Showing papers on "Perovskite (structure) published in 1989"

Magnetoresistance measurements on the magnetic semiconductor Nd0.5Pb0.5MnO3

Abstract: We present magnetoresistance data up to 20 T and magnetisation measurements on the mixed-valence cubic perovskite semiconductor Nd 05 Pb 05 MnO 3 , which demonstrate that the material orders ferromagnetically below 184 K, and that around and above this temperature the conductivity is dominated by hopping of localized magnetic polarons, with an activation energy ∼95 meV

Structure and Reactivity of Perovskite-Type Oxides

Abstract: Publisher Summary This chapter discusses the structure and reactivity of perovskite-type oxides. Perovskite-type oxides have the general formula ABO 3 (A, cation of larger size) and are structurally similar to CaTiO 3 , the mineral that gave its name to that group of compounds. These materials are first studied because of their important physical properties such as ferro-, piezo-, and pyroelectricity, magnetism and electrooptic effects. The most numerous and most interesting compounds with the perovskite structure are oxides. Some hydrides, carbides, halides, and nitrides also crystallize with this structure. The chapter reviews only the study of oxides and their behavior in the gas solid interface and in heterogeneous catalysis. An important characteristic of perovskites, mentioned in the chapter, is their susceptibility of partial substitution in both A and B positions. This provides a wealth of isomorphic compounds that can easily be synthesized. Given the extensive range of possibilities in the tailoring of their chemical and physical properties, there is no doubt that new reactions can be studied, where these oxides can participate as catalytic agents.

Simulating the core‐mantle boundary: An experimental study of high‐pressure reactions between silicates and liquid iron

Abstract: Experiments with the laser-heated diamond cell show that (Mg,Fe)SiO3 perovskite reacts chemically with liquid iron at pressures greater than 70 GPa and temperatures above 3700 K X-ray diffraction analyses of quenched samples demonstrate that the reaction products include SiO2 stishovite and iron alloys (Fe,Mg)xO and FexSiy, at the interface between the Fe and silicate Our results suggest that similar chemical reactions occur at the Earth's core-mantle boundary, forming chemical heterogeneities composed of silicate-rich and iron alloy-rich regions in the D′ layer This scenario is consistent with seismological observations

Stability and equation of state of CaSiO3-perovskite to 134 GPa

Abstract: The stability and P-V equation of state of CaSiO3 have been investigated using in situ diamond-anvil X ray diffraction techniques to 134 GPa, a pressure equivalent to that at the core-mantle boundary. Samples were heated by YAG laser at each pressure increment at high pressures to accelerate phase transitions. X ray diffraction measurements were carried out at 300 K using both energy-dispersive synchrotron and sealed-tube film techniques. Quenched CaSiO3-perovskite was observed to remain metastable close to 0.1 MPa, and to convert rapidly to an amorphous phase on pressure release. The simple cubic perovskite phase of CaSiO3 was found to be the stable phase for all lower mantle pressure conditions. All 47 P-V data points were used to obtain a third-order Birch-Murnaghan equation of state with zero-pressure parameters: unit cell volume V0 = 45.37±(0.08) A3, density ρ0 = 4.252(±0.008) Mg/m3, and bulk modulus K0 = 281(±4) GPa, with an assumed bulk modulus pressure K0′ = 4. These parameters are close to those of (Mg0.88Fe0.12)SiO3-perovskite and to those inferred by the Preliminary Reference Earth Model for the lower mantle. Hence, CaSiO3 must be considered an invisible component, in terms of density and bulk modulus constraints, in the lower mantle. Mantle composition models with both high and low calcium content can satisfy existing seismological constraints for the lower mantle.

Single crystal X-ray diffraction study of MgSiO3 perovskite from 77 to 400 K

Abstract: Single crystal X-ray diffraction study of MgSiO3 perovskite has been completed from 77 to 400 K. The thermal expansion coefficient between 298 and 381 K is 2.2(8) × 10-5 K-1. Above 400 K, the single crystal becomes so multiply twinned that the cell parameters can no longer be determined.

Process for making sol-gel deposited ferroelectric thin films insensitive to their substrates

Abstract: A method for producing a thin film of a ferroelectric perovskite material having the steps of providing a first substrate; depositing a first layer of a sol-gel perovskite precursor material wherein the crystallization of this precursor material to the pervoskite phase is insensitive to the first substrate; depositing a second layer of a sol-gel perovskite precursor material wherein the crystallization is sensitive to the first substrate; and heat-treating the deposited layers to form ferroelectric perovskites. A heat treatment step to form perovskites may optionally follow the deposition of the first layer. The first layer of sol-gel perovskite precursor material is selected to produce a perovskite upon heat treatment of: lead titanate (PbTiO3), or strontium titanate (SrTiO3). The second layer of sol-gel perovskite precursor material is selected to produce a perovskite upon heat treatment of: lead zirconate titanate (Pb(Zr,Ti)O3), lead zirconate (PbZrO3), lead lanthanum titanate ((Pb,La)TiO3), lead lanthanum zirconate ((Pb,La)ZrO3), lead lanthanum zirconate titanate ((Pb,La)(Zr,Ti)O3), lead magnesium niobate (Pb(Mg1/3 Nb2/3)O3), lead zinc niobate (Pb(Zn1/3 Nb2/3)O3), barium titanate (BaTiO3), strontium barium titanate ((Sr,Ba)TiO3), barium titanate zirconate (Ba(Ti,Zr)O3), potassium niobate (KNbO3), potassium tantalate (KTaO3), or potassium tantalate niobate (K(Ta,Nb)O3).

Preparation ofc-Axis-Oriented PbTiO3Thin Films by MOCVD under Reduced Pressure

Abstract: Ferroelectric PbTiO3 thin films were prepared by a simultaneous deposition of TiO2 and PbO on heated substrates under the reduced pressure of 6 Torr. Titanium tetraisopropoxide and tetraethyl lead were used as source materials. The deposition behaviors of TiO2 and PbO were examined independently. Homogeneous nucleations of the reaction species in the vapor phase were depressed under the reduced pressure. The films obtained at 500° to 650°C of consisted of PbTiO3 of the single perovskite phase, and highly c-axis oriented and epitaxal films were grown on MgO(100). The deposition rate of the film available by adjusting the source temperature and the flow rate of the carrier gas is 50 to 1000A/min, the maximum value of which is ten or more times that obtained by the conventional sputtering method.

Elasticity of MgSiO3 in the Perovskite Structure

Abstract: The single-crystal elastic moduli of MgSiO 3 in the perovskite structure, the high-pressure polymorph of MgSiO 3 pyroxene, have been determined. The data indicate that a mantle with either pyrolite or pyroxene stoichiometry is compatible with the seismic models appropriate to the earth9s lower mantle, provided that the shear modulus of MgSiO 3 perovskite exhibits a strong negative temperature derivative. Such a temperature derivative falls outside of the range expected for a well-behaved refractory ceramic and could result if the pressure-temperature regime of the earth9s lower mantle is near that required for a ferroelastic phase transformation of the perovskite phase.

Optical study of the metal-insulator transition on Ba 1–x K x Bi0 3 thin films

Abstract: THE superconducting perovskite (Ba, K)BiO3 (refs 1, 2) has the highest transition temperature known (Tc≈css31 K) aside from the copper oxide compounds3. This system becomes superconducting upon the substitution of monovalent potassium for divalent barium in the parent insulator BaBiO3, analogous to the substitution of strontium for lanthanum in La2CuO4. To understand whether the mechanism of the superconductivity is the same as in the copper oxide compounds, it is important to know the origin of the insulating state of the parent material, as well as the basic character of the metallic state induced by the cation substitution. In the case of its predecessor Ba(Bi,Pb)O3, optical measurements were used to investigate the change in electronic state across the metal-insulator transition at 65% lead doping4. Here we report the successful synthesis of (Ba,K)BiO3 thin films, which has allowed us to measure the optical spectrum, as well as the transport coefficients of (Ba,K)BiO3. Our results, combined with the previous report of the X-ray crystal structure5, strongly suggest that the Peierls gap, responsible for the insulating state of BaBiO3, almost completely vanishes at the metal-insulator transition, resulting in a metallic state described by band theory6.

Thermodynamic Regularities in Perovskite and K2NiF4 Compounds

Abstract: It has been found that the enthalpy of formation of perovskite compounds, ΔfH° (ABO3, B = transition metais), from binary oxides can be well characterized in terms the tolerance factor, t≡(rA+ ro)√2 (rB+ ro), where rA and rB are the radii of A-site ions with 12-coordination and B-site ions with 6-coordination, respectively, and ΔfH°=−168 + 270(1 − t) kJ·mol−1 for AIBVO3, ΔfH°=−125 + 1000(1 − t) kJ·mol−1 for AIIBIVO3, and ΔfH°=− 90 + 720(1 − t) kJ·mol−1 for AIIIBIIIO3. Although the thermodynamic data of K2NiF4 compounds are not extensive, a similar regularity can be found when use is made of the radii of A-site ions with 9-coordination for the K2NiF4 compounds. These correlations will be quite useful in predicting.

Structural characteristics of Sr1−xLaxTi3+δ as a function of oxygen partial pressure at 1400 °C

Abstract: The structural characteristics of Sr1−x Lax TiO3+δ (0≤x≤0.4) at 1400 °C have been investigated as a function of ambient oxygen partial pressure. A modified Rietveld pattern‐fitting structure‐refinement program [H. M. Rietveld, J. Appl. Crystallogr. 2, 65 (1969)] was used to determine the nature of the distortions of the fundamental perovskite unit cell, the degree of lattice perfection, and the cation vacancy concentrations. Specimens equilibrated in forming gas displayed a linear relation between x, the lattice parameters, and the degree of lattice perfection while those samples annealed in air and oxygen deviated significantly from linearity. A monoclinic distortion of the perovskite structure was seen in the samples at low oxygen partial pressures while a second phase or layer type of distortion appeared in samples with x>0.2 under oxidizing conditions.

Preparation of epitaxial ABO3 perovskite‐type oxide thin films on a (100)MgAl2O4/Si substrate

Abstract: Epitaxial thin films of ABO3 perovskite‐type oxides, including PbTiO3, (Pb0.90La0.10) (Zr0.65Ti0.35)0.975O3,BaTiO3, and SrTiO3, have been successfully obtained by rf magnetron sputtering on (100)Si substrate with an intermediate epitaxial layer of MgAl2O4. Only PbTiO3 grew in the tetragonal crystal structure and other materials grew in the cubic structure. The unit axis direction of the perovskite‐type oxide films was coincident with that of the underlying MgAl2O4 films. The tetragonal PbTiO3 films were a mixture of c and a domain. The preferred orientation of the tetragonal PbTiO3 film, that is c to a domain volume ratio, could be controlled by the conditions of sample cooling after the deposition. Highly c‐axis oriented films, which consisted of more than 90% c domains, were produced by cooling the sample at a high cooling rate, typically 30 °C/min, and by maintaining an rf plasma during cooling. The mechanism of the preferred orientation of PbTiO3 film has been explained by a balance of compressive str...

Process for producing sub-micron ceramic powders of perovskite compounds with controlled stoichiometry and particle size

Abstract: A single crystal, solid solution, chemically homogenous powder that comprises a perovskite compound having useful electrical properties and the general formula: ABO₃, wherein the perovskite compound is selected from the group consisting of lead zirconate titanate, lead lanthanum zirconate titanate, lead titanate, lead zirconate, barium titanate, lead magnesium niobate, or lead zinc niobate, and having 0-50 percent total dopant or solid solution substitutions; and a process for making it.

A new phase transition in MnTiO3: LiNbO3-perovskite structure

Abstract: The stable polymorph of MnTiO3 at room temperature and pressure has the ilmenite structure. At high temperatures and pressures, MnTiO3 ilmenite transforms to a LiNbO3 structure through a cation reordering process (Ko and Prewitt 1988). Single crystals of both phases have been studied with X-ray diffraction to 5.0 GPa. We have obtained the first experimental verification of the close relationship between the LiNbO3 and perovskite structures, first postulated by Megaw (1968). MnTiO3 with the LiNbO3 structure (MnTiO3 II) transforms directly to an orthorhombic perovskite structure (MnTiO3 III) between 2.0 and 3.0 GPa. The transition involves a change of volume of -5%, is reversible and has pronounced hysteresis. Only pressure is required to drive the transition because it involves no breaking of bonds; it simply involves rotation of the [TiO6] octahedra about their triad axes accompanied by displacement of the Mn cations to the distorted twelve-coordinated sites formed by the rotations. An unusual aspect of this transition is that twinned MnTiO3 II crystals transform to untwinned MnTiO3 III crystals with increasing pressure. The twin plane of MnTiO3 II, $$\left( {10\bar 1\bar 2} \right)$$ , corresponds to the (001) mirror plane of the orthorhombic perovskite structure. MnTiO3 III examined at 4.5 GPa is very distorted from the ideal cubic perovskite structure. The O(2)-O(2)-O(2) angle describing the tilting in the ab plane is 133.3(7)°, in contrast to 180° for a cubic perovskite and the O(2)-O(2)-O(2) angle describing the tilting in the ac plane is 109.3(4)°, as opposed to 90° in a cubic perovskite.

Preparation and properties of PbO–MgO–Nb2O5 ceramics near the Pb(Mg⅓Nb⅔)O3 composition

Abstract: We prepared and characterized PbO–MgO–Nb2O5 ceramics near the Pb(Mg⅓Nb⅔)O3 stoichiometric composition. We examined microstructures, sintering behavior, and phase compositions in this system and prepared several new compositions, which have a single phase perovskite structure and a high dielectric constant (28000), by low sintering temperature (950–1020 °C). These compositions have a high electrical resistivity (1013 ohm-cm), which is about 1000 times higher than those of Pb(Fe½Nb½)O3 and Pb(Fe⅔W⅓)O3 based ceramics. The new compositions can be readily prepared into a single phase perovskite without a detectable amount of pyrochlore second phase by a simple method of mixing and calcining the constituent oxide and carbonate powders. Our dielectric measurements and XRD data establish the correlation between the decrease in dielectric constant and the presence of pyrochlore second phase. The effects of PbTiO3 dopant were quantitatively measured and our data showed that at a low (10 mole %) PbTiO3 concentration, the Curie temperature increases 6.6 °C for every mole % PbTiO3 dopant in the new compositions. The dielectric constant increases from 18000 in the undoped composition to 27 100 and 29000 by a PbTiO3 dopant concentration of 5% and 10%, respectively. The maximum dissipation factor also decreases from 0.10 in the undoped composition to 0.055 in the 10% PbTiO3 doped composition. The electrical resistivity, ρ, decreases with the PbTiO3 dopant concentration, t, following the ρ = ρ0 exp (–t/t0) relation where ρ0 ≍ (9.6 ± 0.2) × 1012 Ωcm and t0 ≃ 0.044.

Characterization of the mixed perovskite BaSn1–xSbxO3 by electrolyte electroreflectance, diffuse reflectance, and X-ray photoelectron spectroscopy

Abstract: The band gap of the mixed perovskite BaSn1–xSbxO3 has been reported to be 134 kJ mol–1(J-M. Hermann, M. R. Nunes and F. M. A. da Costa, J. Chem. Soc., Faraday Trans. I, 1982, 78, 1983), corresponding to 1.4 eV, which is the optimum value for solar-energy conversion. In order to confirm this value, pressed discs of the mixed perovskite were sintered at 1673 K and made into electrodes, and their electrolyte electroreflectance (e.e.r.) spectra measured. The band gap was determined by fitting the e.e.r. spectra to Aspnes' formula for the low-field regime. The band gap of the pure, insulating perovskite BaSnO3 was determined from its diffuse reflectance spectrum, after application of the Kubelka–Munk formula. The band gap of the mixed perovskite was constant and equal to 330 kJ mol–1 for Sb concentrations in the range x= 0–10%, in agreement with the observed constancy of the lattice parameter for x= 0–17%, the range over which the perovskite exists as a single phase. The high band-gap energy of this material makes it unsuitable for solar-energy conversion. X-Ray photoelectron spectroscopy measurements showed that all the Sb in the mixed perovskite was present as SbV, as would be expected from the high conductivity of the samples. From the atomic ratios it was concluded that probably the surface of the pure BaSnO3 perovskite is enriched in SnO2, and that of the mixed perovskite in Sb2O5.

Defect Chemistry of Relaxor Ferroelectrics and the Implications for Dielectric Degradation

Abstract: Degradation of the electrical resistance of perovskite-related dielectrics under voltage–temperature stress is a function of the concentration of the most mobile ionic species, the oxygen vacancy. Sources of oxygen vacancies in simple perovskite oxides are reviewed, and the effects of the compositional complexity of the relaxor ferroelectrics are discussed. In particular, the possibility of compositional inhomogeneity that results from partial ordering of the cations on the octa hedral sites can greatly affect the properties of the continuous, disordered matrix that controls the degradation behavior. [Key words: ferroelectrics, electrical properties, defects, capacitors, perovskites.]

Phase transformations in diopside CaMgSi2O6 at pressures Up to 25 GPa

Abstract: Phase transformations in diopside have been studied at pressures up to 25 GPa and at temperatures of 1100 and 1500 °C by a multiple anvil apparatus. The following phase assemblages are found to be stable at given pressures; diopside (15 and 18 GPa), Mg2SiO4 spinel + stishovite + CaSiO3 perovskite (19 and 20 GPa), MgSiO3 ilmenite + CaSiO3 perovskite (22 GPa), and MgSiO3 perovskite + CaSiO3 perovskite (24.5 GPa). Electron microprobe analysis reveals that the solubility of the MgSiO3 molecule in CaSiO3 cubic perovskite is limited to between 2–5%. The solubility of the CaSiO3 molecule in MgSiO3 orthorhombic perovskite is more restricted, being of the order of 2% or less. These results are contradictory to earlier work in which diopside is reported to transform into a cubic perovskite structure of the same composition above 22–23 GPa.