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1.
On Hexagonal Perovskites with Cationic Vacancies. XXVII. Systems Ba4?xSrxBIIRe2□O12, Ba4B CaxRe2□O12, and Ba4?xLaxBIIRe2?xWx□O12 with BII = Co, Ni In the systems Ba4?xSrxBIIRe2□O12, Ba4BCaxRe2□O12 and Ba4?xLaxBIIRe2?xWx□O12 (BII = Co, Ni) hexagonal perovskites with a rhombohedral 12 L structure (general composition A4BM2□O12; sequence (hhcc)3; space group R&3macr;m) are observed. With the exception of Ba4NiRe2□O12 the octahedral net consists of BO6 single octahedra and M2□O12 face connected blocks (type 1). In type 2 (Ba4NiRe2□O12) the M ions are located in the single octahedra and in the center of the groups of three face connected octahedra. The two outer positions of the latter are occupied by B ions and vacancies in the ratio 1:1. The difference between type 1 and 2 are discussed by means of the vibrational and diffuse reflectance spectra. 相似文献
2.
R.J.H. Voorhoeve J.P. Remeika L.E. Trimble A.S. Cooper F.J. Disalvo P.K. Gallagher 《Journal of solid state chemistry》1975,14(4):395-406
The new compounds La1?xMxMnO3 (0.05 ? x ? 0.4 for M = K; x = 0.2 for M = Na, Rb) have been prepared. La1?xKxMnO3 (0.05 ? x ? 0.4), LaMnO3.01, LaMnO3.15, La0.8Na0.2MnO3, and La0.8Rb0.2MnO3 have been used as catalysts in the reduction of NO. La0.8K0.2MnO3 has also been used in the catalytic decomposition of NO. The activity of these catalysts is related to the presence of a Mn3+/Mn4+ mixed valence and to the relative ease of forming oxygen vacancies in the solid. The presence of cation vacancies in LaMnO3.15 and the substitution of La3+ by alkali ions in LaMnO3 increases the catalytic activity. The reduction of NO involves both molecular and dissociative adsorption of NO. 相似文献
3.
Electrochemical properties of composite cathodes consisting of La0.8Sr0.2Mn1?x Cu x O3 (LSMCu, 0?≤?x?≤?0.2) and Ce0.8Gd0.2O2?x (GDC) were determined by impedance spectroscopy, and conduction mechanism for the composite cathodes was investigated by a near-edge X-ray absorption fine-structure analysis (NEXAFS). LSMCu–GDC cathodes showed lower polarization resistance (R p) than LSM–GDC up to 750 °C, whereas they exhibited better performance at higher temperature (≥800 °C). The best performance was achieved with the LSMCu10–GDC cathode: 0.27 and 0.08?Ω cm2 at 800 °C and 850 °C, respectively. NEXAFS and refinement results confirmed that Cu doping caused the oxidation of Mn3+ to Mn4+ and lattice contraction. This additional Mn4+ can lead to the formation of oxygen vacancies when Mn4+ is converted to Mn3+ at relatively high temperatures (above 600 °C). This in turn contributes to improved oxygen ion transport in LSM. The LSMCu–GDC composite cathode can thus be considered a suitable potential cathode for SOFC applications. 相似文献
4.
Ti4+ substituted Bi0.8Ba0.2Fe1−xTixO3 for x = 0.0, 0.1 and 0.2 are prepared by modified solid state reaction method. The prepared samples sintered at 850 °C for 1 h show a single phase nature. A structural change was observed on Ti4+ substitutions are confirmed through X-ray Diffraction, Fourier Transform Infrared spectroscopy and Raman spectra. An anomalous phase transition is observed in Bi0.8Ba0.2FeO3 at 1173 K. The absence of ferroelectric transition and enhancement of decomposition temperature is observed in the substituted samples from the thermal analysis. A dielectric spectroscopic measurement shows that on Ti4+ substitutions, the magnitude of dielectric constant and loss tangent (tan δ) value is decreased. Vibrating Sample Magnetometer (VSM) study shows both antiferromagnetic and ferromagnetic phases coexist in the M−H curve. On Ti4+ substitutions in Bi0.8Ba0.2FeO3, the antiferromagnetism dominates over the ferromagnetic phase. In corroboration to magnetisation process, ZFC–FC measurement confirms it that on Ti4+ substitution, the antiferromagnetic behaviour gets dominated. The report suggests that the interplay of strain upon Ti4+ substitution causes the structural and magnetic phase transition. 相似文献
5.
Guerman Popov 《Journal of solid state chemistry》2003,175(1):52-58
(La1−xPbx)1−y□yMnO3 with x=0.05-0.5 and y=0, 0.05, 0.1 (where □ is a vacancy) was studied to evaluate the effects of A-site vacancies on the physical properties. In this system manganese perovskites form with tolerance factors close to 1 and low A-site cation size mismatch due to similarities in the effective ionic radii of La3+ and Pb2+. Increasing vacancy concentration indicates no significant effect on the lattice parameters or volume. However, the vacancies introduce a greater A-site cation size mismatch, which leads to a lowering of the ferromagnetic and metal-insulator transition temperatures, although the transitions are not broadened with increasing vacancy content. Due to the vacancies a distribution of Mn-O-Mn angles and Mn-O distances are created, and long range order in (La1−xPbx)1−y□yMnO3 appears to be determined by Mn-O-Mn angles and Mn-O distances which most distort from 180° and are the longest, respectively, in the structure. 相似文献
6.
Leopoldo Suescun Bogdan Dabrowski Steven Remsen 《Journal of solid state chemistry》2009,182(1):187-5538
LaxSr1−xMnO2.6+δ (x=0.1-0.4) compounds have been obtained by low-temperature annealing of stoichiometric materials in hydrogen. La0.1Sr0.9MnO2.6+δ (δ=0.15) and La0.3Sr0.7MnO2.6, tetragonal (P4/m), and La0.2Sr0.8MnO2.6, pseudo-tetragonal monoclinic (P2/m), structures are isostructural with oxygen-vacancy-ordered Sr5Mn5O13 (, c≈aP). La0.4Sr0.6MnO2.6 shows cubic perovskite structure with disordered oxygen vacancies. In the vacancy-ordered (LaxSr1−x)5Mn5O13 phases four out of five Mn cations are Mn3+ and show a typical Jahn-Teller elongated pyramidal coordination while the fifth one Mn(4−5x)+, in octahedral environment, shows decreasing formal charge from Mn4+ (x=0) to Mn2.5+x=0.3. This unusual selective doping of the octahedral site produces structural strain due to increasing size of the Mn(4−5x)+ and, in the case of (La0.2Sr0.8)5Mn5O13, the unusual compressed octahedral arrangement of oxygen atoms around it. The coordination geometry implies that either the dx2-y2 orbital is occupied, which would be a rare example of inverted occupancy of eg orbitals in manganites, or that disordered Mn3+ apically elongated MnO6 octahedra are present with normal electronic configuration , and the observed bond distances are the average of the long and intermediate in-plane Mn-O bonds. Several structural features favor the second case. 相似文献
7.
V. A. Kolotygin E. V. Tsipis A. I. Ivanov Y. A. Fedotov I. N. Burmistrov D. A. Agarkov V. V. Sinitsyn S. I. Bredikhin V. V. Kharton 《Journal of Solid State Electrochemistry》2012,16(7):2335-2348
Strontium additions in (La1?x Sr x )1?y Mn0.5Ti0.5O3?δ (x?=?0.15–0.75, y?=?0–0.05) having a rhombohedrally distorted perovskite structure under oxidizing conditions lead to the unit cell volume contraction, whilst the total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by surface exchange increase with increasing x. The oxygen partial pressure dependencies of the conductivity and Seebeck coefficient studied at 973–1223?K in the p(O2) range from 10?19 to 0.5?atm suggest a dominant role of electron hole hopping and relatively stable Mn3+ and Ti4+ states. Due to low oxygen nonstoichiometry essentially constant in oxidizing and moderately reducing environments and to strong coulombic interaction between Ti4+ cations and oxygen anions, the tracer diffusion coefficients measured by the 18O/16O isotopic exchange depth profile method with time-of-flight secondary-ion mass spectrometric analysis are lower compared to lanthanum–strontium manganites. The average thermal expansion coefficients determined by controlled-atmosphere dilatometry vary in the range 9.8–15.0?×?10?6?K?1 at 300–1370?K and oxygen pressures from 10?21 to 0.21?atm. The anodic overpotentials of porous La0.5Sr0.5Mn0.5Ti0.5O3?δ electrodes with Ce0.8Gd0.2O2-δ interlayers, applied onto LaGaO3-based solid electrolyte, are lower compared to (La0.75Sr0.25)0.95Cr0.5Mn0.5O3?δ when no metallic current-collecting layers are introduced. However, the polarization resistance is still high, ~2 Ω?×?cm2 in humidified 10?% H2–90?% N2 atmosphere at 1073?K, in correlation with relatively low electronic conduction and isotopic exchange rates. The presence of H2S traces in H2-containing gas mixtures did not result in detectable decomposition of the perovskite phases. 相似文献
8.
E. Pollert J. Hejtmánek K. Kní?ek J.P. Doumerc J. Etourneau 《Journal of solid state chemistry》2004,177(12):4564-4568
The influence of the cobalt substitution for manganese ions in the mixed valence perovskites La0.8Na0.2Mn1−xCoxO3 (0?x?0.2) was investigated by X-ray, electric transport and magnetic measurements. The study carried out on sintered polycrystalline samples revealed the rhombohedral () structure and the insulator-metal transition connected with a ferromagnetic arrangement in the whole concentration range. Increasing concentration of cobalt ions leads to a gradual decrease of PM-FM and I-M transition temperatures. An influence of the cobalt ions on the observed behavior is attributed to charge compensation Mn3+→Mn4+ leading to the formation of stable couples Mn4+-Co2+. Therefore the double-exchange interactions Mn3+-O2−-Mn4+ partly vanish and they are replaced by positive superexchange interactions Mn4+-O2−-Co2+, but of a semiconducting character. 相似文献
9.
H. El Shinawi J.F. Marco F.J. Berry C. Greaves 《Journal of solid state chemistry》2009,182(8):2261-2268
The M4+-containing K2NiF4-type phases La0.8Sr1.2Co0.5Fe0.5O4 and La0.8Sr1.2Co0.5Mn0.5O4 have been synthesized by a sol–gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M4+ to M3+ and of Co3+ to Co2+. The vacancies are confined to the equatorial planes of the K2NiF4-type structure. A partial reduction of Mn3+ to Mn2+ also occurs to achieve the oxygen stoichiometry in La0.8Sr1.2Co0.5Mn0.5O3.6. La0.8Sr1.2Co0.5Fe0.5O3.65 contains Co2+ and Fe3+ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La0.8Sr1.2Co0.5Fe0.5O4, La0.8Sr1.2Co0.5Mn0.5O4 and La0.8Sr1.2Co0.5Mn0.5O3.6 induce spin glass properties in these phases. 相似文献
10.
The M4+-containing K2NiF4-type phases La0.8Sr1.2Co0.5Fe0.5O4 and La0.8Sr1.2Co0.5Mn0.5O4 have been synthesized by a sol-gel procedure and characterized by X-ray powder diffraction, thermal analysis, neutron powder diffraction and Mössbauer spectroscopy. Oxide ion vacancies are created in these materials via reduction of M4+ to M3+ and of Co3+ to Co2+. The vacancies are confined to the equatorial planes of the K2NiF4-type structure. A partial reduction of Mn3+ to Mn2+ also occurs to achieve the oxygen stoichiometry in La0.8Sr1.2Co0.5Mn0.5O3.6. La0.8Sr1.2Co0.5Fe0.5O3.65 contains Co2+ and Fe3+ ions which interact antiferromagnetically and result in noncollinear magnetic order consistent with the tetragonal symmetry. Competing ferromagnetic and antiferromagnetic interactions in La0.8Sr1.2Co0.5Fe0.5O4, La0.8Sr1.2Co0.5Mn0.5O4 and La0.8Sr1.2Co0.5Mn0.5O3.6 induce spin glass properties in these phases. 相似文献
11.
Perovskite-structure oxides La1?x Sr x FeO3?y (x = 0, 0.2, 0.6, 1) were synthesized by the mechanochemical method. In order to refine the stoichiometric composition and the charge state of ions, these samples were studied by X-ray photoelectron spectroscopy (XPS). An investigation of perovskites with x = 0, 0.2, and 0.6 in air at room temperature showed that these samples do not contain oxygen vacancies (y = 0), i.e., they are fully oxidized. Hence, to produce electrical neutrality, these samples should contain iron(4+) cations in an amount proportional to the degree of substitution (x) of strontium(2+) for lanthanum(3+). However, no Fe4+ cations were found in the oxides. All perovskites contain only Fe3+ cations, oxygen ions O2? along with oxygen ions with reduced electron density (O?). These data provid evidence of the possible electron density redistribution from oxygen ions to iron cations. The fact that the oxides contain oxygen ions with reduced electron density suggests that weakly bound lattice oxygen in substituted perovskites is represented by O? ions. 相似文献
12.
Zulhadjri B. Prijamboedi N. Mufti T.T.M. Palstra 《Journal of solid state chemistry》2011,184(5):1318-1323
Doping of manganese (Mn3+/Mn4+) into the Aurivillius phase Pb1−xBi4+xTi4−xMnxO15 was carried out using the molten salt technique for various Mn concentrations (x=0, 0.2, 0.4, 0.6, 0.8, and 1). Single phase samples could be obtained in the composition range with x up to 0.6 as confirmed by X-ray and neutron diffraction analysis. Dielectric measurements show a peak at 801, 803, 813 and 850 K for samples with x=0, 0.2, 0.4, and 0.6, respectively, related to the ferroelectric transition temperature (Tc). The main contribution of the in-plane polarization for x≤0.2 which was calculated from the atomic positions obtained by the structure analysis is the dipole moment in the Ti(1)O6 layer; however, for x≥0.4 the polarization originates from the dipole moment in the Ti(2)O6 layer. Mn doping in the Pb1−xBi4+xTi4−xMnxO15 does not show any long range magnetic ordering. 相似文献
13.
S. Kemmler-Sack 《无机化学与普通化学杂志》1981,483(12):126-139
Photoluminescence of Trivalent Rare Earths in Perovskite Stacking Polytypes Ba2La2?x RE MgW2□O12, Ba6Y2?x RE W3□O18, and Sr8SrGd2?xRE W4□O24 Rhombohedral 12 L stacking polytypes Ba2La2?xREMgW2□O12 show with RE3+ = Pr, Sm, Eu, Tb, Dy, Ho, Er, Tm; the 18 L stacking polytypes Ba6Y2?xREW3□O18 and the polymorphic perovskites Sr8SrGd2?xREW4□O24 with RE3+ = Sm, Eu, Dy, Ho, Er visible photoluminescence. The concentration dependence and the influence of the coordination number of the rare earth are reported. 相似文献
14.
S. Kemmler-Sack 《无机化学与普通化学杂志》1982,489(1):55-66
On Hexagonal Perovskites with Cationic Vacancies. XXXII. Photoluminescence of Trivalent Rare Earth in the Systems Ba2?ySryLa2?xRExMgW2□O12 In the series Ba2?ySryLa2?xRExMgW2□O12 the Ba2+ can be completely substituted by Sr2+. All compounds crystallize in the rhombohedral 12 L-type (space group R3 m; sequence (hhcc)3). By doping the stacking polytypes with some of the trivalent rare earths efficient visible photoluminescence is obtained. The simultaneous incorporation of two different rare earth ions leads to two-color-phosphors, which, according to the excitation energy used, emit either mainly the typical spectrum from one or the other activator; the corresponding luminescence mechanism are discussed. 相似文献
15.
C. Ostos L. Mestres M.L. Martínez-Sarrión J.E. García A. Albareda R. Perez 《Solid State Sciences》2009,11(5):1016-1022
A-site deficient rare-earth doped BaZrxTi1?xO3 (BZT) ceramics were prepared from a soft-chemistry route and by solid-state reaction (SSR). Perovskite-like single-phase diagrams for the BaTiO3–La2/3TiO3–BaZrO3 system were constructed for each method of synthesis. Infrared spectroscopy on (Ba1?yLa2y/3)ZrxTi1?xO3 solid solution revealed a dramatic stress on the M–O (M = Ti, Zr) bonds due to the combined effect of A-site vacancies and the lower ionic radius of La3+ than that of Ba2+. A relationship between the M–O stretching vibration (υ) and the tolerance factor (t) was determined. (Ba1?yLn2y/3)Zr0.09Ti0.91O3 (Ln = La, Pr, Nd) samples synthesized by SSR were selected for detailed studies. X-ray diffraction data were refined by the Rietveld method. Scanning electron microscopy on sintered compacts detected abnormal crystal growth and grain sizes in the range of about 1 μm up to 10 μm when the dopant concentration is 6.7 at. %. Impedance measurements exhibited that ferroelectric to paraelectric phase-transition temperature shifted to lower values as increasing rare-earth content. (Ba1?yLn2y/3)Zr0.09Ti0.91O3 system showed a diffuse phase transition with a relaxor-like ferroelectric behaviour. Furthermore, the dielectric constant was enhanced with respect to non-doped BZT system. 相似文献
16.
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)CrIVx′CrIII(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
17.
Magnetic properties were measured on the cubic perovskite systems SrCoO3?δ, (La1?xSrx)CoO3 (0.5 ≦ x ≦ 1.0), and Sr(Co1?xMnx)O3 (0 ≦ x ≦ 1.0). It is found that S2+ and La3+ ions strongly affect the spin state of the Co2+ ion and that the Mn4+ ion located at the octahedral site affects the spin state of Co4+ ion. The magnetic properties (Tc, Tθ, and σ) are explained by the magnetic interaction Co3+OCo3+, Co3+OCo4+, Co4+OCo4+, Mn4+OMn4+, and Mn4+OCo4+ in these systems. 相似文献
18.
In order to determine the stability of some potential NOx reduction catalysts (La0.8M0.2MnO3, M Na, K, Rb) the accelerated reduction of these catalysts in H2, N2 atmospheres was studied. La0.8K0.2MnO3 goes through a reversible oxygen loss at about 350°C corresponding to the reduction of the available Mn4+ to Mn3+ in H2, N2 atmospheres. By reduction at higher temperatures a previously unreported phase La2MnO4 is formed. The most reducing conditions (10% H2 in N2, >940°C) formed only La2O3 and MnO. Between 700 and 880°C in 10% H2 in N2 potassium was eliminated from the sample by reduction to the metal and evaporation. Analogous results were found for Na and Rb substituted LaMnO3 except that the intermediate phase La2MnO4 was not observed in the reduction of La0.8Rb0.2MnO3. 相似文献
19.
Aamir Iqbal Waidha Humera Khatoon Siddiqui Dr. Yuji Ikeda Dr. Maren Lepple Dr. Sami Vasala Manuel Donzelli Dr. A. D. Fortes Dr. Peter Slater Dr. Blazej Grabowski Dr. Ulrike I. Kramm Dr. Oliver Clemens 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(38):9763-9767
A new vacancy ordered, anion deficient perovskite modification with composition of BaCoO2.67 (Ba3Co3O8□1) has been prepared via a two-step heating process. Combined Rietveld analysis of neutron and X-ray powder diffraction data shows a novel ordering of oxygen vacancies not known before for barium cobaltates. A combination of neutron powder diffraction, magnetic measurements, and density functional theory (DFT) studies confirms G-type antiferromagnetic ordering. From impedance measurements, the electronic conductivity of the order of 10−4 S cm−1 is determined. Remarkably, the bifunctional catalytic activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is found to be comparable to that of Ba0.5Sr0.5Co0.8Fe0.2O3–y, confirming that charge-ordered anion deficient non-cubic perovskites can be highly efficient catalysts. 相似文献
20.
《Solid State Sciences》2012,14(7):782-788
First principles calculations have been performed to study the effects of the La3+ and Mn3+ substitutions in the multiferroic BiFeO3. The real compositions Bi1−xLaxFeO3 and BiFe1−xMnxO3 with x = 0.0, 0.1, 0.2, 0.3 were modeled by substitution of one, two and three Bi3+ or Fe3+ by La3+ or Mn3+ in the orthorhombic BiFeO3 structure, respectively. Density functional theory within the generalized gradient approximation with Hubbard correction of Dudarev (GGA + U) and plane wave pseudo-potential approach has been used to track the changes that occur in the structural parameters, electronic structure, magnetic, optical and polarization properties of the modified BiFeO3. The substitution of one Bi3+ with La3+ increases the band gap energy whereas the augmentation of La3+ substitutes decreases it. The substitutions of Fe3+ with Mn3+ do not change the band gap energy. The calculations predicted larger polarization of the modified BiFeO3, antiferromagnetism for Bi1−xLaxFeO3 and small ferrimagnetism for BiFe1−xMnxO3. Better multiferroic properties are expected for BiFe1−xMnxO3 materials (x = 0.1, 0.2) due to the increasing polarization and ferrimagnetic behavior. The optical properties were estimated by the calculated imaginary and real parts of the dielectric function. The increase of La3+ and Mn3+ substitutes lead to lower absorption intensity at energy range 2–7 eV. 相似文献