This review focuses on key aspects of the thermal decomposition of multinary or mixed hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of such hydrides, Tdec. An attempt is also made to predict the thermal stability of as-yet unknown, elusive or even unknown hydrides. The future of a particularly promising class of materials for hydrogen storage, namely the catalytically enhanced complex metal hydrides, is discussed. The predictions are supported by thermodynamics considerations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition.

Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen.

Actinide elements produce a plethora of interesting physical behaviors due to the $5f$ states. This review compiles and analyzes progress in the understanding of the electronic and magnetic structure of the $5f$ states in actinide metals. Particular interest is given to electron energy-loss spectroscopy and many-electron atomic spectral calculations, since there is now an appreciable library of core $d\ensuremath{\rightarrow}\text{valence}$ $f$ transitions for Th, U, Np, Pu, Am, and Cm. These results are interwoven and discussed against published experimental data, such as x-ray photoemission and absorption spectroscopy, transport measurements, and electron, x-ray, and neutron diffraction, as well as theoretical results, such as density-functional theory and dynamical mean-field theory.

Nature of the 5f states in actinide metals

(1979). The Chemical Thermodynamics of Actinide Elements and Compounds. Nuclear Technology: Vol. 42, No. 1, pp. 114-115.

The Chemical Thermodynamics of Actinide Elements and Compounds

Hydrogen dissociation and diffusion on transition metal (= Ti, Zr, V, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H$_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab-initio density functional theory investigation of H$_2$ dissociation and subsequent atomic H diffusion on TM(=Ti,V,Zr,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TM's on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TM's on the right of the periodic table don't bind H, however, they do not reduce the barrier to dissociate H$_2$ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.

Hydrogen Dissociation and Diffusion on Transition Metal(=Ti,Zr,V,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) Surfaces

The physico-chemical properties of the actinide metals, alloys, and compounds show interesting parallels and contrasts with the rare earths, beyond uranium. At first there is a transition region where the unique bonding properties of the early actinides become less prominent, due to progressive f-electron localization. Nevertheless, in contrast to the rare earths, f-electron energies remain close to the Fermi level, resulting in complex behavior as a function of temperature, pressure and structure. Particularly interesting in this region are the metallic hydrides, whose chemistry is clearly rare-earth like, but whose electronic properties are entirely different. At americium a major localization and f-band narrowing occurs, but the explanation of americium behavior is obscured by the occurrence of the unique f/sup 6/ non-magnetic solid-state configuration. Beyond americium, it would appear that real rare-earth-like behavior finally begins; this has been born out by recent studies on the thermodynamics and cohesive energies of curium, berkelium, californium and einsteinium metals. However, a new complication arises almost immediately, in the onset of incipient stabilization of the divalent state, which already appears in californium, whose physico-chemical properties are remarkably similar to samarium. Einsteinium appears to be fully divalent, thus heralding the beginning of a mini-series of truly divalentmore » metals.« less

Electronic structure and bonding in transuranics: Comparison with lanthanides☆

The ionivation and appearance potentials of parent and fragment ions from the neutral molecules PuF6 and PuF5 were measured using electron impact ionivation. From this data the bond dissociation energies of the molecules PuFn-F (n = 0 to 5) were calculated. Vapor pressures of PaO2(g), PaO(g) and Pa(g) were measured above the two-phase system Pa(l) + PaO2−y(s) and the congruently vaporiving composition PaO2−v(s). From these measurements, the enthalpies of formation of Pa(g), PaO(g) and PaO2(g) were calculated to be respectively 131.7 ± 6.1 (551 ± 26), 2.4 ± 4.1 (10 ± 17) and −122.7 ± 4.0 (−513 ± 17) kcal mol−1 (kJ mol−1). Bond dissociation energies of PaO-O and Pa-O were found to be respectively 184.6 ± 4.1 (772 ± 17) and 188.4 ± 4.1 (788 ± 17) kcal mol−1 (kJ mol−1).

Thermochemical studies on the plutonium fluorides and protactinium ovides

The resistivity and magnetic susceptibility from 1–300 K have been measured on five compositions of plutonium hydride, PuHx. The resistance vs. temperature data show metallic to semiconductive behavior as x is varied from 1.93 to 2.65 in the cubic dihydride phase field. The samples order ferromagnetically between 44 and 67 K as an increasing function of x. The effective paramagnetic moments are 0.83 to 0.70 μB, and the ordered moments about half these values. X-ray photoemission measurements place the hybridized 5f-6d band 1.8 eV below the Fermi energy. The core level 4f spectra and the magnetic data are consistent with a trivalent plutonium configuration.

Electronic properties and structure of the plutonium-hydrogen system

Problems that can arise during the cycling steps for a hydride storage system usually involve events at surfaces. Chemisorption and reaction processes can be affected by small amounts of contaminants that may act as catalytic poisons. The nature of the poisoning process can vary greatly for the different metals and alloys that form hydrides. A unifying concept is offered, which satisfactorily correlates many of the properties of transition metal, rare earth and actinide hydrides.

Electronic structure, bonding and chemisorption in metallic hydrides

Pressure-Composition-Temperature (PCT) data are presented for the plutonium-hydrogen (Pu-H) and plutonium-deuterium (Pu-D) systems in the solubility region up to terminal solubility (precipitation of PuH2). The heats of solution for PuHS and PuDS are determined from PCT data in the ranges 350-625°C for gallium alloyed Pu and 400-575°C for unalloyed Pu. The solubility of high purity plutonium alloyed with 2 at.% gallium is compared to high purity unalloyed plutonium. Significant differences are found in hydrogen solubility for unalloyed Pu versus gallium alloyed Pu. Differences in hydrogen solubility due to an apparent phase change are observable in the alloyed and unalloyed solubilities. The effect of iron impurities on Pu-Ga alloyed Pu is shown via hydrogen solubility data as preventing complete homogenization.

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John W. Ward

Papers

Electronic structure and bonding in transuranics: Comparison with lanthanides☆

Thermochemical studies on the plutonium fluorides and protactinium ovides

Electronic properties and structure of the plutonium-hydrogen system

Electronic structure, bonding and chemisorption in metallic hydrides

The solubility of hydrogen and deuterium in alloyed, unalloyed and impure plutonium metal