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1.
The non-linear thermal expansion behaviour observed in Ce1−yPryO2−δ materials can be substantially controlled by Gd substitution. Coulometric titration shows that the charge compensation mechanism changes with increasing x, in the system GdxCe0.8−xPr0.2O2−δ. For x=0.15, charge compensation is by vacancy formation and destabilises the presence of Pr4+. At x=0.2, further Gd substitution is charge compensated by additionally raising the oxidation state of Pr rather than solely the creation of further oxygen ion vacancies. Oxygen concentration cell e.m.f. measurements in an oxygen/air potential gradient show that increasing Gd content decreases ionic and electronic conductivities. Ion transference numbers measured under these conditions show a positive temperature dependence, with typical values to=0.90,0.98 and 0.80 for x=0,0.15 and 0.2, respectively, at 950 °C. These observations are discussed in terms of defect association. Oxygen permeation fluxes are limited by both bulk ambipolar conductivity and surface exchange. However, the composition dependent trends in permeability are shown to be dominated by ambipolar conductivities, and limited by the level of electronic conductivity. At the highest temperatures, oxygen permeability of composition x=0.2 approaches that of composition x=0, Ce0.8Pr0.2O2−δ, with specific oxygen permeability values approximately 2×10−9 mol s−1 cm−1 at 950 °C, but offering much better thermal expansion properties.  相似文献   

2.
Oxides in the system PrCo1−xMgxO3 (x=0.0, 0.05, 0.10, 0.15, 0.20, 0.25) were synthesized by citrate technique and characterized by powder X-ray diffraction and scanning electron microscope. All compounds have a cubic perovskite structure (space group ). The maximum ratio of doped Mg in the system PrCo1−xMgxO3 is x=0.2. Further doping leads to the segregation of Pr6O11 in PrCo1−xMgxO3. The substitution of Mg for Co improves the performance of PrCoO3 as compared to the electrical conductivity measured by a four-probe electrical conductivity analyzer in the temperature range from 298 to 1073 K. The substitution of Mg for Co on the B site may be compensated by the formations of Co4+ and oxygen vacancies. The electrical conductivity of PrCo1−xMgxO3 oxides increases with increasing x in the range of 0.0-0.2. The increase in conductivity becomes considerable at the temperatures ?673 K especially for x?0.1; it reaches a maximum at x=0.2 and 1073 K. From x>0.2 the conductivity of PrCo1−xMgxO3 starts getting lower. This is probably a result of the segregation of Pr6O11 in PrCo1−xMgxO3 , which blocks oxygen transport, and association of oxygen vacancies. A change in activation energy for all PrCo1−xMgxO3 compounds (x=0-0.25) was observed, with a higher activation energy above 573 K and a lower activation energy below 573 K. The reasons for such a change are probably due to the change of dominant charge carriers from Co4+ to Vö in PrCo1−xMgxO3 oxides and a phase transition mainly starting at 573 K.  相似文献   

3.
The microstructure and phase stability of nanocrystalline mixed oxide LuxCe1−xO2−y (x=0-1) are described. Nano-sized (3-4 nm) oxide particles were prepared by the reverse microemulsion method. Morphological and structural changes upon heat treatment in an oxidizing atmosphere were studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman and Yb3+ emission spectroscopy, the latter ion being present as an impurity in the Lu2O3 starting material. Up to 950 °C, the samples were single phase, with structure changing smoothly with Lu content from fluorite type (F) to bixbyite type (C). For the samples heated at 1100 °C phase separation into coexisting F- and C-type structures was observed for 0.35<x<0.7. It was also found that addition of Lu strongly hinders the crystallite growth of ceria during heat treatment at 800 and 950 °C.  相似文献   

4.
The local structure of In2O3 cosubstituted with Zn and Sn (In2−2xSnxZnxO3, x≤0.4 or ZITO) was determined by extended X-ray absorption fine structure (EXAFS) for x=0.1, 0.2, 0.3 and 0.4. The host bixbyite In2O3 structure is maintained up to the enhanced substitution limit (x=0.4). The EXAFS spectra are consistent with random substitution of In by the smaller Zn and Sn cations, a result that is consistent with the “good-to-excellent” conductivities reported for ZITO.  相似文献   

5.
Five series of perovskite-type compounds in the system La1−xCaxCr1−yTiyO3 with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO3-LaCrIIIO3-CaCrIVO3 quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCrIVO3-CaTiO3-LaCrIIIO3. The solid solution La(1−x′−y)Ca(x′+y)CrIVxCrIII(1−x′−y)TiyO3 exists by up to 0.6-0.7 mol fractions of CaCrIVO3 (x<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La0.1Ca0.9TiO3, La0.2Ca0.8TiO3, La0.4Ca0.6Cr0.2Ti0.8O3 and La0.3Ca0.7Cr0.2Ti0.8O3 in the system La1−xCaxCr1−yTiyO3 were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO2=6×10−16-0.21×105 Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10−15-0.21×105 Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr3+]/[Cr4+] and [Ti4+]/[Ti3+]. The maximum electrical conductivity at 900 °C and pO2=10−15 Pa was found for the composition La0.1Ca0.9TiO3 (near 50 S/cm) and in air at 900 °C for La0.5Ca0.5CrO3 (close to 100 S/cm).  相似文献   

6.
Electrical conductivity, thermopower and oxygen content were measured for La1−xSrxFeO3−δ (x=0.2, 0.5, 0.9) within the oxygen partial pressure range 10−4-0.5 atm and at temperatures 750-950 °C. The dominating charge carriers under these experimental conditions are electron holes. The results of oxygen nonstoichiometry measurements are used to estimate the concentration of holes and to analyze data on conductivity and thermopower. The changes in thermopower are described by the model assuming that the number of sites accessible for migration of holes is independent on oxygen content. The mobility of electron holes is calculated, and it is found to be virtually independent on temperature in the compositions with x<0.5 while compositions with x>0.5 exhibit thermally activated mobility and mobility values about 0.1 cm2 V−1 s−1 or smaller. It is suggested that the main contribution to the composition dependent variations in electron hole mobility are associated with Coulomb interactions at small x's, whereas at high degrees of doping the mobility of holes is most strongly affected by the increasing amount of oxygen vacancies.  相似文献   

7.
Doped-rutile has been traditionally used in ceramic pigments for its intense optical properties. In this paper, we compare the classical ceramic synthesis of Ti1−2xNbxNixO2−x/2 system with the sol-gel methodology, which allows a reduction of the anatase-rutile transformation temperature. The composition was optimised in order to obtain a unique rutile phase with the minimum amount of pollutant Ni(II) and enhanced chromatic coordinates. Incorporation of the doping ions in the rutile structure was corroborated by XRD and Rietveld refinements. The species responsible for the colour mechanism were studied by different techniques. UV-VIS spectroscopy showed the characteristic features of Ni2+ ions, whose existence was corroborated by EPR and magnetic measurements. From these results, (Ni,Nb)doped-TiO2 powder samples can be now shaped as thin films, monoliths, etc. by using sol-gel methodology without modifying their properties. This study introduces new possibilities of coloured TiO2-based solid solutions in new combined advanced applications (colouring agent and photocatalyst, etc.).  相似文献   

8.
A structural, magnetic and electronic study of the cobaltocuprate CoSr2Y2−xCexCu2Oδ (x=0.5-0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO4 tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5-0.8; TM increases with x. A change in the dimensionality of the magnetic order occurs at x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co3+ is partially oxidised to Co4+ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.  相似文献   

9.
Fifteen perovskite-type compounds Sr0.9−xCaxCe0.1MnO3, x=0-0.9 in steps as fine as 0.05, have been synthesised by solid state methods, and the room temperature structures characterised using X-ray synchrotron powder diffraction. At low Ca contents (x?0.45) the structures are tetragonal in space group I4/mcm and at high Ca contents (x?0.55) the compounds are orthorhombic in space group Pbnm. At room temperature these two phases co-exist in the compound with x=0.5. XANES measurements show the Ce to be present as Ce4+ in all the oxides. High temperature structures are reported for selected members.  相似文献   

10.
The solid solution of YMn1−xFexO3 (x=0.0, 0.1, 0.2, 0.3, 0.5, 1.0) was synthesized from the citrate precursor route. The hexagonal crystal structure related to YMnO3 was stable for x?0.3. Rietveld refinement was carried out on the composition for x=0.3 and was refined to a major hexagonal phase (∼97%) with 3% of orthorhombic Y(Fe/Mn)O3 phase. The a-axis lattice constant increases and the c-axis lattice constant decreases with x for x?0.2. The increase in the c-axis lattice constant at x=0.3 could be due to the doping of significant amount of d5 ion (high spin Fe3+ ion) in a trigonal bipyramidal crystal field. The detailed structural, magnetic and dielectric properties are discussed.  相似文献   

11.
The 6H BaRu1−xMnxO3 with the hexagonal BaTiO3 structure was synthesized using high-pressure sintering method. It is found that the lattice parameter deviates from Vegard's law at x=0.3 for the solid solutions due to the charge transfer effects at B-site. The substitution of Mn for Ru cations gives rise to the short-range magnetic ordering, due to the disordered arrangement of Ru and Mn cations. The compounds are weak ferromagnetic in the x range 0.05-0.40, with the maximal Curie temperature Tc 175.2 K at x=0.10. They are of spin-glass-like magnetism at lower temperature at x?0.1. With Mn doping, the 6H BaRuO3 transforms to a semiconductor from the primal metal at x=0.30. The resistance as a function of temperature below about 70 K follows the two-dimensional variable-range hopping conduction mechanism in BaRu0.50Mn0.50O3.  相似文献   

12.
The KxBa1−xGa2−xGe2+xO8 (x=0.6−1.0) solid solutions undergo a structural phase transition that has a significant effect on their sintering behavior and their microwave dielectric properties. The crystal structures of both phases within the solid-solution region were determined by the Rietveld method using powder X-ray diffraction data. We found that the low-temperature-stable phase is isostructural with the pseudo-orthorhombic KGaGe3O8 (space group P21/a), while the high-temperature-stable phase has a typical monoclinic feldspar structure (space group C2/m). Due to the topological differences between the two structures, the T-O bonds within the tetrahedra must be partially recombined to make a new framework, which causes an endothermic effect during the P21/a to C2/m phase transition. The correlation between the crystal structures, the microwave dielectric properties and the phase-transition behaviors were discussed in terms of the crystallographic features, the lattice parameters, and the strain-induced anisotropic peak-broadening.  相似文献   

13.
X-band and high-frequency EPR spectroscopy were used for studying the manganese environment in layered Li[MgxNi0.5−xMn0.5]O2, 0?x?0.5. Both layered LiMg0.5Mn0.5O2 and monoclinic Li[Li1/3Mn2/3]O2 oxides (containing Mn4+ ions only) were used as EPR standards. The EPR study was extended to the Ni-substituted analogues, where both Ni2+ and Mn4+ are paramagnetic. For LiMg0.5−xNixMn0.5O2 and Li[Li(1−2x)/3NixMn(2−x)/3]O2, an EPR response from Mn4+ ions only was detected, while the Ni2+ ions remained EPR silent in the frequency range of 9.23-285 GHz. For the diamagnetically diluted oxides, LiMg0.25Ni0.25Mn0.5O2 and Li[Li0.10Ni0.35Mn0.55]O2, two types of Mn4+ ions located in a mixed (Mn-Ni-Li)-environment and in a Ni-Mn environment, respectively, were registered by high-field experiments. In the X-band, comparative analysis of the EPR line width of Mn4+ ions permits to extract the composition of the first coordination sphere of Mn in layered LiMg0.5−xNixMn0.5O2 (0?x?0.5) and Li[Li(1−2x)/3NixMn(2−x)/3]O2 (x>0.2). It was shown that a fraction of Mn4+ are in an environment resembling the ordered “α,β”-type arrangement in Li1−δ1Niδ1[Li(1−2x)/3+δ1Ni2x/3−δ1)α(Mn(2−x)/3Nix/3)β]O2 (where and δ1=0.06 were calculated), while the rest of Mn4+ are in the Ni,Mn-environment corresponding to the Li1−δ2Niδ2[Ni1−yMny]O2 () composition with a statistical Ni,Mn distribution. For Li[Li(1−2x)/3NixMn(2−x)/3]O2 with x?0.2, IR spectroscopy indicated that the ordered α,β-type arrangement is retained upon Ni introduction into monoclinic Li[Li1/3Mn2/3]O2.  相似文献   

14.
The aim of this work was to determine structural parameters of the Y10−xLaxW2O21 (x=0-10) solid solution series and investigate their electric properties. Crystallographic data shows a gradual increase in symmetry with increasing La content, as the structure evolves from orthorhombic, Y10W2O21, towards the pseudo-cubic structure of Y5La5W2O21. The solubility limit of La2O3 was found to be 50% (x=5). Above this level two phases were observed, La6W2O15 and (La,Y)10+xW2−xO21−δ. The conductivity of Y rich samples was very low, with σ of the order 2×10−5-5×10−5 S cm−1 at 1000 °C, whilst ionic conductivity was observed for most La rich doped samples. The highest conductivity was observed for La10W2O21 and its doped analogues, at 1×10−3-5×10−3 S cm−1 at 1000 °C. Unit cell parameters were determined as a function of temperature from 0 to 1000°C, and thermal expansion of these materials was determined from temperature studies carried out at the Australian Synchrotron facility in Melbourne, Victoria, Australia.  相似文献   

15.
High-temperature electrical conductivity measurements, structural data from powder X-ray diffraction and 57Fe Mössbauer spectroscopy were combined to study the interrelationship of oxygen ion transport and p- and n-type transport in Sr2(Fe1−xGax)2O5, where x=0, 0.1 and 0.2. Although gallium substitution generally decreases the total ion-electron transport, the transition of the orthorhombic brownmillerite structure to a cubic phase on heating results in the recurrence of the conductivity to the same high level as in the parent ferrite (x=0). The changes of the partial contributions to the total conductivity as a function of x are shown to reflect a complicated interplay of the disordering processes that develop in the oxygen sublattice on heating in response to replacement of iron with gallium.  相似文献   

16.
Antimony-doped K1−xTi1−xSbxOPO4, x=0.23, crystals have been prepared by spontaneous nucleation from the flux in the quaternary system K2O-TiO2-P2O5-Sb2O5. Crystal structure observation with TEM method reveals the presence of superstructure ordering. Core level electronic parameters have been studied by X-ray photoelectron spectroscopy. Strong effect of Sb doping has been detected for inner shells of Ti4+ ions. Prominent decreasing of the binding energy difference Δ(O 1s-Ti 2p3/2) correlates with the shortening of mean oxide bond length L(Ti−O) at x=0.23 that suggests increased ionicity of Ti−O bonds in K1−xTi1−xSbxOPO4 solid solutions.  相似文献   

17.
Polycrystalline Sr2−xNdxFeMoO6 (x=0.0, 0.1, 0.2, 0.4) materials have been synthesized by a citrate co-precipitation method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature-dependent NPD data shows that the compounds (x=0.0, 0.1, 0.2) crystallize in the tetragonal symmetry in the range 10-400 K and converts to cubic symmetry above 450 K. The unit cell volume increases with increasing Nd3+ concentration, which is an electronic effect in order to change the valence state of the B-site cations. Antisite defects at the Fe-Mo sublattice increases with the Nd3+ doping. The Curie temperature was increased from 430 K for x=0 to 443 K for x=0.4. The magnetic moment of the Fe-site decreases while the Mo-site moment increases with electron doping. The antiferromagnetic arrangement causes the system to show a net ferrimagnetic moment.  相似文献   

18.
Zr1−xLnxW2O8−x/2 solid solutions (Ln=Eu, Er, Yb) of different substitution fractions x have been synthesized. Their X-ray diffraction (XRD) patterns have been indexed and lattice parameters calculated based on the α-ZrW2O8 structure. The coefficients of thermal expansion (CTEs) of these solid solutions were estimated to be −10.3×10−6 K−1 in temperature range of 30-100 °C. The solubility of lanthanide ions in these solid solutions decreases linearly with the increase in the radius of substituted lanthanide ions. Based on the concentration dependence of phase transition temperatures, a novel method for determination of solubility of the lanthanide ions in Zr1−xLnxW2O8−x/2 solid solutions has been developed. This method seems to be more sensitive as compared with that based on XRD technique.  相似文献   

19.
Nitrogen substituted yellow colored anatase TiO2−xNx and Fe-N co-doped Ti1−yFeyO2−xNx have been easily synthesized by novel hydrazine method. White anatase TiO2−δ and N/Fe-N-doped samples are semiconducting and the presence of ESR signals at g ∼1.994-2.0025 supports the oxygen vacancy and g∼4.3 indicates Fe3+ in the lattice. TiO2−xNx has higher conductivity than TiO2−x and Fe/Fe-N-doped anatase and the UV absorption edge of white TiO2−x extends in the visible region in N, Fe and Fe-N co-doped TiO2, which show, respectively, two band gaps at ∼3.25/2.63, ∼3.31/2.44 and 2.8/2.44 eV. An activation energy of ∼1.8 eV is observed in Arrhenius log resistivity vs. 1/T plots for all samples. All TiO2 and Fe-doped TiO2 show low 2-propanol photodegradation activity but have significant NO photodestruction capability, both in UV and visible regions, while standard Degussa P-25 is incapable in destroying NO in the visible region The mid-gap levels that these N and Fe-N-doped TiO2 consist may cause this discrepancy in their photocatalytic activities.  相似文献   

20.
A-site substituted cerium orthovanadates, Ce1−xSrxVO4, were synthesised by solid-state reactions. It was found that the solid solution limit in Ce1−xSrxVO4 is at x=0.175. The crystal structure was analysed by X-ray diffraction and it exhibits a tetragonal zircon structure of space group I41/amd (1 4 1) with a=7.3670 (3) and c=6.4894 (1) Å for Ce0.825Sr0.175VO4. The UV-vis absorption spectra indicated that the compounds have band gaps at room temperature in the range 4.5-4.6 eV. Conductivity measurements were performed for the first time up to the strontium solid solution limit in air and in dry 5% H2/Ar with conductivity values at 600 °C ranging from 0.3 to 30 mS cm−1 in air to 30-45 mS cm−1 in reduced atmosphere. Sample Ce0.825Sr0.175VO4 is redox stable at a temperature below 600 °C although the conductivity is not high enough to be used as an electrode for solid oxide fuel cells.  相似文献   

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