Showing papers on "Perovskite (structure) published in 1978"

Magnetic Studies of the Metallic Perovskite-Type Compounds of Manganese

Abstract: In this study of metallic perovskites Mn 3 MX (M=metal, X=C, N) we emphasize the influence of the (outer) electron concentration on the sequences of crystallographic transitions, on the magnetic ordering temperatures and on the constant (Pauli-type like) part of the magnetic susceptibility. A great variety of observed magnetic structures and transitions (including spin rotations) are described for carbides and nitrides. The Labbe-Jardin tight binding model which invokes a p-d II hybridization between N and Mn orbitals having their energy levels near to the Fermi surface provides the possibility of great variations of the density of states versus temperature and is in qualitative agreement with experiment. It also explains that small substitutions on X have a more drastical effect than those can M.

Structure and crystal chemistry of perovskite-type MgSiO 3

Abstract: Synthetic clinoenstatite (MgSiO3) has been converted to a single phase with the perovskite structure in complete reactions at approx. 300 kbar in experiments that utilize the laser-heated diamond-anvil pressure apparatus. The structure of this phase after quenching was determined by powder X-ray diffraction intensity measurement to be similar to that of the distorted rare-earth, orthoferrite-type, orthorhombic perovskites, but it is suggested that such distortion from ideal cubic perovskite would diminish at high pressure. The unit cell dimensions and density of perovskite-type MgSiO3 at ambient conditions (1 bar, 25°C) are a=4.780(1) A, b=4.933(1) A, c=6.902(1) A, V=162.75 A3, and ρ=4.098(1) g/cm3. This phase is 3.1% denser than the isochemical oxide mixture [periclase (MgO)+stishovite (SiO2)]. The small crystal-field stabilization energy of the cation site in the perovskite structure may play an important role in limiting the high-pressure stability field of perovskites that contain transition metal cations. Approximate calculations of the crystal-field effects indicate that perovskite of pure FeSiO3 composition may become stable at 400–600 kbar; pressures greater than 800 kbar would be required to stabilize CoSiO3 or NiSiO3 perovskite.

Diffuse phase transition in lead zinc niobate

Abstract: The behavior of the diffuse phase transition in a Pb(Zn1/3Nb2/3)O3 crystal of the disordered perovskite type was investigated. The electrooptic effect, spontaneous polarization, dielectric constant and thermal expansion were measured carefully as a function of temperature in the region of the phase transition. Using the technique suggested by Rolov, statistical treatments based on a Gaussian distribution function for the local Curie temperature were applied in interpreting the temperature dependence of various physical properties of Pb(Zn1/3Nb2/3)O3. Good agreement was obtained between the calculated and experimental results.

Magnetic Properties of New Cubic-Perovskite Mixtures; (La 0.5 Na 0.5 )RuO 3 and (Ca 1-x A x )RuO 3 (A=Mg, Sr)

Abstract: To clarify the nature of the ferromagnetic interactions in SrRuO 3 , the Ru 4+ – –Ru 4+ exchange couplings in cubic-perovskite type ARu 4+ O 3 (A=nonmagnetic cation) were investigated Magnetic measurements were done on these sample oxides (including single crystals of CaRuO 3 ) by using a magnetic balance technique In contrast to the ferromagnetic behaviors in (Ca 1- x Sr x )RuO 3 ( x ≧04), both the (La 05 Na 05 )RuO 3 metallic and (Ca 1- x Mg x )RuO 3 ( x =01, 03 and 05) semiconductive systems were found to be Curie-Weiss paramagnets having negative asymptotic Curie points On the (Ca 08 Sr 08 )RuO 3 sample, an anomalous hump of χ vs T curve was observed, which could be interpreted as being due to ferromagnetic clusters of Sr 2+ It was suggested that, besides the 180° superexchange coupling, another indirect-exchange interaction via Sr 2+ would be indispensable for the ferromagnetism in SrRuO 3

Electrical properties of lead‐zirconate–lead‐titanate ferroelectric thin films and their composition analysis by Auger electron spectroscopy

Abstract: Lead‐zirconate–lead‐titanate (PZT) thin films with perovskite and pyrochlore structures were successfully fabricated on several kinds of substrates by rf diode sputtering. Semiquantitative compositional analysis of the deposited thin films were made with Auger electron spectroscopy (AES). The films were deposited from targets consisting of mixed powder oxides. One target had a composition corresponding to 10 mol% PbO‐modified PZT 52/48 (Zr to Ti ratio in the PZT, target A), while the other had the composition corresponding to 10 mol% PbO‐modified PZT 42/58 (target B). The films in the perovskite structure had ferroelectricity, while the films in pyrochlore had none. The perovskite films deposited from target A had a dielectric constant of 751, a remanent polarization of 20.4 μC/cm2, and a coercive field of 23.3 kV/cm. The films deposited from target B had a dielectric constant of 654, a remanent polarization of 6.37 μC/cm2, and a coercive field of 12.2 kV/cm. The composition of the former film was PZT 60/...

Mixed-oxide and perovskite-structure model mantles from 700-1200 km

Abstract: Summary. The mantle between 700 and 1200 km is modelled using mixed oxides (MgO + FeO + SiO2 (stishovite)) having olivine, pyroxene, and peridotite (67 per cent ol-33 per cent px) stoichiometries and third-order finite strain theory. It is possible to satisfy the densities, compressional and shear velocities, and bulk and shear moduli of seismic models B1 and PEM and to obtain adiabatic temperature profiles consistent with literature profiles with olivine and peridotite stoichiometries, but not with pyroxenes. Estimates of K and μ for stishovite satisfying Bl and PEM profiles indicate that the stishovite data of Mizutani, Hamano & Akimoto and Liebermann, Ringwood & Major are inconsistent with the assumed mixed-oxide assemblages, even with a large range of pressure and temperature derivatives for K and μ. Assuming perovskite structure materials below 700 km shows that pyroxenes transformed to the perovskite structure are too dense by 1–2.5 per cent. Taking a wide range of P and T derivatives for the K and μ of MgSiO3 (perovskite) indicates that the seismic models can be satisfied with olivines and peridotites transformed to perovskites and oxides. The MgSiO3 (perovskite) K and μ estimates bracket the systematics estimates of Liebermann, Jones & Ring-wood. We find only marginal evidence for relaxation of the shear modulus. The mean atomic weight calculated for the mixed-oxide models agrees with the results of Watt, Shankland & Mao for the same region and with previous estimates for the upper mantle. Thus, an increase in iron content below 700 km appears unlikely. This result and the corresponding temperatures (lower than the Clark & Ringwood conduction geotherm) are consistent with convection below 700 km.

Polymer compositions for electrical use

Abstract: Novel polymeric compositions having non-linear resistance characteristics, together with electrical equipment comprising such compositions. Any polymer may be used; the polymer contains substantial proportions of certain particulate fillers of a given structure, for example the perovskite and spinel structures.

The Reduction of Oxygen on Teflon‐Bonded Perovskite Oxide Electrodes

Abstract: The electrochemical reduction of oxygen on Teflon‐bonded electrodes in 45% was studied as a function of temperature and oxygen partial pressure. The activation energy was 10.75 kcal/mole and the relationship between and obeys the relationship , suggesting that the rate of oxygen chemisorption is the rate‐controlling step, Galvanostatic oxygen stripping showed that the surface coverage of oxygen was only 1% for both Sr‐doped and , and that the coverage was independent of temperature in the range 25°–80°C.

Barium- and ruthenium-containing perovskite catalysts

Abstract: Catalytic compositions having the general formula, ABO 3 , and a perovskite crystal structure, wherein the A sites are substantially fully occupied by barium ions. About 1% to 20% of the Type B cation sites are occupied by ions of at least one platinum group metal selected from ruthenium, osmium, rhodium, iridium, palladium and platinum, with ruthenium being present and occupying up to about 10% of the Type B cation sites. The balance of 80% to 99% of the Type B cation sites are occupied by ions of at least one metal having an atomic radius between 0.4 to 1.4 Angstroms. The compositions are useful in gaseous oxidation and reduction reactions.

Ruthenium-containing perovskite catalysts

Abstract: Catalytic compositions having the general formula, ABO 3 , and a perovskite crystal structure, wherein the A site cations consist essentially of ions of a metal having an atomic number of 11 to 51, 57 to 71 or 89 to 103. About 1% to 20% of the Type B cation sites are occupied by ions of at least one platinum group metal selected from ruthenium, osmium, rhodium, iridium, palladium and platinum, with ruthenium being present and occupying up to about 10% of the Type B cation sites. The balance of 80% to 99% of the Type B cation sites are occupied by ions of at least one metal having an atomic radius between 0.4 to 1.4 Angstroms. The compositions are useful in gaseous oxidation and reduction reactions.

Bi2TiO4F2–-a new one-layer perovskite-like ferroelectric

Abstract: Dielectric investigations of the one-layer perovskite-like crystal Bi2TiO4F2 were carried out. It was shown that this material is ferroelectric with the Curie temperature Tc =11°C ± 0.5°C. The ferroelectric phase transition is accompanied by the change of unit cell symmetry C 20 2ν-Imm2 to D 4h-I4/mmm.

Photoelectric phenomena in single crystals of some perovskite oxides

Abstract: Photoconductivity (PQ and thermally stimulated conductivity (TSC) were investigated on some single-crystal perovskite oxides. Using PC spectral dependence, band gap and impurity-level depths were found. The TSC method gave further information about the energy spectrum of local levels. Due to a significant PC and deep local levels, a photoelectret state was achieved in crystals of some compounds. Illumination of lead titanate crystals by light from the intrinsic photoconductivity region was found to induce screening of spontaneous polarization by non-equilibrium carriers.

Luminescence of Rare Earth‐Activated Tellurate Perovskites

Abstract: , where and , crystallize with ordered (Fm3m) or ordered‐distorted perovskite structures, and can be used as hosts for rare earth activated luminescence. Rare earths (Ln3+) were introduced according to the substitution, . The Eu3+ emission depends strongly on the ordering of the perovskite structure. The transition is dominant when Eu3+ occupies a site with symmetry in the Fm3m ordered perovskite, whereas the Eu3+ emission is the transition in the ordered‐distorted perovskite structure. Only is an effective host for Tb3+‐, Dy3+‐, and Er3+‐activated luminescence.

Determination of the Crystal Structure of Perovskites with the Space Group I4/mcm

Abstract: A method is proposed for determining volume ratios of domains in a poly-domain specimen of perovskite crystals with the space group I 4/ m c m ; the ratios are fundamental parameters in determining the crystal structure. The method is based on measuring relative intensities of triplets of Bragg reflexions with special indices and is applied to structure determinations of SrTiO 3 and KMnF 3 . A discussion is given of refinement of the structure; a difficulty in a least squares calculation owing to a correlation between structural parameters is emphasized. The use of accurate lattice-constant data together with a conventional structure analysis is recommended.

Synthesis, crystal structure, and mössbauer study of a series of perovskite-like compounds [ACu3] (M, Fe)4O12

Abstract: A series of cubic perovskite-like compounds with the general formula [ACu3](M, Fe)4O12 is synthesized. The A-sites are occupied by large cations such as Th4+ or rare earths R3+. The B-sites are statistically occupied by Fe3+ cations and by tetravalent M4+ cations such as Mn4+ or Ti4+ The structure of [NdCu3](Ti3Fe)O12 (space group Im3) is refined from room temperature neutron diffraction data. Mossbauer spectroscopy data performed on [NdCu3](Ti3Fe)O12 and on [ThCu3] · (Mn2Fe2)O12 show that a magnetic order takes place below 14 K for the first compound and below 200 K for the second one. Es wurde eine Reihe von kubischen perovskit-ahnlichen Verbindungen mit der allgemeinen Formel [ACu3](M, Fe)4O12 hergestellt. Die A-Platze werden von grosen Kationen wie Th4+ oder Seltenen Erden R3+ besetzt. Die B-Platze werden statistisch besetzt durch Fe3+-Kationen und durch tetravalente M4+-Kationen wie Mn4+ oder Ti4+. Die Struktur von [NdCu3](Ti3Fe)O12 (Raumgruppe Im3) wird aus Neutronenbeugungswerten bei Zimmertemperatur gewonnen. Mosbauer-spektroskopiewerte an [NdCu3](Ti3Fe)O12 und an [ThCu3](Mn2Fe2)O12 zeigen, das eine magnetische Ordnung unterhalb 14 K fur die erste Verbindung und unterhalb 200 K fur die zweite Verbindung stattfindet.

Point defects and electrical properties of ferroelectrics

Abstract: A classification of the electrophysical properties of perovskite ferroelectrics is suggested. A group of representative oxides is selected, for which a number of physical properties are analysed. The proposed scheme of local energy levels is useful for explaining the type of conduction in the three characteristic sub-groups of perovskite oxides. Point defects are shown to play an important part in the origin of dielectric and semiconducting behavior of ferroelectrics.