L. Eyring

A compositional and structural rationalization of the higher oxides of Ce, Pr, and Tb

Abstract Cerium, praseodymium and terbium exhibit a family of related mixed-valent binary oxides which are anion-deflcient and fluorite-related. The recent structural refinement of five members of this series from neutron diffraction data have made it possible for the first time to generalize the main features of their structures, and to propose a structural principle as well as a generic formula covering all known members of the homologous series and their polymorphs. The structural principle also permits the modeling of the five refined structures correctly (and it is believed uniquely), hence, the confident prediction of the, as yet undetermined, structures of series members for which the composition and suitable diffraction patterns are available. The principle flows from the way fluorite unit-cell modules which may contain vacant oxygen sites can pack together. Rules for packing these modules and examples of their application are presented.

Calculation, display and comparison of electron microscope images modelled and observed

Abstract The calculation and comparison of electron micrographs from periodic structures has been enhanced by the development of an integral and dedicated computer system. The essential elements are a 32K minicomputer with an image display unit and 20 megabyte storage disk that is directly linked to a microdensitometer for the digitising of experimental images. The interactive nature of the programs allows the user to test a large number of models for calculating the best match for an experimental image. Complex defects may be modelled easily with an atom plotting program. The calculated images can be saved on disk for direct comparison with the experimental micrographs. The experimental images may be compared to the calculated maps on a unit cell basis. The lattice directions are determined from a Fourier transform of the digitised image. The image is then rotated to a standard orientation and averaged onto a single unit cell. The intensity histograms of the experimental and calculated maps are then matched to ensure similar contrast levels. Agreement factors and difference maps can then be calculated. Examples of three systems are presented. The defect modelling is demonstrated for a case of microtwinning in beta rhombohedral boron, and also a planar defect in the hexagonal bronze structure of K 0.25 VF 3 . The image comparison is illustrated quantitatively for a niobium block structure oxide, Nb 12 O 29 .

On the PrnO2n−2 series of oxides and the structure of Pr24O44: An investigation by high-resolution electron microscopy

Abstract The ordered intermediate rare earth oxides of praseodymium in the composition range between PrO 1.714 and PrO 1.833 (Pr n O 2 n −2 , n = 7, 9, 10, 11, 12) are easily transformed into each other by the variation of temperature and/or oxygen pressure. X-ray powder diffraction patterns of all the intermediate phases are fluorite-related and it is supposed that there are structural relationships between members of the homologous series. Group-subgroup relationships are discussed within this framework, and the probable structure of Pr 24 O 44 is proposed. The unit cell, reported as monoclinic from electron diffraction studies, is assumed to contain two clusters, each with contents Pr 7 O 30 , but no definitive structure analysis has been made. From the contrast variation observed in a high-resolution electron microscope (HREM) image of Pr 24 O 44 the relative positions and orientation of these clusters can be inferred, which allows selection of a unique model from those possible. This same image further suggests that the unit cell is actually triclinic. The atomic coordinates have been calculated for the two defect clusters within the unit cell, and have been used for image calculations and simulation comparisons.

Dynamic edge and surface processes in terbium oxide

Abstract Dynamic events occuring near and on the surfaces of terbium oxide crystals have been characterized using high-resolution surface profile imaging and real-time videotape recording. Although the surface morphology of the native crystals emphasized {111} faces, greater atomic activity occurred on other faces during observation. The removal of a twin boundary and the accretion of small, chemically and physically differentiated regions into the larger bulk crystal were also observed.

The evolution of a thin, amorphous neodymium hydroxide film to the C-type oxide—an HREM study

High-resolution electron microscopy accompanied by the collection of atomic-level images at video rates, followed by frame-by-frame analysis of the video record has been used to study the decomposition of Nd(OH)3 to the oxide. The reaction is observed to go through the intermediate NdOOH to C-type Nd2O3. This sequence is shown to be in agreement with thermal and X-ray analyses. Analysis of the video record shows an amorphous phase crystallizing to the hydroxide which, in turn, decomposes to the oxyhydroxide in the [-1 1 1] zone. From this a crystal of the C-type sesquioxide in the [7 1 11] zone emerges with a minimal reconstruction except the removal of a tilt boundary that existed in the oxyhydroxide. Efforts are being made to propose an atomic-level mechanism for the structure orientational observations resulting from the loss of water.

Time-resolved X-ray diffraction by synchrotron radiation: The thermal decomposition of Cd((OH)2) powders

Abstract The application of time-resolved X-ray diffraction by synchrotron radiation to thermal decomposition of Cd(OH) 2 is reported. In this demonstrative and preliminary work, we follow the reaction by measuring the (100) reflection of Cd(OH) 2 and the (111) reflection of CdO as a function of time. The results are consistent with the reported martinsitic nature for the formation of the oxide. The Cd(OH) 2 (100) reflection undergoes narrowing that is not consistent with a diffraction domain size effect alone.

Electron beam-induced reduction of higher oxides of the rare earths: A high-resolution electron microscopic study

Many compounds are unstable in the beam of the electron microscope or can be made so by increasing the beam intensity. Observations of beam-induced decompositions of the higher oxides of the rare earths were made under circumstances where the details of reduction in the vacuum of the microscope can be examined. The ζ and ι phases of praseodymium oxide (Pr9O16, Pr7O12) and the ι phase of terbium oxide were observed to reduce directly to the ф phase (R2O3), revealing the texture of intermediate stages of the reaction. The nature of the ZrO2Sc2O3 system in the same intermediate composition region is also revealed and compared to the binary rare earth systems. Particular attention is paid to the appearance of composite crystals in these semicoherent phases during reaction. From these varied observations speculations on the mechanism of the decomposition are presented.

Vacancy ordering in Sc1−xS

This is a study of scandium vacancy ordering in slowly cooled Sc1−xS using high-resolution electron microscopy. Previous X-ray analysis had shown vacancy segregation into alternate (111) planes of the rocksalt substructure. The present study reveals a further incommensurate ordering of the vacancies within the (111)c planes in approximately every third (110)rhombohedral plane. The incommensurate order was easily destroyed by the irradiation and heating of the electron beam. These observations are discussed in terms of the termination of the wave vector within the Brillouin zone and described in terms of a possible continuous transition to an incommensurate Sc atom population wave.

The prediction of the structure of members of the homologous series of the higher rare earth oxides

Abstract The structural principle supported by the determination of the structures of the anion-deficient, fluorite-related, homologous series of higher rare earth oxides, is reviewed. The principle is applied to predict the structure of the β(2) phase, one of the members of the homologous series, R n O 2 n −2 m . Addition of the rule that during reaction or phase transformation, oxygen or vacant oxygen sites move in close-packed layers, allows the steps in the transformation of β(2) to its homologue β(3) to be shown. This transformation has been observed at atomic-resolution in the electron microscope and is interpreted in terms of intermediate forms that appear as modulated distortion waves sweeping the specimen.

Hydrogen production from methane and water by lattice oxygen transfer with Ce0.70Zr0.25Tb0.05O2−x

Hydrogen is a prime prospect for an alternative fuel. In this paper a cyclical method of production of hydrogen is proposed. The method consists of hydrogen production from methane reduction of fluorite-type Ce 0.70 Zr 0.25 Tb 0.05 O 2 to Ce 0.70 Zr 0.25 Tb 0.05 O 2-x followed by reduction of water by Ce 0.70 Zr 0.25 Tb 0.05 O 2-x to Ce 0.70 Zr 0.25 Tb 0.05 O 2 .