We have performed a systematic study on the crystal structures and electronic properties of two ternary hydrides, ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$, under pressure, using the particle swarm optimization method and first-principles calculations. As a result of extensive structure searches, metallic ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$ are thermodynamically stable between 195–237 and 170–300 GPa, respectively. Interestingly, in ${\mathrm{YSH}}_6$ eight neighboring hydrogen atoms form octagons, and the octagons in different layers are connected by four sulfur atoms, forming a cagelike structure with a Y atom at the center, while those octagons in the same layer form polyphenylene-like chains via one shared side. In ${\mathrm{LaSH}}_6$, however, hydrogen atoms form both curved “${\mathrm{H}}_5$” chains or straight chains when bonded to sulfur atoms. Furthermore, electron-phonon coupling calculations indicate that ${\mathrm{YSH}}_6$ and ${\mathrm{LaSH}}_6$ are promising superconductors with estimated $T_c$ values of 91 and 35 K at 210 and 300 GPa, respectively. These results provide guidance for future experimental studies and stimulate more exploration on ternary hydrides.

• Received 29 June 2019

DOI:https://doi.org/10.1103/PhysRevB.100.184502