Perovskite-like Phase Research Articles

Electron Microscopy Study of a New Cation Deficient Perovskite-like Oxide: Ba3MoNbO8.5

A new ordering scheme in cationic-deficient perovskite-like phases of the Ban(Mo,Nb)n-δO3n-x system has been described for the composition Ba3MoNbO8.5, which has an oxygen stoichiometry between those of the 9R polytype and the palmierite structure. An electron diffraction and high-resolution electron microscopy study shows that the oxygen defect with respect to the 9R polytype (hhc)3 leads to an ordered distribution of octahedra and tetrahedra in a 3:2 ratio in the cubic layers. The flexibility of the (hhc)3 stacking sequence allows the formation of a modulated structure through an ordered shearing mechanism along c by introducing mixed BaO2.6 cubic layers in the 9R polytype.

Rutherford backscattering spectrometry (RBS) characterization of oxide scale formed on (AISI-304) steel after surface deposition of lanthanum

Oxidized AISI-304 steel samples coated with lanthanum have been investigated by Rutherford backscattering spectrometry (RBS) in order to determine the composition profile of the oxide scale. In this sense, RBS technique has been revealed as a very reliable and nondestructive method for depth profile analysis. Lanthanum deposition, specially over preoxidized specimens, has proven to improve their resistance to high temperature oxidation. The formation of a LaCrO 3 perovskite-like phase uniformly distributed along the oxide scale is proposed as being responsible for this behaviour.

An X-ray Powder Diffraction Study of Sr2LnTaCu2O8, Ln = Nd, Sm, Eu, Gd

A series of oxides of the composition Sr2LnTaCu2O8, Ln = Nd, Sm, Eu, Gd has been prepared and characterized by X-ray powder diffraction. Samples are tetragonal, space group P4/mmm and isostructural with a perovskite-like phase Ba2LaTaCu2O8. The unit-cell parameters have been determined using Mo or Si as internal standard. The structure of Sr2SmTaCu2O8 has been refined by the Rietveld method. X-ray diffraction patterns of the samples and the crystal structure of Sr2SmTaCu2O8 showed ordering of Ta and Cu ions: the octahedrally coordinated Ta ion is on the Cu(1) site, while Cu ions remain on the Cu(2) sites of the Ba2YCu3O7−δ-type structure. (Powder Diffraction File Nos. 46–1490 for Sr/NdTaCu2O8, 46-1489 for Sr2SmTaCu2O8, 46-1488 for Sr2EuTaCu2O8 and 46-1487 for Sr2CdTaCu2O8.

The Effect of Additives on the Perovskite-like Phase Formation from Bi-Ti Complex Alkoxide Solutions

Bi-Ti複アルコキシド溶液の部分加水分解の際に添加する触媒種と量が, Bi4Ti3O12 薄膜の結晶相の出現にどのような影響を与えるか調査した。その結果, 0.01mol の水のみを添加した条件でペロブスカイト-類縁相の出現率が高いことが明らかになった。本研究で合成した Bi-Ti 複アルコキシド溶液のように酢酸基を含むアルコキシド溶液の場合は,添加剤の効果が顕著であり,添加する触媒種により複アルコキシドの構造と加水分解および重縮合反応,さらには析出したフィルムのゲル構造が大きく変化したと考えることができる。またこれらの結果から,得られるセラミックス薄膜の結晶相がコントロールできる可能性が示された。

Size effect in rare earth stabilization of Bi 2BaLnCuO 6+δ family

Using the solution calorimetry technique, the formation enthalpies for the Bi 2MLaCuO 6.5 (M  Ba, Ba 0.5Sr 0.5 and Sr) phases at 323 K were obtained. The values are equal to 2201, 2217 and 2227 kJ mol −1 for M  Ba, Ba 0.5Sr 0.5 and Sr respectively. On the basis of these data and an estimation of the lattice energies for perovskite-like BaBi 0.5Ln 0.5O 3 phases, a simulation of the reaction Bi 2BaLnCuO 6.5=BaBi 0.5Ln 0.5O 2.5+CuO+Bi 1.5Ln 0.5O 3 for Ln  La, Pr, Nd, Sm, Eu, Gd, Dy and Y was performed. The steeper dependence of the lattice energies of perovskite-like phases on the rare earth metal radii relative to the BaBi 0.5Ln 0.5O 3 phases determines the possibility of the formation of 2201-type structures for different rare earth elements in the BiBaLnCuO system near the 2:1:1:1 composition.

Onset of perovskite formation in the catalytic system La2O3/?-Al2O3

Molecular dynamics simulations of model systems for the surface interaction of lanthanum oxide supported on γ-alumina have been carried out at 1500 K. The onset of formation of perovskite-like phases has been analysed in samples containing four different concentrations of lanthanum oxide. A mechanism of the formation of perovskite-like polyhedra is proposed. This mechanism essentially involves a displacement of an oxide ion associated to an octahedral aluminum by a lanthanum ion and appears to be independent of La2O3 loadings.

Influence of the size of RE 3+ ions on the stability of new perovskite-like phases in the most basic region of the systems BaRECuO ( RE ≡ Ln, Y)

The influence of size of the ionic radius of RE 3+ ions ( RE ≡ La, Nd, Sm, Gd, Dy, Ho, Yb, Y) on the formation of the different and mutually exclusive phases Ba 2 − x RE x CuO 3 + δ, or Ba 4RECu 3O 9 and Ba 6RECu 3O 11 in the BaO rich zones of the BaRECuO systems was examined. The structures of the different phases were characterized by X-ray diffraction and their derivation from formerly examined phases of perovskite or layered perovskite type was examined. The importance of the preparation method used is stressed, in particular the exclusion of traces of CO 3 2− and CO 2 from the starting materials and from the gaseous atmosphere.

Correlation between the composition, crystal structure and superconducting properties of metal-oxide phases

Two models for the crystal structure formation of perovskitelike phases in Ln-Ba-Cu-O, Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O systems are proposed. On the basis of these models all calculated phases with paramters a≈3.8 Å and c = 9–43 A ̊ were experimentally obtained. The decrease of mean distance ϵ̄ between layers correlates with the rise of T c. The extrapolation of T c (ϵ̄) dependences for systems with Bi and Tl gives T max c≈150 K at ϵ ̄ = 0.95 . On the basis of the Hume-Rothery, Matthias and Yvon criteria a new criterion for superconductivity of metal-oxide phases is proposed, based on a correlation between T c and ρ k/ K electronic concentration per cation.

High Tc superconductivity and crystal structure of Pb Sr Y,Ca Cu O system

The search for high Tc superconductors in PbSrY,CaCuO system has been carried out based on crystal considerations as well as formal valency relationships. The four perovskite-like phases were detected in samples with different compositions. Superconducting resistive anomalies were detected in some samples at about 80 K, however it was not clear what phase was really superconducting one.

EPR spectrum and magnetic properties of the K 3FeTe 3O 12 perovskite

The magnetic behaviour and EPR spectrum of the perovskite-like phase K 3FeTe 3O 12 have been investigated. The results are consistent with a high spin configuration for the Fe(III) ion. This material provides a new example of an isotropic EPR line with g=4.30, which suggests a great distortion of the iron-oxygen polyhedra in this lattice.

Preparation and characterization of alkaline-earth substituted superconducting La 2CuO 4

Metallic-oxygen deficient compounds in ternary systems MOz.sbnd;La 2O 3CuO with M = Ca, Sr, Ba have been prepared by coprecipitation and subsequent low-temperature solid-state reaction, as well as by high-temperature reaction from the corresponding oxides and carbonates. Substitution of trivalent La by divalent alka-line-earth ions results in sharp drops in resistivity in the 30 – 40 K range, first discovered for the Ba-La-Cu-O system. Room-temperature X-ray powder diffraction shows the presence of a perovskite-like layer-type phase of K 2NiF 4-type to be responsible for the high T c super-conductivity. For single-phase samples of La 2CuO 4:M composition, the resistivity drop and especially the onset of the diamagnetic susceptibility are enhanced, and occur near the composition where an orthorhombic-to-tetragonal structural phase transition is detected by X-ray diffraction.

Approaches to the synthesis of High-TCsuperconducting oxides in La–Ba–Cu–O and Y–Ba–Cu–O systems

Abstract High-TC superconducting oxides of nominal La1.85Ba0.15 CuO4 and YBa2 Cu3 O7 have been prepared by using nitrate, carbonate, oxalate/malonate and citrate precursors. While the samples in the Y-system are generally monophasic YBa2Cu3O7−δ with TC around 90K, the preparations in the La-system are biphasic containing K2NiF4-like La1.85Ba0.15 CuO4 (TC = 30K) and a perovskite-like phase with' a much higher TC (200–300K). Effect of Ca, Zr, Ce as well as S substitution in YBa2Cu3O7−δ has also been investigated

Preparation of Superconductive Ceramics Containing Rare Earth Oxides by Using Yttria-Concentrate

Mixture of BaCO3, CuO and yttria-concentrate was heat-treated at 900°C in an open electric furnace, and a single phase material in the system Ln-Ba-Cu-O (Ln=rare earth elements, Y, Ce, La, Eu, Gd, Dy, Er, Ho, Yb, Sm, Nd and Pr) was prepared. X-ray diffraction study revealed that the structure of the material is similar to that of the perovskite-like phase as reported by Izumi et al. The superconducting state of this material was observed at liquid nitrogen temperature, 77K. Further heat-treatment at 900°C in oxygen gas (0.6l/min) increased the critical current density at 77K by 3 times.

Synthesis of a Superconducting Perovskite-Like Phase in the System Ba-Y-Cu-O

The effect of oxygen partial pressure on the synthesis of a supereonducting perovskite-like phase from a mixture of BaCO3, Y2O3, and CuO was studied. The rate of perovskite formation reaction and critical current density in the superconducting state at 77K increased with increasing oxygen partial pressure.

Synthesis, composition, and magnetic properties of the ferrimagnetic NdCu 3− xMn 4+ xO 12 perovskite-like phases

A magnetic oxide with composition close of NdCu 3Mn 4O 12 with a perovskite-related cubic structure ( a ⋍ 7.30 Å, space group Im3, Z = 2) has been synthesized by using either the high-pressure or the hydrothermal technique. The composition is strongly dependent on the synthesis conditions. A partial reduction of Mn 4+ in the octahedral sites, resulting in a partial substitution of Cu 2+ by Mn 3+ in the Jahn-Teller sites, leads to the actual formula Nd(Cu 2+ 3− x Mn 3+ x )(Mn 4+ 3− x Mn 3+ 1+ x )O 12. For the compound synthesized at 650°C/2 kbar, the value of the substitution parameter x, as determined by neutron diffraction, is 0.32. For samples synthesized at higher temperatures, larger values of x are obtained. The compound is ferrimagnetic with Néel temperature of 390 K and a spontaneous magnetization of 93 emu/g at 4 K (52 emu/g at room temperature). For larger x values, magnetizations up to 118 emu/g at 4 K are obtained.

Oxydes de type perovskite du systeme CaNbO

Three perovskite-like phases of the CaNbO system have been isolated, Ca x NbO 3 (0.9 ⩽ x ⩽ 1), Ca(Ca x Nb 1−x) O 3 (0 ⩽ x ⩽ 1 3 ) , and Ca 3Nb 2O 8− x . X-ray powder diffraction and electron microscopy and diffraction were used to obtain crystallographic data of these phases. The high temperature form of Ca(Ca x Nb 1− x )O 3 perovskites, where calcium ions occupy partially the octahedral sites, presents an orthorhombic symmetry and quasi-periodic twin bands like Ca x NbO 3 compounds. Two monoclinic forms (for x = 1 3 and x = 1 4 ) were found by heating Ca(Ca x Nb 1− x )O 3 compounds at lower temperature for long periods; ordering of calcium and niobium ions in octahedral sites is proposed for that structure. The Ca 3Nb 2O 8− x compound presents a quadratic cell: ordering of the cations derived from (NH 4) 3FeF 6 structure is proposed in agreement with the observed space group P4 nnc . A new multiple-phase family, with general formula Ca n Nb n O 3 n+2 , was obtained for n = 4.25, 4.5, and 5; electron microscopy shows intergrowth phenomena.

Synthesis of Barium Ferrates in Oxygen.

Phase relations in the system BaO-Fe2O3-"FeO2" at 1 atm (1 atm = 1.013 × 105 N/m2) oxygen and between 800 to 1050 °C are shown in the vicinity (high Ba portion) of the 1:1 cation-cation ratio composition. A hexagonal BaTiO3-like phase, BaFeO3-X (a = 5.676Å, c = 13.934Å), exists below 960 °C. A perovskite-like phase BaFeO3-X' occurs between 960 and 1050 °C. Single-phase tetragonal perovskite can be obtained at room temperature by quenching compositions with Ba-Fe ratios of 67:66 (a = 3.985 Å, c = 4.005 Å, 1000 °C) and 27:26 (a = 3.988 Å, c = 4.003 Å, 1000 °C) from above 960 °C. At the 17:16 ratio, single phase cubic perovskite (a = 3.994 Å, 1000 °C), can be synthesized. Influence of temperature, additions of barium, and use of Pt-containers on phase relations are discussed.

Phase Transformation in Barium Orthoferrate, BaFeO3‐x

The compound BaFeO3‐x exists in many forms, the hexagonal phase having a wide range in oxygen content (BaFeO2.63–2.92). The other phases have the limited compositions and the distinct structures of perovskite: triclinic I, BaFeO2.50; triclinic II, BaFeO2.64–2.67; rhombohedral I and II, BaFeO2.62–2.64; and tetragonal, BaFeO2.75–2.81. The phases contain Fe4+ ions correlating with the oxygen content. The hexagonal phase shows a continuous change in oxygen content with temperature down to BaFeO2.63. The perovskitelike phases, however, show characteristic transformations. The triclinic I phase oxidizes to the triclinic II form at 320° to 500°C and to the hexagonal form at 720° to 915°C in oxygen. These transformations are related to oxidation‐reduction of iron ions (Fe3+⇌ Fe4+).