Showing papers on "Perovskite (structure) published in 1986"
IBM1
TL;DR: In this paper, Ba−La−Cu−O system, with the composition BaxLa5−xCu5O5(3−y) have been prepared in polycrystalline form, and samples with x=1 and 0.75,y>0, annealed below 900°C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound.
Abstract: Metallic, oxygen-deficient compounds in the Ba−La−Cu−O system, with the composition BaxLa5−xCu5O5(3−y) have been prepared in polycrystalline form. Samples withx=1 and 0.75,y>0, annealed below 900°C under reducing conditions, consist of three phases, one of them a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity, then an approximately logarithmic increase, interpreted as a beginning of localization. Finally an abrupt decrease by up to three orders of magnitude occurs, reminiscent of the onset of percolative superconductivity. The highest onset temperature is observed in the 30 K range. It is markedly reduced by high current densities. Thus, it results partially from the percolative nature, bute possibly also from 2D superconducting fluctuations of double perovskite layers of one of the phases present.
10,272 citations
TL;DR: In this paper, catalytic oxidation of methane was carried out over various perovskite-type oxides and compared with Pt/alumina catalyst at a conversion level below 80%.
487 citations
TL;DR: In this article, the 57Fe Mossbauer effect was applied to the SrFeOx system with 2.5 ≤ x ≤ 3.0 and the results indicated the ideal series of SrFeO 3− 1 n, where n = ∞, 8, 4, and 2 give x = 3, 2.75, 2, and 1.50, correspondingly.
400 citations
TL;DR: The thermal expansion coefficient of perovskite provides an important constraint on possible compositional models for the lower mantle as discussed by the authors, which suggests that the upper and lower mantle are chemically distinct.
Abstract: A pressure of 24 GPa and temperatures of about 2,000–3,000 K correspond to the observed 670-km seismic discontinuity which separates the upper and lower mantle. In such conditions the major minerals of the Earth's upper mantle, olivine ((Mg, Fe)2SiO4), pyroxene ((Mg, Fe)SiO3) and garnet ((Mg, Fe, Ca)3Al2Si3O12) transform to a distorted (orthorhombic) perovskite-structured mineral ((Mg, Fe)SiO3), or to a perovskite-dominated assemblage (refs 1–4). Because silicate perovskite is stable to at least 70 GPa, it is thought to be the most abundant mineral in the lower mantle and, possibly, in the entire Earth. Despite its importance, silicate perovskite was only discovered in 1976, and little is known about its physical properties because of the difficulty in achieving the conditions of pressure and temperature required for the synthesis of this phase. For example, the value of the thermal expansion coefficient of perovskite provides an important constraint on possible compositional models for the lower mantle5–7. We have recently produced sufficient amounts of (Mg0.9, Fe0.1)SiO3 perovskite to measure its zero-pressure thermal expansion to 840 K by X-ray diffraction. At high temperatures, the average thermal expansion coefficient is 4×l0−5K−1. Such a large value for the thermal expansion coefficient implies that standard models of upper mantle composition, such as pyrolite or garnet peridotite (Mg value ≍ 0.89), yield zero-pressure densities that are about 2% lower than that of the density of the lower mantle extrapolated to zero pressure conditions. This result suggests that the upper and lower mantle are chemically distinct, in accord with layered models of the thermal and convective state of the mantle.
224 citations
145 citations
TL;DR: The unit cell of HCa 2 Nb 3 O 10, as determined from powder X-ray diffraction, is tetragonal with a = 3.851 A and c = 14.38 A and contains one formula unit as mentioned in this paper.
Abstract: The alkali cations in the layered perovskites MCa 2 Nb 3 O 10 (M ≡ K, Rb, Cs) are exchanged by protons in aqueous acid in a topochemical reaction to give HCa 2 Nb 3 O 10 . The unit cell of HCa 2 Nb 3 O 10 , as determined from powder X-ray diffraction, is tetragonal with a = 3.851 A and c = 14.38 A and contains one formula unit. The anhydrous hydrogen compound hydrates to form a composition HCa 2 Nb 3 O 10 ·1.5H 2 O which is also tetragonal with a = 3.854 A and c = 16.23 A . The hydration-dehydration behavior at 24 °C as a function of the partial water pressure shows significant hysteresis as a result of the expansion and contraction of the unit cell along the c axis.
136 citations
TL;DR: In this paper, the properties of structurally ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases.
Abstract: Ordering behaviour and the factors which influence the ordering have been studied in both single crystals and hot-pressed ceramics of Pb(Sc1/2Ta1/2)O3 using TEM techniques, including low-temperature microscopy. The characteristics of the structurally-ordered domains are described and also the observation by electron diffraction of new superlattice reflections and diffuse scattering in both the paraelectric and ferroelectric phases. The superlattice reflections are interpreted in terms of Pb2+ cation displacements. The non-interaction of ordered domains and ferroelectric domains is explained within this model.
128 citations
TL;DR: The intercalation-deintercalation process of weak interaction organic molecules such as 1-chloronaphthalene and o-dichlorobenzene into highly oriented polycrystalline (C10H21NH3)2CdCl4 films and he...
Abstract: The intercalation-deintercalation process of weak interaction organic molecules such as 1-chloronaphthalene and o-dichlorobenzene into highly oriented polycrystalline (C10H21NH3)2CdCl4 films and he...
74 citations
TL;DR: In this article, a color center associated with Pb 2+ as impurity in the cubic perovskite KMgF 3 was found to be tunable from 855 to 965 nm.
67 citations
TL;DR: In this paper, a single-crystal X-ray diffraction method was used to study the crystal structure of a perovskite-derivatives constructed from Fe 2 O 3 -and Fe 2 Fe 6 O 5 -slabs perpendicular to the b-axis.
63 citations
TL;DR: In this paper, the pyrochlore phase was calculated to be Pb/sub 2.25/Mg/sub 0.79/O/sub 7.27/Nb/ sub 1.
Abstract: Lead-magnesium niobate ceramics were processed with excess MgO and by precalcining MgO and Nb/sub 2/O/sub 5/. Transmission electron microscopy revealed submicrometer MgO inclusions in the perovskite phase. The pyrochlore phase exists as submicrometer rectangular crystals in an amorphous PbO matrix. The composition of the pyrochlore phase was calculated to be Pb/sub 2.25/Mg/sub 0.27/Nb/sub 1.79/O/sub 7/.
TL;DR: The Mott theory, applied to a single-configuration-coordinate, linear-coupling model with parameters inferred from emission spectra, fails by 20 orders of magnitude to explain the observed radiationless-transition rate.
Abstract: The temperature dependence of chromium photoluminescence spectra and lifetimes have been measured in ordered perovskite (${\mathrm{Cs}}_{2}$${\mathrm{NaYCl}}_{6}$, ${\mathrm{K}}_{2}$${\mathrm{NaScF}}_{6}$, and ${\mathrm{K}}_{2}$${\mathrm{NaGaF}}_{6}$) host crystals. Analysis of lifetime data yields thermal-quenching activation energies \ensuremath{\Delta}E of 4250, 7270, and 9240 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The Mott theory, applied to a single-configuration-coordinate, linear-coupling model with parameters inferred from emission spectra, fails by 20 orders of magnitude to explain the observed radiationless-transition rate.
Journal Article•
TL;DR: In this article, the dielectric properties of PbMg/sub 1/3/Nb/sub 2/3//O/sup 3/-type ceramics can be improved both by using a preferred sintering cycle, which prevents the formation of a pyrochlore layer at the surface of the pellets, and by adding the correct amount of excess PbO.
Abstract: Dielectric properties of PbMg/sub 1/3//Nb/sub 2/3//O/sup 3/-type ceramics can be improved both by using a preferred sintering cycle, which prevents the formation of a pyrochlore layer at the surface of the pellets, and by adding the correct amount of excess PbO, which results in dense ceramics with a nearly pure perovskite phase at a low sintering temperature (900/sup 0/C).
TL;DR: In this article, it was shown that the low temperature phase of Sr 2 Co 2 O 5 has a rhombohedral structure (R), being the stable phase at room temperature of this strontium cobalite oxide.
TL;DR: In this article, a series of perovskite-type oxides were measured by temperature programmed reduction (TPR) technique and the relation among Tmax in TPR, catalytic activity and surface oxygen binding energy was discussed.
Abstract: Reducibility of a series of RECoO3 (RE=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy) perovskite-type oxides were measured by temperature programmed reduction (TPR) technique. Catalytic activity for CO oxidation of them were investigated. The relation among Tmax in TPR, catalytic activity and surface oxygen binding energy is discussed.
Journal Article•
TL;DR: In this paper, the authors decrit de two techniques de preparation de poudres de solutions solides de niobate de plomb et zinc and de titanate of plomb.
Abstract: On decrit deux techniques de preparation de poudres de solutions solides de niobate de plomb et zinc et de titanate de plomb. On etudie la cinetique de transformation de phase perovskite→pyrochlore. Ces poudres sont d'excellentes matieres de charge pour des composites a proprietes piezoelectriques
TL;DR: In this paper, a single crystal of the orthorhombic perovskite modification of MgSiO3 was grown from a melt at 27 GPa and 1830°C.
Abstract: Single crystal of the orthorhombic perovskite modification of MgSiO3 have been grown from a melt at 27 GPa and 1830°C. The starting material of orthoenstatite was sealed in a platinum capsule with water. Crystals of good quality up to 200 microns in size were obtained, which are promising for the structural refinement, Brillouin spectroscopy, and other measurements. Some crystals, however, exihibit twinning on the (110) plane.
TL;DR: In this article, microstructural observations of the products of the transformation of (Mg, Fe)2SiO4 olivine and spinel and Ca2GeO4 spinel were made by transmission electron microscopy (TEM) in a laser-heated diamond anvil cell (DAC).
Abstract: The 700-km seismic discontinuity which defines the top of the Earth's lower mantle is generally attributed to the transformation of the upper mantle (Mg, Fe)2 SiO4 spinel into magnesiowustite (Mg, Fe)O and (Mg, Fe)SiO3 with perovskite structure1. Large-volume high-pressure experiments2 have provided important thermodynamical information on this transition, but its mechanisms and kinetics are still unknown. Here we report microstructural observations of the products of the transformation of (Mg, Fe)2SiO4 olivine and spinel and Ca2GeO4 olivine, effected in a laser-heated diamond anvil cell (DAC). The observations were made by transmission electron microscopy (TEM), a technique that has already proved useful in investigating the olivine-spinel transition in the DAC3,4. We find that, in all cases, the transformed product has a eutectoid-like appearance (as in pearlitic carbon steels), with alternating lamellae of the rock salt and perovskite phases. The thickness and spacing of the lamellae increase with temperature until a granular structure forms. These observations suggest that the transformation starts as a eutectoid transformation, whose physical mechanisms might be as effective in the Earth's mantle as in the DAC.
TL;DR: In this paper, the crystal structure of LaTiO3 was investigated at five temperatures from 298 K to 10 K by time-of-flight neutron diffraction to look for any structural origin to the above phenomena.
Abstract: LaTiO3 shows a transition from temperature-independent paramagnetism to antiferromagnetism at 125 K. The temperature coefficient of the electrical resistivity changes at the same temperature from positive (i.e. metal-like) to negative (i.e. semiconductor-like). The crystal structure of LaTiO3 was investigated at five temperatures from 298 K to 10 K by time-of-flight neutron diffraction to look for any structural origin to the above phenomena. Analysis of the 298 K data confirm and improve on an existing X-ray structure determination in Pbnm. The same orthorhombic cell and space group are consistent with the data at all temperatures studied. The cell constants show anisotropy as b and c decrease with temperature and a increases slightly. There are no significant changes in Ti-O bond lengths or Ti-O-Ti angles with temperature. In the La-O polyhedron the eight nearest-neighbour bond distances decrease with decreasing temperature while the next four increase demonstrating a tendency to eightfold coordination. There is no structural anomaly of any significance near 125 K. This is not inconsistent with an itinerant antiferromagnetic model for the transition.
TL;DR: A Mossbauer resonance study of 57 Fe doped Sr 2 Co 2 O 5 has firstly confirmed brownmillerite structure and high spin configuration of cobalt in the high temperature variety, and secondly in the low temperature form, it has shown the existence of two kinds of crystallographic sites, a very distorted octahedral site and two sites of lower coordination as discussed by the authors.
TL;DR: In this paper, the synthesis of perovskite Pb(Mg1/3Nb2/3)O3 from an equimolar mixture of Pb 3Nb 208 and MgO was studied by solid-state reaction techniques.
Abstract: The synthesis of perovskite Pb(Mg1/3Nb2/3)O3 from an equimolar mixture of Pb3Nb208 and MgO was studied by solid-state reaction techniques. An addition of 1 wt% excess MgO to the stoichiometric composition enhances the formation of the cubic perovskite phase. The absence of free PbO in the initial starting materials minimizes the volatilization loss during firing, thereby reducing the possibility of any compositional change and resulting in a substantial improvement of the perovskite phase purity over the conventional mixed-oxide processing.
TL;DR: In this article, the authors investigated the oxygen sensitivity of SrTiO3 doped with MgO in the exhaust gas of propane-oxygen combustion as a function of the mgO content.
Abstract: The oxygen sensitivity of SrTiO3 doped with MgO has been investigated in the exhaust gas of propane-oxygen combustion as a function of the MgO content. Among the specimens studied, SrTi0.6Mg0.4O3−δ. appeared to be a suitable material for a lean-burn oxygen sensor, because it exhibited a high oxygen sensing performance in the lean-burn region and an excellent stability in the rich-burn region.
TL;DR: In this article, the structure and elastic properties of MgSiOa, a major mantle-forming phase, have been simu- lated using computer models which predict the minimum energy structure by using interatomic pair potentials to describe the net forces acting between the atoms.
Abstract: The structure and elastic properties of MgSiOa, a major mantle-forming phase, have been simu- lated using computer models which predict the minimum energy structure by using interatomic pair potentials to describe the net forces acting between the atoms. Four such interatomic potentials were developed in this study, and are compared with potential NI of Miyamoto and Takeda (1984). The most successful potential (W3) was derived by fitting the short range potential parameters to both the experimentally obtained structural and elastic properties of MgSiO 3 perovskite. The relative stabilities of some of the possible perovskite polymorphs, the orthorhombic, cubic, and tetragonal phases and hexa- gonal polytypes, were evaluated at 0 K and between 1 bar and 2 Mbar. The orthorhombic phase is found to be stable at all but the highest pressures, where the cubic phase may be stable. The temperature of the ortho- rhombic to cubic transition may decrease with increasing pressure. The energy of a stacking fault on (110) in the cubic phase was estimated using the ANNNI model and found to be about 1.95 J m -2 using potential W3. The distance of separation of partial dislocations of this type is predicted to increase with increasing pressure from 8.4 A at 1 bar to 9.2 A at 1 Mbar.
TL;DR: A polycrystalline CaTiO3 (perovskite) was investigated under static pressures up to 38 GPa and temperatures up to 1000°C by using a diamond anvil pressure cell, a YAG laser, and the ruby fluorescence pressure calibration system.
TL;DR: In this article, the reduction of LnCoO3(Ln = La-Eu) in a hydrogen atmosphere of 2×106 Pa at 25 to 600°C was investigated by X-ray diffraction and thermogravimetric analysis.
Abstract: The reduction of LnCoO3(Ln = La-Eu) in a hydrogen atmosphere of 2×106 Pa at 25 to 600° C was investigated by X-ray diffraction and thermogravimetric analysis, which were carried out to pursuein situ the reaction of reduction of LnCoO3. The amount of lattice oxygen consumed at 600° C increases on going from LaCoO3 to EuCoO3. It was shown that the reduction process proceeded through the formation of a series of oxygen-deficient structures in keeping the perovskite structures; for example, the final X-ray powder diffraction pattern of NdCoO3−x (x=1.1) could be indexed on a cubic cell (a = 0.39 nm) and an orthorhombic cell (a0=0.522 nm,b0 = 0.559nm, c0 = 0.795nm), the relationship with the cubic cell beinga0 ≃ 21/2ac,b0 ≃ 21/2ac,c ≃ 2ac.
TL;DR: In this article, the textural properties of perovskite-type binary oxides corresponding to the formulae LaMeO3 (Me ≡ Fe, Co, Ni, Cr) and La0.85Me′0.15CoO3 were investigated.
Abstract: The results of investigation of the textural properties of perovskite-type binary oxides corresponding to the formulae LaMeO3 (Me ≡ Fe, Co, Ni, Cr) and La0.85Me′0.15CoO3 (Me′ ≡ Ca, Ba, Ce) are presented. The texture of the samples was studied by physical adsorption and capillary condensation of nitrogen, and is discussed in relation to the method of preparation of the samples.
Patent•
02 Apr 1986
TL;DR: In this paper, a powder material of perovskite or its solid solution represented by the formula: ABOsub3 is described, where A is one or more metal elements coordinated with 12 oxygen atoms, and B is a metal element coordinated with 6 oxygen atoms.
Abstract: A process for producing a powder material of perovskite or its solid solution represented by the formula: ABOsub3 wherein A is one or more metal elements coordinated with 12 oxygen atoms, and B is one or more metal elements coordinated with 6 oxygen atoms, which comprises contacting an aqueous or alcohol solution of either component A or component B with a precipitating solution to form precipitates, then adding an aqueous or alcohol solution of the other component to form precipitates, and drying the precipitates, followed by calcining at a temperature of from 400° to 1200° C
TL;DR: La1−xSrxCoO3, a perovskite-type mixed oxide, was successfully formed on the surface of cordierite by impregnation with an aqueous solution of starting materials and subsequent calcination as mentioned in this paper.
Abstract: La1−xSrxCoO3, a perovskite-type mixed oxide, was successfully formed on the surface of cordierite by impregnation with an aqueous solution of starting materials and subsequent calcination, when the cordierite was precoated with adequate amount of lanthanum oxide. Effect of Sr substituton for La was similar to the perovskite bulk itself, and a high catalytic activity for oxidation of propane was obtained at x = 0.2.
TL;DR: In this paper, the cubic perovskite structure of the compound YRh3B was found to be a superconductor whose transition temperature is 0.76 K. The upper critical-field-temperature curve indicates that this compound appears as an ordinary, type II super-conductor.
Abstract: The compound YRh3B with cubic perovskite structure is found to be a superconductor whose transition temperature is 0.76 K. The upper-critical-field-temperature curve indicates that this compound appears as an ordinary, type II superconductor.
TL;DR: In this article, the authors used electron spectroscopic investigations to show that defects at various concentrations are inherently present on surfaces of KNbO3 crystals and their concentration at the surface is determined by the bulk concentration as well as by diffusion.
Abstract: Electron spectroscopic investigations are used to show that defects at various concentrations are inherently present on surfaces of KNbO3 crystals. Their concentration at the surface is determined by the bulk concentration as well as by diffusion. These defects are important for most classical measurements on KNbO3 and similar materials.