovskite-type one. The X-ray diffraction (XRD) experiments revealed that the samples with x=0.05 and 0.10 were the mixed phase of Sm 2O 3-type and perovskite-type structure, and changed to perovskite phase in the range of x0.20. From oxygen partial pressure dependence of the electrical conductivity, it was found that both the Sm 2O 3-type and the perovskite-type single phases showed hole conduction, but the mixed phase did oxide-ion one. The electrical conductivity of the LaY 1−xIn xO 3 ( x=0.0–0.7) system increased with increasing x, and showed the maximum value in the range of x=0.05–0.10, and then decreased with increasing x. The occurrence of oxide-ion conduction was discussed from the viewpoint of lattice distortion in the mixed phase. 相似文献
6.
Oxygen tracer diffusion ( D*) and surface exchange rate constant ( k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La 1−xSr xFe 0.8Cr 0.2O 3−δ ( x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10 −21 atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV ( x=0.2), 1.53 eV ( x=0.4) and 1.21 eV ( x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10 −8 cm 2 s −1 for x=0.2 to 10 −7 cm 2 s −1 for x=0.4 and 0.6. The activation energies determined at constant H 2O/H 2 ratio are 1.21 eV ( x=0.2), 1.59 eV ( x=0.4) and 0.82 eV ( x=0.6). The surface exchange rate constant of oxygen for the H2O molecule is similar in magnitude to that for the O2 molecule and both increase with increasing Sr concentration. 相似文献
7.
The n = 2 Aurivillius phase Bi 2 − xPb xSr 1 − xNd 2O 9 was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2 1am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi 3+ by the isoelectronic Pb 2+. The ferroelectric transition temperature Tc = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with Tc = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected. 相似文献
8.
In the system Bi 2−wPb wSr 2−xCa 1−yNd x+yCu 2O 8+z different fractions of Nd are substituted on either Sr of Ca sites in order to introduce intrinsic insulating pinning centres. It is shown that a Nd concentration around x or y = 0.2 is likewise favourable with an average Nd---Nd distance in the range of the coherence length in the a, b-plane. However, clear evidence of flux pinning is only present for charge compensation with Pb 2+. A simultaneous substitution of the Bi-based 2212 superconductor with moderate amounts of Nd 3+ and Pb 2+ improves the superconducting properties by strengthening the flux pinning forces. 相似文献
9.
The mixed oxides Bi xEu 1−xVO 4 and Bi yGd 1−yVO 4 crystallize in a zircon-type structure, for 0 < x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6 s2 of Bi 3+ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi xEu 1−xVO 4 spectra are ascribed to intra-configurational 4 f – 4 ftransitions of the Eu 3+ ion. The broad absorption shift in BiLnVO 4 (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO 4 lattice, is ascribed to charge transfer processes in a Bi-VO 4 center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi 3+. The concept of an internal pressure of Bi 3+ ions is also used to explain the broad A-band shifts. 相似文献
10.
The polarized Raman spectra of Nd 1+xBa 2−xCu 3O 7−δ (−0.023≤ x≤0.107) and Pr 1+xBa 2−xCu 3O 7−δ (0.01≤ x≤0.15) single crystals have been investigated. It was found that the Cu(2) Ag mode softens by 6 cm −1 in Nd 1:2:3 and 4 cm −1 in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x( yy)
geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains. 相似文献
11.
Cation deficient spinels Ni xMn 3−x□ 3δ/4O 4+δ (0≤ x≤1) have been prepared by thermal decomposition of mixed oxalates Ni x/3Mn (3−x)/3(C 2O 4)· nH 2O in air at 623 K. They have been characterised by temperature programmed reduction (TPR) under H 2, the reaction being followed by gravimetric and powder X-ray diffraction measurements. It has been shown that TPR proceeds in several steps. The first steps correspond to the loss of nonstoichiometric oxygen leading to the formation of a stoichiometric oxide. During the following stages the manganese cations are reduced, causing the spinel structure to be destroyed, and the formation of solid solution of NiO in a cubic MnO. Subsequently, Ni 2+ cations undergo a reduction to metallic nickel, and, finally, a mixture of nonstoichiometric MnO 1−δ and metallic nickel is formed. These oxides contain a high level of vacancies which vary with the nickel content with a maximum of δ≈1 near x=0.6. This nonstoichiometry is ascribed both to the presence of Ni 3+ and excess of Mn 4+. 相似文献
12.
Pr concentration dependence of the superconducting transition temperature Tc in the Ho 1−xPr xBa 2Cu 3O 7−δ system is determined from measurements of DC electrical resistance. This dependence coincides with that for the parallely studied Y 1−xPr xBa 2Cu 3O 7−δ reference system. Both systems have the same value of the critical concentration xc=0.58, in accordance with nearly equal ionic radii of Ho 3+ and Y 3+ ions. It has been shown that the Tc( x) curve can be described with a single mechanism based on a decreasing number of sheet holes trapped by Pr IV-ions, if one takes also into account that the number of these ions changes with x. 相似文献
13.
Tracer diffusion of 18O in dense, polycrystalline La 1−xSr xCoO 3 for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10 5 Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La 0.9Sr 0.1CoO 3 is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/ RT] cm 2/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La 0.8Sr 0.2CoO 3 exhibits a higher diffusivity at 500 °C than does La 0.9Sr 0.1CoO 3. 相似文献
15.
Oxy-anion (PO 43−) substituted phases of the composition REBaSrCu 2.8(PO 4) 0.2O y have been synthesized. Single phase formation is achieved only for the larger rare earths (La → Ho and Y) and all the phases crystallize in the tetragonal 123 structure. IR studies confirm the presence of a phosphate group in the structure. The total oxygen content (including that associated with the oxy-anion PO 43−) has been found to be in the range 7.01 to 7.10 per formula unit. Except for RE = La, Nd, Yb and Lu, all the phases exhibit superconductivity and the Tc,zero values vary from 18 to 38 K. The Tc,zero increases with decreasing size of RE 3+. ErBaSrCu 3O 7 is multiphasic and single phase formation is achieved for the first time by phosphate substitution at the copper site. Single phase formation is encountered for x = 0.10 and 0.15 with Tc,zero values of 54 and 37 K respectively. 相似文献
16.
A 19F-NMR investigation of the Pb 1 − xAl xF 2 + x (0 ≤ x ≤ 0.12) solid solution and of the ordered Pb 9Al 2F 24 phase is undertaken as a function of temperature. Several F − ion groups become progressively mobile at the NMR time scale at increasing temperature, the first, (F i) pb and (F n) pb, are located in the surroundings of Pb 2+ cations, the second ones, (F rd) AI and (F ra) Al, in that of Al 3+ cations. Four types of fluoride ions are distinguished in the 19F-NMR spectra of rigid lattice at low temperature ( T = 175 K) and the distribution of fluoride ions among the four sites is determined by deconvolution of the spectra registered. The sites occupied by the (F n) pb anions are identified with the normal anionic sites of the fluorite network, the others with different interstitial sites. 相似文献
17.
The superconductivity of Tl 0.5Pb 0.5Sr 2−xNd xCuO 5−δ was observed for x ≥ 0.3, although the calculated Cu valence assuming Tl 3+ and Pb 4+ is slightly above or below 2.0. The binding energy of the Tl and Pb 4f core levels measured by XPS shifted to higher energy than those of Tl 3+ and Pb 4+ in the reference materials, showing that the Tl and Pb valences are lower than 3 + and 4 +, respectively. These observations strongly suggest that holes in the CuO 2 sheet are created by charge transfer from the (Tl,Pb)O layer, similar to the double-layered Tl---Ba cuprates. In contrast, the temperature variation of electrical conductivity of Tl0.5Pb0.5Sr2CuO5−δ changed from metallic to semiconducting with increasing oxygen deficiency, δ, and no superconductivity was observed through any control of δ. Although XPS measurement also suggested that Tl and Pb valences lowered with increasing oxygen deficiency, δ, the reason why the system did not show superconductivity can be understood by the depletion of oxygen from the CuO2 plane during deoxygenation. 相似文献
18.
The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa 2Cu 3−xCo xO 6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10 −5 and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa 2Cu 3−xCo xO 6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa 2Cu 3O 6+δ 相似文献
19.
The flux pinning behavior of a Nd 1+xBa 2−xCu 3O 7−δ (Nd123) single crystal, which exhibited a peak effect, has been studied by monitoring the time decay of the magnetic moment. The apparent pinning energy ( U0*) was deduced from flux creep data on the basis of the Anderson-Kim model. The magnetic field dependence of U0* showed maxima at peak fields which depended on the temperatures, in a similar manner to those of critical current densities. In addition, the temperature dependence of U0* showed several features. To explain the increase in U0* with the field as well as its temperature and field dependence, we made a numerical calculation by introducing an additional pinning energy which increased with increasing field. The results are in good agreement with the experimental data, especially at temperatures above 60 K, where the contribution of field induced pinning centers is believed to be dominant. 相似文献
20.
A series of apatite-type La–Ge–O ceramics were prepared and their cation-defect at the 4f+6h sites and oxide ion-defect at 2a site were investigated. In La xGe 6O 12+1.5x ceramics of x=6–12, the higher conductivities were obtained in the region of apatite composition, La x(GeO 4) 6O 1.5x−12 ( x=8–9.33), and the highest conductivity was achieved for La 9(GeO 4) 6O 1.5 ( x=9), where the number of cation (La 3+) occupying the 4f+6h sites is 9 and the number of oxide ion occupying the 2a site is 1.5. The ceramics with cation- and oxide ion-defects were La 9−0.66xSr x(GeO 4) 6O 1.5 ( x=0–1), La 9−1.33xZr x(GeO 4) 6O 1.5 ( x=0–1), La 9−xSr x(GeO 4) 6O 1.5−0.5x ( x=0–3), La 9−xZr x(GeO 4) 6O 1.5+0.5x ( x=0–1), La x(GeO 4) 3x−21(AsO 4) 27−3xO 1.5 ( x=0–3), La x(GeO 4) 33−3x(AlO 4) 3x−27O 1.5 ( x=0–3), La 9(GeO 4) 6−x (AlO 4) xO 1.5−0.5x ( x=0–3), La 9(GeO 4) 6−x(AsO 4) xO 1.5+0.5x ( x=0–1), La 9.33−xSr x(GeO 4) 6O 2−0.5x ( x=0–1.2) and La x(GeO 4) 4.5(AlO 4) 1.5O 1.5x−12.75 ( x=8.8–9.83), which were prepared by the partial substitution of La 3+and GeO 44−of the basic apatite La 9(GeO 4) 6O 1.5 with Sr 2+ or Zr 4+ and AlO 45− or AsO 43−. Such substitutions lowered the conductivity of La 9(GeO 4) 6O 1.5. These results were discussed by the electrostatic interaction between Sr 2+, Zr 4+, AlO 45− or AsO 43− and oxide ion as a conductive species. 相似文献
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