Theoretical Study of Phase Separation of Scandium Hydrides under High Pressure

We report theoretical calculations of the static ground-state structures and pressure-induced phase transformations of three scandium hydrides: ScH, ScH₂, and ScH₃. For the monohydride, ScH, we predict several phases to be more stable at 1 atm than the previously suggested rock-salt structure, in particular one of <i>P</i>4₂/<i>mmc</i> symmetry. The NaCl-type structure for ScH takes over at 10 GPa and dominates over a wide pressure range until it is replaced by a <i>Cmcm</i> structure around 265 GPa. Under pressure, the experimental <i>P</i> = 1 atm CaF₂-type structure of ScH₂ should transform to a <i>C</i>2/<i>m</i> structure around 65 GPa, which then is likely to disproportionate to NaCl-type ScH and face-centered cubic ScH₃ above 72 GPa. According to theory, as the pressure is elevated, ScH₃ moves through the following sequence of phases: <i>P</i>6₃ → <i>F</i><i>m</i>3̅<i>m</i> → <i>P</i>6₃/<i>mmc</i>(YH₃-type) → <i>Cmcm</i>; the corresponding transition pressures are calculated to be 29, 360, and 483 GPa, respectively. The predicted disproportionation tendencies of ScH₂ are fascinating: stable to decomposition to ScH and ScH₃ at low pressures, it should begin to disproportionate near 72 GPa. However, the process is predicted to reverse at still higher pressures (above 300 GPa). We also find ScH to be stable to disproportionation to Sc and ScH₂ above ∼25 GPa. The three hydrides are metallic, except for (at low pressures) ScH₃

New Insights into the Crystal Structures of Plutonium Hydrides from First-Principles Calculations

One of the important research contents on hydrogen corrosion of plutonium is the determination of the complex crystal structures of plutonium hydrides and the bonding interactions between plutonium and hydrogen. However, it is very difficult to carry out the structural characterization of plutonium hydrides because of their high activity, high toxicity, and radioactivity. In this work, the crystal structures, lattice vibrations, and bonding properties of plutonium hydrides under ambient pressure are investigated by means of the density functional theory + <i>U</i> approach. Results show that PuH₃ exhibits many competition phase structures. After considering spin polarization, strong correlation (<i>U</i>), and spin–orbit coupling effects on the total energy and lattice dynamics stability, it is found that PuH₃ at ambient pressure is more likely to be hexagonal <i>P</i>6₃<i>cm</i> or trigonal <i>P</i>3<i>c</i>1 structure, instead of the usual supposed structures of hexagonal <i>P</i>6₃/<i>mmc</i> (LaF₃-type) and face-centered cubic (BiF₃-type). The calculated electronic structures clearly indicate that <i>P</i>6₃<i>cm</i> (<i>P</i>3<i>c</i>1) PuH₃ is a semiconductor with a small band gap about 0.87 eV (0.85 eV). The Pu–H bonds in Pu hydrides are dominated by the ionic interactions

High Hydrides of Scandium under Pressure: Potential Superconductors

In a systematic investigation of scandium hydrides with high hydrogen content we predict seven phases of scandium hydrides (ScH_4, ScH₆, ScH₇, ScH₈, ScH₉, ScH₁₀, and ScH₁₂), which are stable above 150 GPa. Zero point energies are essential in determining the phases and pressure ranges within which they are stable. The interconversion of the various hydrides is intriguing; in one case there is a “return” to a lower hydrogen content hydride with increasing pressure. We argue that these hydrides may be synthesized by compressing mixtures of ScH₃ and H₂ above 150 GPa. New H bonding motifs are uncovered, including “H₅” pentagons or “H₈” octagons in ScH₉, ScH₁₀, and ScH₁₂. High <i>T</i>_cs are predicted for ScH₆, ScH₇, ScH₉, ScH₁₀, and ScH₁₂, with superconducting transition temperatures (<i>T</i>_cs) of 120–169 K above 250 GPa, as estimated by the Allen–Dynes modified McMillan equation