Initial decomposition mechanisms and the inverse effects of temperature and PH2 on the thermodynamic stability of UH3.

The thermodynamic stability of uranium hydrides is of broad interest and fundamental importance for understanding the hydriding corrosion of uranium, and the storage and isotope separation of hydrogen. Based on the first-principles calculations, we reveal the initial decomposition mechanism, interpret the experimental pyrolysis results, and discuss the inverse effects of temperature and hydrogen pressure (PH2) on the thermodynamic stability of ß-UH3. The decomposition mechanism of ß-UH3 is found to be closely related to the changes of U-H bonding properties in UH12 cages. Specifically, at the beginning it is difficult to break the first U-H covalent bond in each UH12 cage, which brings in the existence of a concave region in the experimental PH2-C-T curve; however, it boosts the itinerant character of U-5f electrons. Thereafter, the formation energy of H-vacancies in the degraded UH11 cages is almost changeless when the H/U atom ratio decreases, resulting in the van't Hoff plateau of the PH2-C-T curve. Based on the above mechanisms, we propose a theoretical method to evaluate the thermodynamic stability of ß-UH3. The calculated PH2-C-T curve is consistent with experiment, showing that temperature promotes ß-UH3 decomposition and PH2 plays an opposite role. Moreover, this method is independent of experimental calibration and is applied to discuss the isotope effect of hydrogen in ß-UH3. This work provides new insight and a practical method for the scientific studies of uranium hydride, which is also essential to industrial applications in hydrogen isotope separation.

Novel Valence Transition in Elemental Metal Europium around 80 GPa.

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Equations of state and transport properties of warm dense beryllium: a quantum molecular dynamics study.

We have calculated the equations of state, the viscosity and self-diffusion coefficients, and electronic transport coefficients of beryllium in the warm dense regime for densities from 4.0 to 6.0 g/cm(3) and temperatures from 1.0 to 10.0 eV by using quantum molecular dynamics simulations. The principal Hugoniot curve is in agreement with underground nuclear explosive and high-power laser experimental results up to ~20 Mbar. The calculated viscosity and self-diffusion coefficients are compared with the one-component plasma model, using effective charges given by the average-atom model. The Stokes-Einstein relationship, which connects viscosity and self-diffusion coefficients, is found to hold fairly well in the strong coupling regime. The Lorenz number, which is the ratio between thermal and electrical conductivities, is computed via Kubo-Greenwood formula and compared to the well-known Wiedemann-Franz law in the warm dense region.