Structures and stabilities of trivalent and tetravalent rare earth ions in sevenfold and eightfold coordination in fluorite-related complex oxides

This paper reports the preparation and characterization of a series of oxides containing 3+ or 4+ lanthanide (M = Ce, Pr, or Tb) ions, with different ionic sizes and varying M{sup 4+}/M{sup 3+} reduction potentials, in nearly cubic coordination. The objective of the study was to demonstrate how oxidation-reduction characteristics and ionic size trends explain the properties of these oxides and to compare the oxidation-reduction stability of M{sup 3+} and M{sup 4+} lanthanide ions in high (CN 7 or 8) coordination in fluorite-related oxides versus low (CN 6) coordination in perovskite oxides. Efficient preparative methods are reported, as well as powder diffraction and thermogravimetric measurements for oxides CaMTi{sub 2}O{sub 7-x} and CaMZr{sub 2}O{sub 7-x}. These oxides were characterized by X-ray powder diffraction and by thermogravimetric analysis. CaCeTi{sub 2}O{sub 7} is a pyrochlore, a = 10.142(4) {Angstrom}, with Ce{sup 4+} much more easily reducible than in the perovskite BaCeO{sub 3}. By contrast, a preparation with the stoichiometry CaPbTi{sub 2}O{sub 7-x}'' is a two-phase mixture-of perovskite CaTiCo{sub 3} and a presumably Pr{sup 3+}-rich pyrochlore Pr{sub 2}Ti{sub 2}O{sub 7}( ). CaTbTi{sub 2}O{sub 7-x} appears to be a Tb{sup 3+} pyrochlore, a = 10.149(2) {Angstrom}. CaCeZr{sub 2}O{sub 7} is a pyrochlore, a = 10.524(1) {Angstrom}. A preparation of CaPbZr{sub 2}O{sub 7-x}'' also appeared to yield a two-phase mixture, perovskite CaZrO{sub 3} and pyrochlore Pr{sub 2}Zr{sub 2}O{sub 7}. In this paper, the structures, f-element ion sites, and M(4)-M(3) stability trends in the CaMTi{sub 2}O{sub 7-x} and CaMZr{sub 2}O{sub 7-x} oxides are compared with the structural and stability trends in the perovskites BaMO{sub 3} which have M{sup 4+} ions in sixfold (tilted octahedra) coordination

Flux through a hole from a shaken granular medium

We have measured the flux of grains from a hole in the bottom of a shaken container of grains. We find that the peak velocity of the vibration, vmax, controls the flux, i.e., the flux is nearly independent of the frequency and acceleration amplitude for a given value of vmax. The flux decreases with increasing peak velocity and then becomes almost constant for the largest values of vmax. The data at low peak velocity can be quantitatively described by a simple model, but the crossover to nearly constant flux at larger peak velocity suggests a regime in which the granular density near the container bottom is independent of the energy input to the system.Comment: 14 pages, 4 figures. to appear in Physical Review

Engaging and empowering first-year students through curriculum design: perspectives from the literature

There is an increasing value being placed on engaging and empowering first-year students and first-year curriculum design is a key driver and opportunity to ensure early enculturation into successful learning at university. This paper summarises the literature on first-year curriculum design linked to student engagement and empowerment. We present conceptualisations of ‘curriculum’ and examples from first-year curriculum design. We also note the limited literature where students have been involved in designing first-year curricula. The results of the literature review suggest that key characteristics of engaging first-year curricula include active learning, timely feedback, relevance and challenge. The literature also points to the importance of identifying students' abilities on entry to university as well as being clear about desired graduate attributes and developmental goals. Acknowledging realities and constraints, we present a framework for the first-year curriculum design process based on the literature

Enthalpies of formation of rare earths and actinide(III) hydroxides: Their acid-base relationships and estimation of their thermodynamic properties

This paper reviews the literature on rare earth(III) and actinide(III) hydroxide thermodynamics, in particular the determination of their enthalpies of formation at 25{degree}C. The hydroxide unit-cell volumes, lanthanide/actinide ion sizes, and solid-solution stability trends have been correlated with a generalized acid-base strength model for oxides to estimate properties for heterogeneous equilibria that are relevant to nuclear waste modeling and to characterization of potential actinide environmental interactions. Enthalpies of formation and solubility-product constants of actinide(III) hydroxides are estimated

Enthalpy of formation of americium sesquioxide; systematics of actinide sesquioxide thermochemistry

The enthalpy of formation of hexagonal Am/sub 2/O/sub 3/ at 298.15 K has been determined to be -1690.4 +- 7.9 kj mol/sup -1/ by solution microcalorimetry. Systematic comparison of the measured enthalpies of formation of hexagonal Pu/sub 2/O/sub 3/ and Am/sub 2/O/sub 3/, monoclinic Cm/sub 2/O/sub 3/, and all lanthanide sesquioxides has been utilized to develop a predictive understanding of stability relationships among all actinide sesquioxides and other actinide species

Nuclear structure and shapes from prompt gamma ray spectroscopy of fission products

Many nuclear shape phenomena are predicted to occur in neutron-rich nuclei. The best source for the production of these nuclides is the spontaneous fission which produces practically hundreds of nuclides with yields of greater than 0.1 % per decay. Measurements of coincident gamma rays with large Ge arrays have recently been made to obtain information on nuclear structures and shapes of these neutron- rich nuclei. Among the important results that have been obtained from such measurements are octupole correlations in Ba isotopes, triaxial shapes in Ru nuclei, two-phonon vibrations in {sup 106}Mo and level lifetimes and quadrupole moments in Nd isotopes and A=100 nuclei. These data have been used to test theoretical models

Competing C–Br and C–C Bond Fission Following 1[n(O),π∗(C=O)] Excitation in Bromoacetone: Conformation Dependence of Nonadiabaticity at a Conical Intersection

These experiments investigate the competition between C-C and C-Br bond fission in bromoacetone excited in the (1)[n(O),pi(*)(C=O)] absorption, elucidating the role of molecular conformation in influencing the probability of adiabatically traversing the conical intersection along the C-C fission reaction coordinate. In the first part of the paper, measurement of the photofragment velocity and angular distributions with a crossed laser-molecular beam time-of-flight technique identifies the primary photofragmentation channels at 308 nm. The time-of-flight spectra evidence two dissociation channels, C-Br fission and fission of one of the two C-C bonds, BrH2C-COCH3. The distribution of relative kinetic energies imparted to the C-Br fission and C-C fission fragments show dissociation is not occurring via internal conversion to the ground electronic state and allow us to identify these channels in the closely related systems of bromoacetyl- and bromopropionyl chloride. In the second part of the work we focus on the marked conformation dependence to the branching between C-C fission and C-Br fission. Photofragment angular distribution measurements show that C-Br fission occurs primarily from the minor, anti, conformer, giving a beta of 0.8, so C-C fission must dominate the competition in the gauche conformer. Noting that the dynamics of these two bond fission pathways are expected to be strongly influenced by nonadiabatic recrossing of the reaction barriers, we investigate the possible mechanisms for the conformation dependence of the nonadiabatic recrossing with low-level ab initio electronic structure calculations on the C-Br reaction coordinate and qualitative consideration of the conical intersection along the C-C reaction coordinate. The resulting model proposes that C-C bond fission,cannot compete with C-Br fission in the anti conformer because the dissociation samples regions of the phase space near the conical intersection along the CC fission reaction coordinate, where nonadiabaticity inhibits C-C fission, while from the gauche conformer C-C fission can proceed more adiabatically and dominate C-Br fission. A final experiment confirms that the branching ratio changes with the relative conformer populations in accord with this model

EXAFS studies of lanthanide coordination in crystalline phosphates and amorphous phytates

As part of the DOE Efficient Separations and Processing Integrated Program, techniques are being developed to stabilize radioactive and hazardous contaminants to prevent their migration from buried wastes. This report is part of a study to immobilize actinide ions in the near-surface environment by reacting them with organophosphorus complexants that decompose to phosphates. Nd and Gd ions were used as models for Pu, Am, and Cm ions; phytic acid was used as the complexant. Solid Nd and Gd phytates were prepared and the coordination characterized by EXAFS