Showing papers on "Perovskite (structure) published in 1985"

Oxygen permeation through perovskite-type oxides

Abstract: The rate of oxygen permeation through La1−xSrxCo1−yFeyO3−δ was found to increase with an increase in Sr or Co content, showing that the permeability was mainly controlled by the amount of oxygen vacancies. The results obtained indicate that mixed conductive perovskite-type oxides are promising materials for oxygen permeation at elevated temperatures.

Physicochemical properties of rare earth perovskite oxides used as gas sensor material

Abstract: The change of the conductivity in the rare earth perovskite oxide took place after the chemisorption of flammable gases. The sensitivity for methanol was highest. From the conductivity change of these perovskite oxides after the injection of methanol, the energy needed to promote an electron from a conducting to a nonconducting state, ΔE=E c-E t, could be derived from the equationσ=A σ 0 exp (−ΔE/kT). LnCoO3 had the smallest ΔE and ΔH (metal-O), which is the binding energy of oxygen coordinating to the metal ions, but exhibited the highest activity for gas sensing. The gas sensing mechanism was also considered.

Garnet-perovskite transformation in CaGeO3: In-situ X-ray measurements using synchrotron radiation

Abstract: The garnet-perovskite phase transformation in CaGeO3 was investigated in the pressure-temperature region to 6.5 GPa and 1200°C using a cubic anvil type of high-pressure apparatus combined with synchrotron radiation. In-situ measurements with an energy dispersive x-ray diffraction system enable us to carry out dynamical observation of the transformation. The equilibrium phase boundary between the garnet and perovskite phases was determined as P(GPa)= 6.9 - 0.0008T(°C). The negative P-T slope definitely established in the present study is in reasonable agreement with the value, −0.0023(8) GPa/°C, that was calculated from the thermochemical data on the enthalpy of transition. The molar volume change accompanied with this transformation was estimated to be about 13% at about 6 GPa and 1000°C.

OXYGEN-SORPTIVE PROPERTIES OF DEFECT PEROVSKITE-TYPE La1−xSrxCo1−yFeyO3−δ

Abstract: The substitution of Fe for Co of La1−xSrxCoO3−δ makes it possible to preserve perovskite-type structure at higher Sr contents. While the amount of oxygen reversibly absorbed or desorbed was mostly determined by the Sr content, the Fe substitution was found to enhance the oxygen desorption particularly in a lower temperature region as well as catalytic activities.

Enhancement of humidity sensitivity for Perovskite-type oxides having semiconductivity.

Abstract: The humidity sensitivities of SrSnO3 and BaSnO3 were enhanced slightly By means of calcining their hydroxides, which were prepared by a coprecipitation method. The partial substitution of the A site element of the perovskite-type oxides with lanthanum was more useful for the enhancement of the sensitivity. The highest sensitivity was found in Sr0.95La0.05SnO3 among the specimens studied.

Organic-inorganic layer compounds: physical properties and chemical reactions

Abstract: In contrast with intercalation compounds, which can exist both with and without organic molecules between the planes of inorganic material, ‘molecular composite’ compounds have organic groups covalently or ionically bound to inorganic layers. In such crystals the aim is to combine magnetic or optical properties characteristic of the inorganic solid state, like magnetism and luminescence, with properties found in the organic solid state like mesomorphism or polymerization. This contribution surveys experiments from our laboratory on the structural, physical and chemical properties of one series of ‘molecular composites’, the layer perovskite halide salts (RNH 3 ) 2 MX 4 (R = organic group; M = Cr, Mn, Cd; X = Cl, Br). When R is an n -alkyl group these compounds undergo structural phase transitions triggered by variations in the hydrogen-bonding of the −NH 3 to the inorganic layer. Optical microscopy is used to follow the phase transitions and map ferroelastic domains. Compounds with M = Cr are ferromagnets with strongly temperature-dependent visible absorption spectra, whose intensity correlates with the degree of magnetic order. In Mn and Cd salts more elaborate functional groups can be inserted in the organic sidechains with the aim of making topochemical polymerizations in the solid state.

Lattice dynamics and structural distortions of CaSiO3 and MgSiO3 perovskites

Abstract: The phonon spectra, distortion mechanisms, and thermoelastic properties of CaSiO3and MgSiO3 in the cubic perovskite structure are investigated as a function of pressure using a lattice dynamic approach. The bonding forces are derived from a parameter-free rigid-ion electron-gas formulation. At low pressures, CaSiO3 is found to be dynamically stable in the cubic perovskite structure; however, the phonon spectrum exhibits soft modes at the Brillouin zone boundary which ultimately result in a dynamic instability of the lattice near 80 GPa. The computed phonon spectrum of cubic MgSiO3 perovskite exhibits complex frequencies along parts of the zone boundary at all densities investigated. These vibrational instabilities include coupled octahedral rotations which produce the observed distorted structure of MgSiO3 perovskite. The measured bulk modulus of MgSiO3 perovskite compares well with our calculated value for the cubic structure.

Ab initio structural and thermoelastic properties of orthorhombic MgSiO3 perovskite

Abstract: We present results of an ab initio study of the structural and thermoelastic properties of orthorhombic (Pbnm) MgSiO3 perovskite as a function of temperature and pressure. Self-consistent free energies are computed using quasiharmonic lattice dynamics with interatomic potentials derived from an electron-gas formulation. At high temperatures the orthorhombic phase undergoes successive second-order transitions to tetragonal and cubic phases. The transition temperatures increase with pressure such that the orthorhombic phase is stable throughout most of the lower mantle although the higher symmetry phases could occur near the top of the lower mantle. At zero pressure the calculated bulk modulus and thermal expansion are in excellent agreement with the available data. We find that for high-temperature isotherms, a finite-strain decompression of the high-pressure equation of state of perovskite substantially overestimates its zero-pressure density and bulk modulus; hence, constraints on the chemical composition of the lower mantle, based on a comparison of the decompressed seismological properties with zero-pressure laboratory data may significantly overestimate the proportion of perovskite in the lower mantle.

Perovskite-type oxides having semiconductivity as oxygen sensors.

Abstract: Among the perovskite-type oxides examined, SrTiO3 showed a high sensitivity to oxygen in the “lean-burn” region. The resistivity characteristics of the specimen were also investigated in the exhaust gas of propane-oxygen combustion. SrSnO3 is promising for a combustion monitoring sensor.

Magnetic exchange interactions in perovskite solid solutions. Part 5. The unusual defect structure of SrFeO3 –y

Abstract: Mesures d'effet Mossbauer 57 Fe, de diffraction RX et de susceptibilite magnetique, entre 4,2 et 900 K, sur SrFeO 3−y pour 0,15

Method of preparation of perovskite-type compounds

Abstract: The present invention relates to a low temperature method of preparing a compound of the formula: ABO.sub.3 wherein A=Ba, Sr, Ca and Pb; and B=Ti, Zr and Hf, by (a) reacting a B-alkoxide, with a predetermined amount of aqueous A-hydroxide, (b) heating the reaction mixture to an initial temperature of 100° C.-250° C. at 1-40 atmospheres of pressure; (c) cooling the reaction mixture and thereafter, heating it to a subsequent temperature of approximately 40° C. to 60° C.; (d) dialyzing the cooled reaction product and recovering the tenate.

Fabrication and characterisation of ferroelectric perovskite lead indium niobate

Abstract: Fabrication techniques are investigated with a view to producing single phase perovskite lead indium niobate (PbIn1/2Nb1/2O3). The physical properties of ceramics produced using the various techniques are investigated. The dielectric relaxation of lead indium niobate is also characterised.

Dielectric Properties of Modified Lead Zinc Niobate Ceramic

Abstract: Crystal structures, dielectric properties and electrical resistivities for partially substituted lead zinc niobate [Pb(Zn1/3Nb2/3)O3] ceramic were investigated. This ceramic, sintered at 1000–1050°C, was composed mainly of perovskite phase and showed a high dielectric constant (3500–5000) and a high resistivity (approximately 1013Ωcm) when partial substitutions of the Pb site by Ba or Sr and the (Zn1/3Nb2/3) site by Ti in the Pb(Zn1/3Nb2/3)O3 were carried out. The reason why the perovskite phase was stable in the sintered body is explained qualitatively by introducing the concepts of an electronegativity difference and a tolerance factor.

Oxygen Vacancy Ordering In Superlattices Of The 2 Novel Oxides,La2ni2o5 And La2co2o5, Prepared By Low-Temperature Reduction Of The Parent Perovskites

Abstract: Controlled reduction of LaBO3(B = Co or Ni) perovskites gives rise to ordered structures of composition La2B2O5, belonging to the homologous series LanBnO3n–1, which retain the original features of the perovskite; the co-ordination of the transition metal ions differ, with Ni2+ ions showing octahedral and square-planar co-ordination and the Co2+ ions showing octahedral and tetrahedral co-ordination.

The Al2O3-Nd2O3 Phase Diagram

Abstract: A tentative phase diagram for the system Al203-Nd2O3 is presented. Three compounds were obtained: a β-A12O3-type compound, the perovskite NdAlO3, and Nd4Al2O9. The perovskite melts congruently (mp 2090°C), and the two other compounds exhibit incongruent melting behavior: β-Nd/Al2O3, mp 1900°C; Nd4Al2O9, mp 1905°C. Two eutectics exist with the following compositions and melting points: 80 mol% Al2O3, 1750°C; 23 mol% Al2O3,1800°C. Nd4Al2O9 decomposes in the solid state at 1780°C.

Electrical Properties of Perovskite Type Oxide Thin-Films Prepared by RF Sputtering

Abstract: Oxide thin-films with perovskite structure are prepared by rf magnetron sputtering in argon-oxygen mixed gas at substrate temperature below 500°C using the Sr(Zr, Ti)O3 and Ba(Sn, Ti)O3 solid solution ceramic targets. The dielectric properties and the microstructure of the sputtered thin-films depend on the substrate temperature, oxygen partial pressure, and target compositions. The dielectric constant and the electrical breakdown strength of the thin-films sputtered by using Sr(Zr0.2Ti0.8)O3 and Ba(Sn0.2Ti0.8)O3 targets is about 100 and 3 MV/cm, respectively. These thin-films with the high dielectric constant and the high electrical breakdown strength are useful for the dielectic layer of the ac TFEL panel.

Superconductivity in uranium compounds with Cu3Au structure

Abstract: PHYSICAL REVIEW VOLUME 31, NUMBER 3 Superconductivity in uranium H. R. Ott, Laboratorium fiir Festkorperphysik, compounds F. Hulliger, Eidgenossische FEBRUARY 1985 with Cu3Au structure and H. Rudigier Technische Hochschule, Honggerberg, 8093 Zurich, Switzerland Z. Fisk Los A/amos National Lab oratory, Los Alamos, New Mexico 87545 (Received 5 September 1984) Superconductivity has been observed in uranium compounds with partly disordered Cu3Au-type U distances of more than 4 A. Low-temperature specific-heat crystal structure and interatomic U — experiments reveal no anomalous enhancement of the electronic specific heat thus distinguishing the present materials from the unconventional superconductors UBe» and UPt3. Until very recently, only a few uranium compounds were known to be superconducting at low temperatures, namely, U6X compounds with X =Mn, Fe, Co, and Ni, UCo, ' and U2PtC2. For elemental uraniu, supercon- ductivity is claimed for its y (Ref. 3) and its a modifica- tion, but the latter under external pressure only. In these materials superconductivity was ascribed to the formation of f-electron-derived energy bands due to direct 5f function overlap between adjacent nearest-neighbor U atoms which are less than about 3. 6 A apart. Newer work on uranium intermetallics has shown that is also possible when the U atoms are superconductivity farther apart than 4 A. So far, the most extreme example in this respect is UBe&3, where the nearest-neighbor U U distance is 5. 13 A. In UPt3 (Ref. 7) this distance is 4. 13 A, considerably larger than that found in the sub- stances mentioned above. In these latter two cases, how- ever, superconductivity out of a rather develops normal state and involves electrons with anomalous unusually heavy effective masses, giving rise to enormous electronic specific heats at low temperatures. For both UBe» (Refs. 8 and 9) and UPt3 (Ref. 9) it has been reasoned that superconductivity is not primarily due to conventional electron-phonon interaction, and therefore the superconducting state may be characterized by i&0 paired electrons as opposed to the conventional l =0 BCS where / is the electron-pair angular- superconductors, momentum quantum number. With these observations in mind, it seemed of interest to check whether the mentioned interatomic U — U dis- tance is a crucial parameter in determining the low- temperature behavior of metallic uranium compounds and we also simply aimed at finding additional superconduct- In the course of this mark we ing U compounds. discovered superconductivity in U compounds crystalliz- ing in the cubic Cu3Au structure. Superconductivity was probed by measuring the magnetic susceptibility between 0. 02 and 4 K, using a low-frequency ac mutual- inductance system with a primary-field amplitude of less than 0. 1 Ge. For some of the compounds the magnetic susceptibility was also measured between 1. 5 and 300 K, using a sample-moving magnetometer. In order to obtain values for the electronic specific-heat parameter y, we also measured the temperature dependence of the specific heat between 1. 5 and 12 K for two typical compounds. For further characterization we also measured the electri- cal resistivity p of three compounds between 1. 5 and 300 K. The samples were synthesized by arc melting under argon. The purity of the starting elements Al, Ga, Si, and Ge were 99.99% or better, that of Ru and Rh was 99.9% (Johnson-Matthey Co. , London), and that of U was 99.7% with carbon as the main impurity. Of the ma- terials studied (see Table I), UA12Si2 and UA12Ge2 had been reported by Zygmunt' to crystallize with primitive cubic cells and a =4. 145(2) and 4. 217(5) A, respectively. From the experimental density we derive one formula unit per unit cell. This leads to PI3m as the probable space group, i. e. , a partly disordered perovskite or a Cu3Au structure. The former structure, however, would require a much higher density. Thus we are left with the possibility of partially disordered Cu3Au structures as met in ThPd~ and UPd4. . In the case of UA12Si2 the Guinier photo- graphs indicated a slight variation of the lattice constant (see Table I). All UA12Si2 powder patterns contained some foreign lines, though no lines from unreacted Si. The Guinier pattern of UAlzGe2 revealed traces of free Ge in addition to the pure Cu3Au-type phase. The same holds for the UGa2Ge2 sample which surprisingly had al- most the same lattice constant as the corresponding Al compound. Based on the lattice constants ap of the Cu3Au end members UX3(X=A1, Ga, Si, and Ge) and assuming the validity of Vegard's rule, we would expect the observed lattice constants at the following compositions (with the la positions (0,0,0) fully occupied by U atoms only, and the X' atoms statistically distributed on the 3c po- sitions ( —, , —, 0; —, 0, —, ';0, —, —, ' ): UA1& 6&Sit 32 (4. 145 A), UAli 47Sii 53 (4. 163 A), UA12 5Geo s (4. 219 A), and UGa2 iGeo 9 (4.218 A). In the case of the U-Al-Si com- pound one might accept this interpretation, but not for the U-Al-Ge and U-Ga-Ge phases. Since according to the x-ray photographs these latter phases only contained Ge as a second phase we measured their density 5 with a pycnometer. For UA12Ge2 we obtained 5 = 7. 73(6) g/cm . Assuming the sample to consist of a mixed purified wave- The American Physical Society

Notiz über einen Perowskit der Zusammensetzung BaInO2,5

Abstract: Feststoffreaktionen fuhrten zu Einkristallen von BaInO2,5. Vierkreisdiffraktometermessungen ergaben eine Perowskitstruktur mit a = 4,219(2) A, (Raumgruppe O–Pm3m, Z = 1). BaInO2,5 ist ein Beispiel fur den seltenen Fall, das ein dreiwertiges Metallion mit Erdalkalimetallionen einen defekten Perowskit bildet. A Non-stoichiometric Perovskite: BaInO2.5 Single crystals of BaInO2.5 were prepared by solid state reactions. Out of X-ray diffraction measurements we found a perovskite structure (a = 4.219(2) A space group Oh1–Pm3m, Z = 1). BaInO2.5 is a rare example of a defect perovskite compound with a pure M3+ metal ion and an alkaline earth element.

Preparation and crystallographic characteristics of the new phase La2Au0.5Li0.5O4

Abstract: The chemical and crystallographic characteristics of a new phase in the La-Au-Li-O system, La2Au0.5Lio0.5O4, have been studied. This compound was obtained by heating a mechanical mixture of metallic Au, La2O3 and Li2O in air at 750 °C. The X-ray powder-diffraction patterns show, in addition to the typical reflections of a K2NiF4-type structure, other lines that can only be indexed with a pseudotetragonal supercell. Li and Au ordering in the perovskite layers implies a transition from the tetragonal space group I4/mmm to the orthorhombic space group Cmmm with the unit cell a = 0.5767(1), b = 1.2472(2), c = 0.5767(1) nm. The structure was refined to an R value of 0.081.