共查询到20条相似文献,搜索用时 15 毫秒
1.
E. Kandyel 《Journal of solid state chemistry》2008,181(11):3137-3144
A new series of Sr-based Hg-1212 superconducting cuprate (Hg1−ySey)Sr2(Y1−xCax)Cu2O6+δ (y=0.25; 0.0?x?0.7) have been successfully synthesized using a highly homogenous and reactive precursor Sr2(Y1−xCax)Cu2Oz prepared by the citrate sol-gel process. This chemical method is fast, cheap, reproducible and more efficient than the traditional solid-state reaction method. X-ray diffraction (XRD) and Energy dispersive X-ray analyses (EDX) studies have shown that Se is required for stabilization of the Sr-based Hg-1212 phase (Hg1−ySey)Sr2YCu2O6+δ; y≈0.25. On the other hand, electrical resistivity and magnetic susceptibility measurements indicated that substitution of Y by Ca is necessary to induce superconductivity in the 1212 (Hg0.75Se0.25)Sr2(Y1−xCax)Cu2O6+δ samples. Superconductivity is observed only for samples with x?0.3 and Tc increases with increasing Ca content as well as O2-annealing. A maximum Tc(onset) of 85 K is found in the (Hg0.75Se0.25)Sr2(Y0.3Ca0.7)Cu2O6.84 sample annealed in an oxygen atmosphere. The structure of O2-annealed samples was investigated by the Rietveld refinement. For all samples, it was found that Se substitutes at the Hg site. Each Se atom is surrounded by four oxygen atoms O(3), but these are not at the ideal site. Rather, these oxygen atoms are shifted along the [110] direction ((0.3989, 0.3989, 0) in the case of x=0.5), implying a four-fold split site with an occupancy of 0.22(2) for each of them. 相似文献
2.
A.A. Markov M.V. Patrakeev A.P. Nemudry Yu.T. Pavlyukhin V.L. Kozhevnikov 《Journal of solid state chemistry》2009,182(4):799-10902
The oxide solid solutions SrFe1−xMoxO3−δ, where x=0.05, 0.1 and 0.2, are studied in this work. It is shown that substitution of iron for molybdenum results in stabilization of a cubic quasi-perovskite locally inhomogeneous structure, which is evidenced by HREM and Mössbauer spectroscopy. The coulometric titration is used in order to determine changes of oxygen nonstoichiometry in the obtained solutions with temperature and ambient oxygen partial pressure. Partial molar thermodynamic functions of the labile oxygen are calculated from the measured data. The variations of partial molar entropy with oxygen content follow the ideal gas model reasonably well thus demonstrating approximately random distribution of oxygen vacancies in the doped ferrite at high temperatures. The partial molar enthalpy is found to increase with doping, which is indicative of a progressive decrease in average values of the bonding energy of labile oxygen ions. The measurements of total conductivity are used in order to determine partial contributions of charge carriers. The oxygen ion component is shown to increase at small level of doping, x=0.05 while further increase in molybdenum content is accompanied with the decline in the ion conductivity. The electron contribution in reducing conditions tends to increase with molybdenum content, which is interpreted as a manifestation of involvement of Mo5+ cations in electron transport. Concentration and mobility of electron carriers are calculated. Some increase in mobility of electron holes at small doping is explained as related to the filling of oxygen vacancies. 相似文献
3.
The structures of compounds in the perovskite series Ba2LnSnxNb1−xO6−δ (Ln=Pr and Tb and x=0, 0.1, 0.2, …, 1.0) have been examined using synchrotron X-ray and neutron diffraction. It was found that niobate members of both series feature full B-site cation ordering but that this order is lost with increasing x. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopies indicate that the oxidation state of the lanthanide cations gradually changes from Ln3+ to Ln4+ with increased Sn4+ doping. This is believed to be the cause of the loss of B-site ordering. Least squares analysis of the XANES spectra suggests that the rate of the transformation of Ln3+ cations to the tetravalent state is such that the Pr series contains no oxygen vacancies while the Tb series may contain a very small amount of vacancies, with δ≈0.02. 相似文献
4.
Paul J. Saines Margaret M. Elcombe Ling-Yun Jang 《Journal of solid state chemistry》2008,181(11):2941-2952
Compounds in the double perovskites series Ba2LnSnxSb1−xO6−δ (Ln=Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba2PrSnxSb1−xO6−δ series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr3+ to Pr4+ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba2TbSbO6 and Ba2TbSnO6−δ are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb3+ to Tb4+. Multiple linear regression analysis of the Pr and Tb LIII-edge XANES indicates that the rate of Ln3+ transforming to Ln4+ is such that there are no oxygen vacancies in Ba2PrSnxSb1−xO6−δ but in Ba2TbSnxSb1−xO6−δ there is a small amount of oxygen vacancies, with a maximum of δ≈0.05 present. 相似文献
5.
Takashi Nakamura Keiji Yashiro Kazuhisa Sato Junichiro Mizusaki 《Journal of solid state chemistry》2009,182(5):1121-299
In order to elucidate the relation between thermodynamic quantities, the defect structure, and the defect equilibrium in La2−xSrxNiO4+δ, statistical thermodynamic calculation is carried out and calculated results are compared to those obtained from experimental data. Partial molar enthalpy of oxygen and partial molar entropy of oxygen are obtained from δ-P(O2)-T relation by using Gibbs-Helmholtz equation. Statistical thermodynamic model is derived from defect equilibrium models proposed before by authors, localized electron model and delocalized electron model which could well explain the variation of oxygen content of La2−xSrxNiO4+δ. Although assumed defect species and their equilibrium are different, the results of thermodynamic calculation by localized electron model and delocalized electron model show minor difference. Calculated results by the both models agree with the thermodynamic quantities obtained from oxygen nonstoichiometry of La2−xSrxNiO4+δ. 相似文献
6.
Liliana Mogni Hugo Ascolani Mario S. Moreno Alberto Caneiro 《Journal of solid state chemistry》2005,178(5):1559-1568
The crystal chemistry, electronic structure, and electrical and magnetic properties of the novel perovskite-related nickel oxides Sr3Fe2−xNixO7−δ with 0?x?1.0 have been studied. X-ray diffraction and selected area electron diffraction (ED) data indicate that the samples have a tetragonal (Space group I4/mmm) structure. ED patterns and high-resolution images reveal the presence of a regular stacking along the c-axis for the x=1.0 sample. The lattice parameters, oxygen content, and average oxidation state of iron and nickel decrease with increasing Ni content. The electronic structure of the x=1.0 sample was studied by M 2p X-ray photoelectron spectroscopy (XPS). An analysis of the spectra using the cluster model indicates that this material is in the negative charge-transfer regime and the ground state is dominated by the 3dn+1L configuration with 2p holes (L) in the oxygen band. The insulator states are stabilized due to a p-p type band gap that arises because the p-d transfer integral Tσ dominates over the O 2p bandwith. Although magnetic measurements reveal the presence of ferromagnetic interactions that lead to magnetic frustration at , no long-range magnetic order was observed for the samples with x?0.3. The electrical resistivity decreases with increasing Ni content as the p-p band gap tend to close due to the reduction of the Tσ value. Negative magnetoresistance (∼−24% for x=0.6 and −7% for x=1.0 at 20 K and 9 T) was observed for the Ni containing samples. 相似文献
7.
Nataliya Melnychenko-Koblyuk Peter Rogl Harald Schmid 《Journal of solid state chemistry》2009,182(7):1754-1760
Phase relations at 700 °C, 800 °C and solidus temperatures have been derived for the clathrate system Ba8CuxGe46−x−y□y via X-ray single crystal and powder diffractometry combined with electron probe micro analysis and differential thermal analysis. The ternary clathrate phase derives from binary Ba8Ge43□3 and extends up to x=6. Structure investigations define cubic primitive symmetry with the space group type consistent with a clathrate type I structure throughout the entire homogeneity region 0<x?6 but defect-free Ba8CuxGe46−x exists for x?5.5. 相似文献
8.
Woosuk Cho 《Journal of solid state chemistry》2006,179(11):3534-3540
Nonstoichiometric spinel oxides, LiAlxMn2−xO4−δ (x=0.1,0.2), were synthesized under controlled partial pressure of oxygen, and their elecrochemical performances were investigated. As an Al content increases, solubility limit of the oxygen nonstoichiometry, δ, increased, while partial molar enthalpy of the formation of oxygen nonstoichiometry decreased.Cycle performance of LiAlxMn2−xO4 showed significant improvement comparing with that of LiMn2O4 cathode. However, the decrease of theoretical capacity was accompanied with Al doping. Nonstoichiometric LiAlxMn2−xO4−δ showed the increase in capacity with keeping good cycle performances as well as stoichiometric LiAlxMn2−xO4. Although the introduction of oxygen nonstoichiometry leads to the increase of Mn3+ which is known as Jahn-Teller ion, DSC curves for LiAlxMn2−xO4−δ showed no exothermic peak due to phase transition arising from Jahn-Teller distortion around room temperature. 相似文献
9.
Takashi Nakamura Keiji Yashiro Kazuhisa Sato Junichiro Mizusaki 《Journal of solid state chemistry》2009,182(6):1533-258
Nonstoichiometric variation of oxygen content in Nd2−xSrxNiO4+δ (x=0, 0.2, 0.4) and decomposition P(O2) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range from 873 to 1173 K and the P(O2) range from 10−20 to 1 bar. Nd2−xSrxNiO4+δ shows the oxygen excess and the oxygen deficient composition depending on P(O2), temperature, and the Sr content. To evaluate the characteristics of oxygen nonstoichiometric behavior, partial molar enthalpy of oxygen was calculated. The value of partial molar enthalpy of oxygen slightly approaches zero as δ increases in the oxygen excess region while that is independent of δ in the oxygen deficient region. Discussion was made by comparing data of this study with nonstoichiometric and thermodynamic data of La2−xSrxNiO4+δ: Nd2−xSrxNiO4+δ show more oxygen excess than La2−xSrxNiO4+δ in the higher P(O2) region, while the nonstoichiometric behavior in the oxygen deficient composition is almost the same. The variation of partial molar enthalpy of oxygen with δ for Nd2−xSrxNiO4+δ in the oxygen excess region is much smaller than that of La2−xSrxNiO4+δ. The oxygen nonstoichiometric behavior of Nd2−xSrxNiO4+δ is more ideal-solution-like than that of La2−xSrxNiO4+δ. 相似文献
10.
M.D. Carvalho L.P. Ferreira J.C. Waerenborgh A.B. Lopes 《Journal of solid state chemistry》2008,181(9):2530-2541
Ca2Fe1−xMnxAlO5 (0?x?1) compounds were prepared by a self-combustion method under air (x=0, 0.1, 0.2 and 0.3) and nitrogen (x=0.5, 0.7 and 1.0). The samples prepared under nitrogen were successfully oxidized after short annealing under air. Both X-ray powder diffraction (XRD) Rietveld analysis and electron diffraction revealed that all compounds adopt the brownmillerite-type structure. All samples present an overall antiferromagnetic behaviour and data from magnetic measurements and Mössbauer spectroscopy allowed to conclude that the transition temperature decreases as Mn content increases for x?0.3 and increases in the case of the x?0.5 compounds. Except for x=1, chemical disorder due to the occupancy of both octahedral and tetrahedral sites by different metals as well as the competition between different moments’ orientation induce a complex magnetic behaviour characterized by magnetic frustration and canted antiferromagnetism. Mössbauer spectroscopy and chemical titrations also allowed to conclude about the preferential oxidation of Mn3+ over Fe3+, obtained by thermal treatment under air of the x=0.5 and 0.7 compositions. 相似文献
11.
Jihong Cheng 《Journal of solid state chemistry》2005,178(1):234-244
A series of perovskites with the general formula La1−xAxCrO3−δ (A=Ca or Sr) have been synthesized in the solid solution range 0.0<x?0.3 and 0.0?δ?0.5x with a variety of heat treatments. High-temperature drop solution calorimetry in molten 2PbO·B2O3 at 1080 K was performed to determine their enthalpies of formation from oxides at room temperature. The enthalpy of oxidation involved in the reaction is roughly independent of oxygen nonstoichiometry (δ) in each series with a given dopant composition, but varies with composition (x). The values change from −620±260 to −280±80 kJ/mol O2 when x=0.1-0.3 for Ca-doped samples, and from −440±150 to −290±50 kJ/mol O2 for Sr-doped ones. This dependence of enthalpy of oxidation on composition suggests oxygen vacancies are increasingly short-range ordered in reduced samples. The higher oxidation state of chromium is stabilized by the substitution of alkaline earth ions, but with increasing doping, the enthalpy of formation of the fully oxidized sample in both Ca and Sr-doped systems becomes more endothermic. This destabilization effect is attributed to the large endothermic enthalpy of oxygen vacancy formation (395±30 kJ/mol of ) for the reaction (A=Ca or Sr) that over-rides the exothermic enthalpies of oxidation. At a given composition, Sr-doped LaCrO3 is more stable than its Ca-doped counterpart, which is consistent with basicity arguments. 相似文献
12.
We report systematic studies on the relations among the CeIV-for-NdIII substitution level (x), oxygen-partial pressure (PO2), oxygen content (4+δ), lattice parameters (a, c) and superconductivity characteristics (Tc, volume fraction) in the (Nd1−xCex)2Cu1−yO4+δ system which includes electron-doped superconductors. Independent of the Ce-doping level x, samples synthesized in air are found oxygen deficient, i.e. δ<0. Nevertheless, reductive annealing is needed to induce superconductivity in the air-synthesized samples. At the same time, the amount of oxygen removed upon the annealing is found very small (e.g. 0.004 oxygen atoms per formula unit at x=0.075), and consequently the effect of the annealing on the valence of copper (and thereby also on the electron doping level) is insignificant. Rather, the main function of the reductive annealing is likely to repair the Cu vacancies believed to exist in tiny concentrations (y) in the air-synthesized samples. 相似文献
13.
A.A. Yaremchenko A.V. Kovalevsky V.A. Cherepanov 《Journal of solid state chemistry》2008,181(11):3024-3032
The substitution of manganese with nickel in LaSr2Mn2O7−δ, where the solubility limit corresponds to approximately 25% Mn sites, enhances the Ruddlesden-Popper phase stability at elevated temperatures and atmospheric oxygen pressure. The total conductivity of LaSr2Mn2−yNiyO7−δ (y=0-0.4) decreases with nickel additions, whilst the average thermal expansion coefficients calculated from dilatometric data in the temperature range 300-1370 K increase from (11.4-13.7)×10−6 K−1 at y=0 up to (12.5-14.4)×10−6 K−1 at y=0.4. The conductivity and Seebeck coefficient of LaSr2Mn1.6Ni0.4O7−δ, analyzed in the oxygen partial pressure range 10−15-0.3 atm at 600-1270 K, display that the electronic transport is n-type and occurs via a small polaron mechanism. Reductive decomposition is observed at the oxygen pressures close to Ni/NiO boundary, namely ∼2.3×10−11 atm at 1223 K. Within the phase stability domain, the electronic transport properties are essentially p(O2)-independent. The steady-state oxygen permeability of dense LaSr2Mn1.6Ni0.4O7−δ membranes is higher than that of (La,Sr)MnO3−δ, but lower if compared to perovskite-like (Sr,Ce)MnO3−δ. Porous LaSr2Mn1.6Ni0.4O7−δ cathodes in contact with apatite-type La10Si5AlO26.5 solid electrolyte exhibit, however, a relatively poor electrochemical performance, partly associated with strong cation interdiffusion between the materials. 相似文献
14.
Konrad ?wierczek Bogdan Dabrowski Leopoldo Suescun Stanislaw Kolesnik 《Journal of solid state chemistry》2009,182(2):280-7116
Structural and magnetic studies are presented for the perovskite type Sr1−xLaxCo0.5Fe0.5O3−δ (0?x?0.5) materials annealed under moderately high-oxygen pressures of ∼200 atm. A detailed analysis of the room temperature neutron time-of-flight diffraction data reveals that the crystal structure of the sample SrCo0.5Fe0.5O2.89(1), previously described as vacancy-disordered cubic, is similar to the formerly reported, oxygen-vacancy ordered Sr8Fe8O23 compound, i.e. Sr8Co4Fe4O23 is tetragonal with the I4/mmm symmetry. With an increase of the La content the studied materials become nearly oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). Low-temperature structural and magnetic measurements show a ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2. 相似文献
15.
H. El Shinawi 《Journal of solid state chemistry》2008,181(10):2705-2712
The K2NiF4 phases LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, and their reduced forms LaSrCo0.5Fe0.5O3.75 and La1.2Sr0.8Co0.5Fe0.5O3.85, have been successfully prepared by solid-state reactions, followed by reduction in 10% H2/N2 in order to produce oxygen-deficient materials. All materials crystallize in a tetragonal K2NiF4 structure (space group I4/mmm) with Co and Fe randomly distributed over the B-sites of the structure. Mössbauer spectra have confirmed the trivalent state of Fe in these materials. In the reduced materials, oxide ion vacancies are confined to the equatorial planes of the K2NiF4 structure and the Co is present almost entirely as Co2+ ions; low-temperature neutron powder diffraction data reveal that these reduced phases are antiferromagnetically ordered with a tetragonal noncollinear arrangement of the moments. The Co3+ ions, present in stoichiometric LaSrCo0.5Fe0.5O4 and La1.2Sr0.8Co0.5Fe0.5O4, inhibit magnetic order and are assumed to be in the low-spin state. 相似文献
16.
Kenji Yoshii Naoshi Ikeda Yusuke Yokota Yasuhiro Yoneda Yoichi Horibe 《Journal of solid state chemistry》2009,182(7):1611-1618
We have investigated the magnetic and dielectric properties of YbFe2−xMnxO4 (0?x?1), which is an Fe-site-substituted system of new multiferroic oxides RFe2O4 (R=Y, Ho-Lu). X-ray diffraction measurements show that a solid solution is formed between YbFe2O4 (x=0) and YbFeMnO4 (x=1) for 0?x?1, whereas only compounds with x=1 (i.e., RM1M2O4; M1 and M2=trivalent and divalent cations, respectively) have been known for the Fe-site substitution in RFe2O4. The valence of the Mn ion is determined to be ∼2+, consistently with the suppression of low-temperature magnetization by the Mn substitution. The magnetic transition temperature (TN) and the dielectric constant (ε′) decrease monotonically with increasing x. This result plausibly confirms that the magnetic and dielectric properties in oxides isostructural with RFe2O4 are governed by the electron transfer between Fe-site ions, unlike ordinary ferroelectrics and dielectrics, in which the ionic displacement plays a key role. The possibility for application is briefly discussed as well. 相似文献
17.
Annika K. Eriksson Gunnar Svensson Paul F. Henry Christopher S. Knee 《Journal of solid state chemistry》2008,181(8):2031-2040
A series of oxygen-deficient Sr0.85Bi0.15Co1−xFexO3−δ (0.0?x?1.0) perovskite phases were prepared using solid-state reaction. Results of neutron powder diffraction analyses show that the introduction of Fe onto the B-site severely effects the long range coherence of the oxygen vacancy ordered, I4/mmm supercell, observed for the x=0.0 sample. For x=0.1 a smaller, a=b≈ap, c≈2ap, P4/mmm supercell gives the best agreement to the diffraction data, whilst phases in the range 0.2?x?0.6 adopt disordered cubic perovskite structures. Pseudo-cubic, a=b≈ap, c≈ap, structures are found for x?0.8. Evidence of weak superstructures, reflecting local oxygen ordering, is also obtained from electron diffraction. For all oxygen-annealed phases the average structure reverts to cubic Pm3¯m. The as-prepared samples show G-type antiferromagnetic order at room temperature. The oxygen annealed x=0.10, 0.25 and 1.0 samples display low-temperature spin-glass transitions. 相似文献
18.
CdVO3−δ and solid solutions of Cd1−xNaxVO3 with the GdFeO3-type perovskite structure were prepared using a high-pressure (6 GPa) and high-temperature technique. No significant oxygen and cation deficiency was found in CdVO3. Cd1−xNaxVO3 are formed in the compositional range of 0?x?0.2. CdVO3 and Cd1−xNaxVO3 demonstrate metallic conductivity and Pauli paramagnetism between 2 and 300 K. A large electronic contribution to the specific heat (γ=13.4 and ) for CdVO3 and Cd0.8Na0.2VO3, respectively) was observed at low temperatures due to the strongly correlated electrons. Crystal structures of CdVO3 and Cd0.8Na0.2VO3 were refined by X-ray powder diffraction: space group Pnma; Z=4; , , and for CdVO3 and , , and for Cd0.8Na0.2VO3. 相似文献
19.
N.V. Tarakina E.A. Nikulina D.G. Kellerman I.F. Berger G. Van Tendeloo 《Journal of solid state chemistry》2007,180(11):3180-3187
In the Mg4−xNixNb2O9 (0?x?4) system two ranges of solid solution have been found. One of the solid solutions has a corundum-related structure type (space group ); the second one adopts the II-Ni4Nb2O9 structure type (space group Pbcn). The unit cell constants and atomic positions have been determined and refined using neutron powder diffraction data. Electron diffraction and high-resolution transmission electron microscopy (HRTEM) from MgNi3Nb2O9 crystals identify the presence of planar defects and the intergrowth of several (structurally related) phases. The magnetic susceptibility of Mg3NiNb2O9, measured in the temperature range T=2-300 K, shows no indications of magnetic ordering at low temperatures, while for MgNi3Nb2O9 there is a magnetic ordering at temperatures below 45.5 K. 相似文献
20.
A polycrystalline sample with SrFe0.5Co0.5O3−δ stoichiometry has been prepared under moderate pressures of 2 GPa in the presence of KClO4 as oxidizing agent. The crystallographic and magnetic structures have been studied from neutron powder diffraction (NPD), complemented with magnetotransport and magnetic susceptibility data. The study of the crystallographic structure confirms that this compound, with the actual stoichiometry SrFe0.5Co0.5O2.88(3), is a simple cubic perovskite at 2 and 295 K, defined in the Pm-3m space group, where Fe and Co atoms are distributed at random over the B positions. The magnetic measurements show that SrFe0.5Co0.5O2.88 is a ferromagnet with , which is also confirmed by neutron diffraction: the magnetic structure is collinear, characterized by a propagation vector k=0; the ordered magnetic moment values for the (Fe4+,Co4+) cations are 1.54(9) and 0.65(15) μB at T=2 and 295 K, respectively. Transport measurements show a semiconducting behaviour, and a negative magnetoresistance (MR) of −6.5% is observed at for . 相似文献