Z.C Kang

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.

The binary higher oxides of the rare earths

Previously published thermodynamic and some structural studies of the higher oxides of cerium, praseodymium and terbium have revealed complex systems that include homologous series of ordered intermediate phases between R2O3 and RO3 at lower temperatures, two widely non-stoichiometric phases at higher temperatures, order-disorder transitions and many other somewhat mysterious solid state phenomena. These oxygen-deficient fluorite-related systems are the most elaborate of this structure type. One of the most important requirements for advancement of understanding of these complex binary oxide systems is a knowledge of the structures and structural principles that relate the members. Recently, high resolution neutron diffraction data have been collected from Tb7O12, Pr9O16, Pr10O18, Tb11O20 and Pr12O22 and their structures have been solved by Rietveld analysis. This makes it possible to advance the outlines of the structural principle involved in understanding the relationships among these fluorite-related phases. In addition, some discussion of the disorder-order transition will be given together with a study of the phases formed when the disordered α phase is cooled rapidly.

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 .

Fluorite structural principles : disordered α-phase to ordered intermediate phases in praseodymia

Abstract Experimental observations of the structural changes effected during rapid cooling of α-phase praseodymium oxide of various compositions in their ambient oxygen pressure have been reported. These results are discussed in terms of the recently discovered structural principles for the anion-deficient, fluorite-related rare earth higher oxides. Two new polymorphs of the β-phase of praseodymia were found in the experiments, and their structures are now predicted using the new structural principles. Furthermore, the structure of one of two new π-phase polymorphs, Pr n O 2 n –2 m , that were discovered, π(1) R 16 O 30 ( n =16, m =1), RO 1.88 , is predicted. The transformation of the established β(1)-phase in the praseodymium oxides to the newly observed β(3)-phase, found in the quenching experiments, is modeled. Finally the surfaces of β(1)- and the α-phase are compared. It is observed that the superstructure of β(1) is clearly reflected in its surface structure.

Real-Time atomic-level observations of in situ chemical reactions and transformations utilizing high-resolution electron microscopy

Three studies ofin situ chemical reactions or transformations utilizing real-time video recording of high-resolution microscopic images to reveal atomic resolution details are reported. In one study, the removal of a twin plane separating a surface promontory consisting of several hundreds of atoms from the main body of a TbOx crystal is observed. In another study of a leached TbOx specimen, the decomposition of a Tb(OH)3 surface impurity is followed as the oxide product is accreted to the main body of the TbO2 crystal. In a third example, a crystal of modified PbO2 grown from acid solution displays, in high resolution, rapid cooperative movement of scores to hundreds of atoms over the crystal surface before finally decomposing, presumably with loss of hydrogen, toβ-PbO2.

Sintering in colloidal particles of rare earth hydroxycarbonate and its decomposition products

Abstract The hydroxycarbonates of binary or ternary rare earths prepared as amorphous colloidal spheres by controlled homogeneous precipitation from urea solutions are shown to sinter without an incoherent interface at each step in their decomposition to the oxide. This behavior is followed whether the decomposition is accomplished by the electron-beam during observation in the high-resolution electron microscope or during heat treatment outside the microscope. The absence of distinct boundaries in clusters of particles without long range order is illustrated. Coherent boundaries are shown to persist even with the substantial morphological change which occurs when the particles become crystalline.

Nanoanalytical characterization of Pr0.5Tb0.5Ox obtained from in situ video observation in the high resolution electron microscope

Abstract This paper illustrates the capability, with the high resolution electron microscope and associated techniques, of characterizing a material with lateral resolution near the atomic level, temporal resolution at 1/30 s, and chemical analysis of volumes no more than 2 nm wide perpendicular to the electron beam. Pr 0.5 Tb 0.5 (OH)CO 3 , in the form of an amorphous colloidal sphere, is transformed through (Pr 0.5 Tb 0.5 O) 2 CO 3 to the oxide, Pr 0.5 Tb 0.5 O x . Chemical analysis through the various stages of decomposition establishes essentially random dispersal of the rare earths. Structural analysis of video frames at time intervals of a few 1/30 s permit progressive characterization of the material as it changes. This is relevant to the promulgation of mechanisms of solid state reactions at the atomic level.

Structures and structural defects in colloidal particles altered in situ in HREM

Abstract In the past, equilibrium tensimetric studies made on the mixed Pr y Tb 1—y O x —O 2 system revealed a marked decrease in the stability, or total absence, of the ordered intermediate phases belonging to the homologous series R n O 2n—2 known in the binary Pr and Tb oxides. The early X-ray studies did, however, indicate that ordered intermediate phases are observed in rapidly cooled samples. The work reported here was designed to obtain a better resolution of the structures of any intermediate phases formed in quite homogeneously mixed specimens. Pr 0.9 Tb 0.1 O x cooled from 1100 °C in He then observed in situ in HREM showed a well-ordered new structure unknown in the binary constituent oxides. Pr 0.5 Tb 0.5 O x obtained in situ in HREM from the electron beam-induced decomposition of Pr 0.5 Tb 0.5 OHCO 3 exhibited a superstructure that suggests one or more of the monoclinic structures with a composition with n equal to or greater than 10. This apparent oxidation in the electron beam has also been reported in the binary oxides. Unfortunately, these results still do not provide information on the structures existing at equilibrium at higher temperatures.