ABSTRACT
Palladium hydrides (PdHx ) have important applications in hydrogen storage, catalysis, and superconductivity. Because of the unique electron subshell structure of Pd, quenching PdHx materials with more than 0.706 hydrogen stoichiometry remains challenging. Here, the 1:1 stoichiometric PdH ( F m 3 ¯ m ) $Fm\bar{3}m)$ is successfully synthesized using Pd nano icosahedrons as a starting material via high-pressure cold-forging at 0.2 GPa. The synthetic initial pressure is reduced by at least one order of magnitude relative to the bulk Pd precursors. Furthermore, PdH is quenched at ambient conditions after being laser heated ≈2000 K under ≈30 GPa. Corresponding ab initio calculations demonstrate that the high potential barrier of the facets (111) restricts hydrogen atoms' diffusion, preventing hydrogen atoms from combining to generate H2 . This study paves the way for the high-pressure synthesis of metal hydrides with promising potential applications.
ABSTRACT
The discoveries of high-temperature superconductivity in H_{3}S and LaH_{10} have excited the search for superconductivity in compressed hydrides, finally leading to the first discovery of a room-temperature superconductor in a carbonaceous sulfur hydride. In contrast to rapidly expanding theoretical studies, high-pressure experiments on hydride superconductors are expensive and technically challenging. Here, we experimentally discovered superconductivity in two new phases, Fm3[over ¯]m-CeH_{10} (SC-I phase) and P6_{3}/mmc-CeH_{9} (SC-II phase) at pressures that are much lower (<100 GPa) than those needed to stabilize other polyhydride superconductors. Superconductivity was evidenced by a sharp drop of the electrical resistance to zero and decreased critical temperature in deuterated samples and in external magnetic field. SC-I has T_{c}=115 K at 95 GPa, showing an expected decrease in further compression due to the decrease of the electron-phonon coupling (EPC) coefficient λ (from 2.0 at 100 GPa to 0.8 at 200 GPa). SC-II has T_{c}=57 K at 88 GPa, rapidly increasing to a maximum T_{c}â¼100 K at 130 GPa, and then decreasing in further compression. According to the theoretical calculation, this is due to a maximum of λ at the phase transition from P6_{3}/mmc-CeH_{9} into a symmetry-broken modification C2/c-CeH_{9}. The pressure-temperature conditions of synthesis affect the actual hydrogen content and the actual value of T_{c}. Anomalously low pressures of stability of cerium superhydrides make them appealing for studies of superhydrides and for designing new superhydrides with stability at even lower pressures.
ABSTRACT
Tailoring the inherent structure of materials is an effective way to improve the hydrogen storage capacity of metal materials. In this work, we report the effect of rhodium (Rh) nanocrystals (NCs) on the hydrogenation reaction. We found that Rh NCs could form rhodium monohydride (RhH) at a lower pressure than the bulk Rh because of its high specific surface area and structure defects. In addition, Rh NCs in the form of icosahedrons exhibited a much higher hydrogen absorption efficiency than Rh nanocubes. Furthermore, much smaller irregular Rh nanoparticles are even partially converted to RhH at lower pressure because of the nanosize effect. We thus believe that it is possible to design materials with excellent hydrogen storage properties under mild conditions.
ABSTRACT
Single crystal diamond produced by chemical vapor deposition (CVD) at very high growth rates (up to 150 microm/h) has been successfully annealed without graphitization at temperatures up to 2200 degrees C and pressures <300 torr. Crystals were annealed in a hydrogen environment by using microwave plasma techniques for periods of time ranging from a fraction of minute to a few hours. This low-pressure/high-temperature (LPHT) annealing enhances the optical properties of this high-growth rate CVD single crystal diamond. Significant decreases are observed in UV, visible, and infrared absorption and photoluminescence spectra. The decrease in optical absorption after the LPHT annealing arises from the changes in defect structure associated with hydrogen incorporation during CVD growth. There is a decrease in sharp line spectral features indicating a reduction in nitrogen-vacancy-hydrogen (NVH(-)) defects. These measurements indicate an increase in relative concentration of nitrogen-vacancy (NV) centers in nitrogen-containing LPHT-annealed diamond as compared with as-grown CVD material. The large overall changes in optical properties and the specific types of alterations in defect structure induced by this facile LPHT processing of high-growth rate single-crystal CVD diamond will be useful in the creation of diamond for a variety of scientific and technological applications.
