Prediction of pressure-induced superconductivity in the ternary systems YScH2n (n=3–6)

Hydrogen-rich ternary compounds are promising candidates for realizing room-temperature superconductivity due to the synergistic effects of crystal structure and electronic properties under high-pressure conditions. Here, the high-pressure structures, electronic properties, and superconductivity of the ternary ${\mathrm{YScH}}_{2n}$ ($n=3$--6) system are investigated by using the prediction method of particle swarm optimization structure combined with first-principles calculations. We find four stable structures, each with different hydrogen sublattices: $\mathit{Pm}\overline{3}\text{\ensuremath{-}}{\mathrm{YScH}}_{6}, P4$/mmm-${\mathrm{YScH}}_{8}$, Cmmm-${\mathrm{YScH}}_{10}$, and $\mathit{Pm}\overline{3}m\text{\ensuremath{-}}{\mathrm{YScH}}_{12}$. All these ${\mathrm{YScH}}_{2n}$ structures are predicted to be high-temperature superconductors. The electron local function (ELF) results indicate a lack of interaction between hydrogen atoms in ${\mathrm{YScH}}_{6}$, while the weak H-H covalent interactions are observed in the other stoichiometric ratios. Strikingly, ${\mathrm{YScH}}_{6}$ maintains dynamic stability down to ambient pressure and keeps a high superconducting critical temperature (${T}_{c}$) of 66 K. At 140 GPa, the pressure-stabilized ${\mathrm{YScH}}_{8}$ and ${\mathrm{YScH}}_{10}$ structures exhibit high ${T}_{c}$ of 110 and 116 K, respectively. Upon further increasing the content of hydrogen, the lowest dynamically stable pressure of ${\mathrm{YScH}}_{12}$ is increased to 200 GPa, and the calculated ${T}_{c}$ is up to 179 K. In all ${\mathrm{YScH}}_{2n}$ structures, $\mathit{Pm}\overline{3}\text{\ensuremath{-}}{\mathrm{YScH}}_{6}$ (stabled from 1 atm to 47 GPa), $P4$/mmm-${\mathrm{YScH}}_{8}$ and Cmmm-${\mathrm{YScH}}_{10}$ (stabled from 140 to 250 GPa), $\mathit{Pm}\overline{3}m\text{\ensuremath{-}}{\mathrm{YScH}}_{12}$ (stabled from 200 to 286 GPa), strong electron-phonon coupling (EPC) and large electronic density of states of hydrogen at the Fermi level play important roles in their high-temperature superconductivity. It is discussed that phonon softening in the midfrequency region induced mainly by Fermi surface nesting effectively enhances the EPC. In this paper, we potentially discover high-temperature superconducting hydrides that can be stable at atmospheric pressure, taking an important step toward understanding the superconductivity and structural stability of ternary hydrides.