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1.
By making use of high-temperature series expansions (HTSE) of the correlation functions, we study the thermal and disorder variation of the short-range order (SRO) in the particular B-spinel ZnCr 2xAl 2−2xS 4. We developed the HTSE for the q-dependent static structure factor S( q) to the order 6 in reciprocal temperature including both the nearest- and next-nearest-neighbour interactions J1 and J2, respectively. Respecting the experimental fact that the broad diffuse peak of the neutron is situated at the particular wave vector q0=[0 0 0.79] and is insensitive to the temperature for a given ratio of dilution x, we have estimated the thermal variation of J1 and J2 in the case of the pure compound. The bond percolation threshold xp of the ZnCr2xAl2−2xS4 is determined by studying the disorder variation of the correlation length ξ. The xp is considered as the concentration at which ξ vanishes. The obtained values are xp=0.27 when only J1 is considered and 0.23 when both J1 and J2 are taken into account. 相似文献
2.
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
3.
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
4.
Phase evolution in the Bi---Sr---Ca---Cu---Al---O system was studied. Two Al-containing phases BiSr 1.5Ca 0.5Al 2O z and (Sr 1−xCa x) 3Al 2O 6 ( x = 0.4 − 0.45) were determined to be chemically compatible with Bi 2.18Sr 2CaCu 2O 8+x (Bi-2212) at temperatures of the samples processing. The phase equilibria in the title system were investigated above the solidus temperature. The BiSr 1.5Ca 0.5Al 2O z was found to be in equilibrium only with the melt and the (Sr 1−xCa x) 3Al 2O 6 phase. This latter aluminate equilibrated with Ca,Sr cuprates, CaO, the Cu-free phase, and the liquid. The melting and solidification in Bi-2212, doped with the aluminate, corresponded to the reversible reaction Bi-2212 + BiSr 1.5Ca 0.5Al 2O z ↔ (Sr 1−xCa x) 3Al 2O 6 + liquid. Two sets of superconducting composite materials with initial compositions Bi-2212 + nBiSr 1.5Ca 0.5Al 2O z and Bi-2212 + m(Sr 1−xCa x) 3Al 2O 6 were prepared by solidification from the partial melt. The former material was composed mostly of large Bi-2212 lamellas separated by the BiSr 1.5Ca 0.5Al 2O z phase, which destroyed superconducting links between Bi-2212 grains. The latter material consisted of a Bi-2212 polycrystalline matrix with high concentration of small (ca. 3 μm) grains of (Sr 1−xCa x) 3Al 2O 6 imbedded in Bi-2212 lamellas. The Bi-2212 + m(Sr 1−xCa x) 3Al 2O 6 materials displayed a trend to enhance flux pinning at T = 60 K with the increase of aluminate phase content. 相似文献
5.
Single crystals with known Tc values of Y 1−xPr xBa 2Cu 3O 7−δ (Y---Pr1:2:3) and YBa 2Cu 3−xZn 3−xZn xO 7−δ (Y---Zn1:2:3) systems are studied by Raman measurements. The Raman spectra for (Y---Pr1:2:3) single crystals show that the frequencies of Ba and O z modes increase as the Pr content increases. The results are consistent with the hole-localization scheme proposed for the suppression of superconductivity in the polycrystalline Y---Pr1:2:3 systems. On the other hand, in the Y---Zn1:2:3 system, all the Raman modes do not change in frequencies. However, the FWHM of the Cu(2) mode increases with the decrease of Tc, indicating strong scattering of charge carriers by the substituted Zn ions in the CuO 2 planes. The induced disorder in the CuO 2 planes may be related with suppression of Tc in the Y---Zn1:2:3 system. Thus, the suppression mechanism in the Y---Zn1:2:3 systems seems to be different from that in the Y---Pr1:2:3 systems. 相似文献
6.
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
7.
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
8.
Thin films of Cu, Y and BaF 2 are co-condensed in ultrahigh vacuum onto SrTiO 3 (1 0 0)-substrates at room temperature without any additional oxygen supply. It is found that the temperature of the ex situ fluorine-oxygen exchange reaction in flowing wet oxygen essentially determines whether the Y 1Ba 2Cu 3O 7 − x-phase ( Tex = 850°C) or the Y 2Ba 4Cu 8O 16 − x-phase ( Tex = 800°C) forms or any mixture of both phases in between. Small-angle X-ray diffraction verifies the strictly epitaxial growth of the superconducting phases. The variation of the composition around the ideal 1,2,3-stoichiometry affects only the superconducting and normal conducting properties which are measured using the four-probe Montgomery method. The values of the normal state resistivity and its temperature dependence are discussed in combination with a reduced cross section of the conduction paths in the films. 相似文献
9.
The dielectric constant (′) and dielectric loss (tan δ) for hexaferrites BaCo 2−xZn xFe 16O 27 have been studied as a function of frequency ( f), temperature ( T) and composition ( x). The experimental results indicate that ′ and tan δ above the relaxation frequency only decrease as the frequency increases and as the temperature decreases. Tan δ shows the dielectric relaxation at certain critical frequencies which rise as temperature increases. The activation energy for the dielectric relaxation ( ED), ′, and tan δ are found to be minimum for x = 0.8. 相似文献
10.
Chemical synthesis routes to Li xMn 2O 4 (0.15≤ x≤1) in non-equilibrium reduction processes were developed to carry out detailed structural analysis. Non-equilibrium Li xMn 2O 4 (0.15≤ x≤1) samples were prepared by chemical lithiation of λ-MnO 2 with LiI for 24 h; longer than 1 week was needed to reach true equilibrium at room temperature. The samples were characterized by X-ray diffraction analysis. The phase diagram was different from that in the equilibrium state; three cubic phases (phases A, B and C) were observed for Li xMn 2O 4 (0.15≤ x≤1). There were two regions of two-phase coexistence: the region of 0.25< x<0.55 (phase B+phase C) and 0.6< x<1.0 (phase A+phase B). In the compositional range of 0.6< x<1.0, the lattice constants of phases A and B change with the lithium composition, this indicates that it is a structural anomaly with a metastable two-phase character in non-equilibrium reduction processes. 相似文献
11.
Oxide ion conductivity of the pure and aliovalent ion substituted rare-earth pyrohafnates in the series RE 2−xSr xHf 2O 7 and RE 2Hf 2−xAl xO 7 (RE=Gd and Nd; x=0–0.2) has been explored in the temperature range 400°C–700°C for the first time. It is seen that, conductivity is enhanced by doping 5 atom% Sr at the rare–earth site in these systems. Well defined impedance plots due to grain interior and grain boundary resistances were obtained in the Gd pyrohafnate with Sr substitution. The results of the conductivity variation for the pure, Sr and Al doped phases are explained on the basis of pyrochlore structure. 相似文献
12.
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
13.
Variation of the complex permeability with frequency of Ni 1−xZn xFe 2O 4 ( x=0.5, 0.6 and 0.7) prepared by a combustion method has been measured over a wide range of frequency, up to 1.8 GHz. Zn content improves permeability but moves the onset of resonance to lower frequencies. The influence on some properties of samples prepared by the insertion of small amounts of Ruthenium, Yttrium and rare-earth cations into the Ni0.5Zn0.5MxFe2−xO4 ferrite has been also investigated. Relative losses and Curie temperature of all the samples have been measured. Ferrites substituted with Ru and Gd improve microwave behavior compared to non-substituted samples. 相似文献
14.
We have made high-temperature (250 K< T<800 K) DC susceptibility measurements in the compounds RuSr 2Eu 2−xCe xCu 2O 10 for x=0.6,0.8, and 1.0 in order to determine the Ru effective magnetic moment. After carefully subtracting all contributions to the magnetic susceptibility except that of the Ru ions, we have been able to fit the Ru susceptibility with a law χ Ru=χ 0+ CRu/( T−Θ Ru). We have found that the Ru effective moment
falls between the values expected for Ru 5+ in spin states
and
. We have also found a dependence of μ eff(Ru) and Θ Ru with the Ce content x. 相似文献
15.
(Ba 1−xLa x) 2In 2O 5+x, whose end member is Ba 2In 2O 5, is an oxygen-deficient perovskite oxide showing high oxide-ion conductivity. In order to clarify the reason why the high oxide ion conductivity appeared in this system, the electrical conductivity was measured as a function of temperature and La content. With an increasing La content, the discontinuous jump of ion conductivity in the Arrhenius plot, which is related to the disordering of the oxygen vacancies, disappeared for the sample with x0.2. Above x=0.12, the ion conductivity linearly increased with La content, while the activation energy remained constant with respect to the La content. Moreover, the conductivity for x=0.6 was 0.042 (S/cm) at 1073 K, which exceeded that of 8 mol% yttria-stabilized zirconia. The higher oxide-ion conductivity of this system could be dominated by the amount of mobile oxygen ions. 相似文献
16.
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
17.
A Bi 2V 1 − x − yU xBi yO 5.5 + 0.5x − y solid solution derived from Bi 4V 2O 11 has been prepared and characterized with x up to 0.125 for y = 0. Partial substitution of U 6+ for V 5+ in Bi 4V 2O 11 leads to the stabilization at room temperature of the high-oxide ion conducting γ-phase, in contrast with other M 6+ dopants which stabilize the β-phase. The lower conductivity in U substituted system compared with BICUVOX.10 is attributed to its higher activation energy. Conductivity values and activation energies of the U substituted phases compare well with Bi 2UO 6. 相似文献
18.
Formation of the La 2Cu 1−xCo xO 4+δ solid solutions with orthorhombic K 2NiF 4-type structure was found to be in the range of 0≤ x≤0.30 at temperatures above 1270 K. Incorporating cobalt into the copper sublattice of lanthanum cuprate leads to increasing oxygen hyperstoichiometry and decreasing electrical conductivity. Thermal expansion coefficients of the La 2Cu 1−xCo xO 4+δ ( x=0.02–0.30) ceramics at 470–1100 K were calculated from the dilatometric data to vary in the range (12.2–13.2)×10 −6 K −1. Studying the dependence of oxygen permeation fluxes through La 2Cu(Co)O 4+δ on the membrane thickness demonstrated that the oxygen transport at the thickness values below 1 mm is limited by both surface exchange rate and bulk ionic conductivity. Oxygen permeability of the La 2Cu 1−xCo xO 4+δ solid solutions was ascertained to increase with cobalt concentration at x=0.02–0.10 and to decrease with further dopant additions, indicating a participation of interstitial oxygen in the ionic transport. 相似文献
19.
Bi-2222 phases were prepared by solid state reactions and were studied using X-ray diffraction, chemical and EDS-analysis. Compounds with composition Bi 2Sr 2R 1.33Ce 0.67Cu 2O 10+δ were obtained for R=Pr, Nd, Sm-Er. The phases Bi 2Sr 2Cu 2O 10+δ with similar structure but without Ce were prepared for R = Pr, Sm-Dy. For Bi 2Sr 2Eu 2−xCe xCu 2O 10+δ the solubility limit was determined and the effect of treatment under different oxygen pressures was investigated. Weak diamagnetic signals (<3%) were detected for some of these samples. 相似文献
20.
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
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