M.A. Alario-Franco

Brownmillerite-type microdomains in the calcium lanthanum ferrites: CaxLa1−xFeO3−y: I. 23 < x < 1

Abstract The study by high resolution electron microscopy and electron diffraction of several oxidized samples of the calcium lanthanum ferrites Ca x La 1− x FeO 3− y , with different values of x and y , prepared at high temperatures in air, shows the existence of a microdomain texture in which brownmillerite-type microdomains with a volume of ∼10 6 A 3 are intergrowth within a Ca 2 LaFe 3 O 8 substructure. This situation was not detected by conventional powder X-ray methods, where a simple cubic perovskite unit cell with constant parameters was observed ( a = 3.849(1) A for x = 0.8 and a = 3.848(1) A for x = 0.9). Electron micrograph images of the microdomain texture show that fringe periodicity is very regular suggesting that the oxygen excess is situated in the domain walls.

A family of non-stoichiometric phases based on Ba2YCu3O7−δ (0≤δ≤1)

Abstract In the light of recent electron diffraction evidence a reappraisal is made of the problem of non-stoichiometry by oxygen deficiency in high-temperature superconducting Ba 2 YCu 3 O 7−δ . On the basis of the ordered states of the oxygen vacancies detected by that technique, a family of phases is proposed as a better description of oxygen stoichiometry and compositional variations in that material. Five phases appear to exist within the usually considered range 0≤δ≤1. All of these phases, which can be considered as consecutive members of a homologous series differing in oxygen content along the CuO rows running along the b axis, are themselves non-stoichiometric and appear to correspond to well differentiated regions in the characteristic T c versus composition phase diagram found by several authors.

Microdomain texture and oxygen excess in the calcium-lanthanum ferrite: Ca2LaFe3O8

Abstract The analysis by TEM and electron diffraction of the anion-deficient perovskite Ca2LaFe3O8 confirms the model previously proposed by J. C. Grenier et al. (Mater. Res. Bull. 11, 1219 (1976) ) with a structure intermediate between perovskite and brownmillerite. The unit cell parameters are ∼√2ac, 3ac, √2ac (where ac is the cubic perovskite unit cell parameter). However, the unit cell is sometimes doubled along the b axis. When the sample is treated in air at temperatures around 1400°C, an oxidation process is observed and the unit cell becomes cubic (ac = 3.848(3) A). Nevertheless, electron diffraction investigations suggest the existence of a much more complex situation in which three-dimensional microdomains intergrow within one crystal. Each of these microdomains appears to have a structure clearly related to the low-temperature sample, but the superstructure is randomly found along each of the three cubic subcell directions (i.e., the unit cell √2ac, √2ac, 3ac alternates randomly with 3ac, √2ac, √2a, and with √2ac, 3ac, √2ac). High-resolution electron microscopy allows one to ascertain this microdomain texture of the real crystal.

Oxygen vacancy ordering and non stoichiometry in the Ba2YCu3O7−x superconductors

Electron diffraction of reduced samples of the high temperature superconductor (HTSC) Ba2YCu3O7−x, show the presence of diffuse scattering in the form of lines parallel to the c∗ axis of the reciprocal cell. A simple model of oxygen-vacancy ordering in ab planes, uncorrelated along the c axis-unit cell: ∼2√2ac × 2√2ac × 3ac, ac ∼ 3.80 A −, seems to account for all experimental evidence. Such a model allows compositional variations to be expected under appropriate thermodynamic conditions in the range of 0 ≤ x ≤ 1.0. In structural terms, these compositional variations produce the breaking of CuOCuO chains running along the b axis in the stoichiometric (x=0) composition. Simultaneously Tc decreases.

Structural intergrowths in the calcium lanthanum ferrites: CaxLa1−xFeO3−Y (23 < x < 1)

Electron microscopy and diffraction studies of annealed samples in the CaxLa1−xFeO3−y system (23 < x < 1) show the existence of disordered intergrowths between Ca2Fe2O5 and (Ca23La13)FeO83. A previous model (Mat. Res. Bull; 11, 1219 (1976)) for the structure of (Ca23La13)FeO83 is confirmed by high resolution images. Two types of interface boundary between both structures have been found.

Structural intergrowth in the CaxLa1−xFeO3−x2 system (0 ≤ x ≤ 1): An electron microscopy study

Abstract An electron microscopy and diffraction study of several samples of the system Ca x La 1−x FeO 3− x 2 shows these solids to be formed by the disordered intergrowth of two out of three of the ordered terms corresponding to x = 0, x = 2 3 , and x = 1 (i.e., LaFeO3, Ca2LaFe3O8, and Ca2Fe2O5). The relative amounts of the intergrowing individuals vary with x and may extend down to unit cell thickness; i.e., to the level of isolated extended defects.

A reassessment of Ba2Fe2O5

Abstract Electron diffraction on samples of pure Ba 2 Fe 2 O 5 shows this phase to be monoclinic with cell parameters multiple of a perovskite basic cell.

Microdomain formation in the CaFexMn1−xO3−y ferrites: I. 0.2 ≤ x ≤ 0.4

Electron microscopy and diffraction show that samples of the title system treated at 1400°C in air are formed by the intergrowth of Ca 2 Fe 2 O 5 -like domains and/or CaMnO 3 -like domains. Six sets of micro-domains appear to be present in the x = 0.4 sample. These results are discussed in terms of oxygen deficiency, compositional variations, and structural relationship within the ferrite-type solid solutions.

The rare-earth H.T.S.C. family Ba2(RE)Cu3O7; structural, electrical and magnetic studies (RE=Y,Nd,Sm,Eu,Gd,Dy,Ho,Er,Tm)

Abstract A systematic study of the title family has been performed in the light of structural, electric and magnetic properties. All samples are orthorhombic and the unit cell parameters show an interesting linear relationship with the lanthanide ion dimension.

Reliable method for determining the oxidation state in chromium oxides.

We show that an appropriate analysis of the electron energy loss spectra in relation to the Cr-O bonds gives a reliable methodology to obtain the oxidation state in chromium oxides. It is based on the energy difference between the Cr L(3) and O K edges, which acts as a measure of the binding energy difference between the Cr 2p(3/2) and O 1s core levels.