Computational Design of Novel Hydrogen-Rich YS-H Compounds

The recent successful findings of H₃S and LaH₁₀ compressed above 150 GPa with a record high T _c (above 200 K) have shifted the focus on hydrogen-rich materials for high superconductivity at high pressure. Moreover, some studies also report that transition-metal ternary hydrides could be synthesized at a relatively low pressure (∼10 GPa). Therefore, it is highly desirable to investigate the crystal structures of ternary hydrides compounds at high pressure since they have been long considered as promising superconductors and hydrogen-storage materials with a high T _c, and can be possibly synthesized at low pressure as well. In this work, combining state-of-the-art crystal structure prediction and first-principles calculations, we have performed extensive simulations on the crystal structures of YSH (n = 1-10) compounds from ambient pressure to 200 GPa. We uncovered three thermodynamically stable compounds with stoichiometries of YSH, YSH₂, and YSH₅, which became energetically stable at ambient pressure, 143, and 87 GPa, respectively. Remarkably, it is found that YSH contains monoatomic H atoms, while YSH₂ and YSH₅ contain a mixture of atomlike and molecular hydrogen units. Upon compression, YSH, YSH₂, and YSH₅ undergo a transition from a semiconductor to a metallic phase at pressures of 168, 143, and 232 GPa, respectively. Unfortunately, electron-phonon coupling calculations reveal that these compounds possess a weak superconductivity with a relatively low T _c (below 1 K), which mainly stem from the low value of density of states occupation at the Fermi level (E _F). These results highlight that the crystal structures play a critical role in determining the high-temperature superconductivity.