High-$T_c$ superconducting hydrides formed by LaH$_{24}$ and YH$_{24}$ cage structures as basic blocks (2103.06690v2)

Abstract: Based on recent studies regarding high-temperature (high-$T_c$) La-Y ternary hydrides (e.g., $P{\bar{1}}$-La$2$YH${12}$, $Pm{\bar{3}}m$-LaYH${12}$, and $Pm{\bar{3}}m$-(La,Y)H${10}$ with a maximum $T_c \sim 253$ K), we examined the phase and structural stabilities of the (LaH$6$)(YH$_6$)$_y$ series as high-$T_c$ ternary hydride compositions using a genetic algorithm and $\it ab$ $\it initio$ calculations. Our evaluation showed that the $Pm\bar{3}m$-LaYH${12}$ reported in the previous study was unstable during decomposition into $R\bar{3}c$-LaH${6}$ + $Im\bar{3}m$-YH${6}$. We also discovered new crystal structures, namely $Cmmm$-LaYH${12}$ ($y=1$), $R\bar{3}c$-LaYH${12}$ ($y=1$), $Cmmm$-LaY$3$H${24}$ ($y=3$), and $R\bar{3}$-LaY$3$H${24}$ ($y=3$), showing stability against such decomposition. While $R\bar{3}c$ ($y=1$) and $R\bar{3}$ ($y=3$) did not exhibit superconductivity owing to the extremely low density of states at the Fermi level, $Cmmm$ phases exhibited a $T_{c}$ of approximately 140~K at around 200~GPa owing to the extremely high electron--phonon coupling constant ($\lambda$ = 1.876 for LaYH${12}$). By the twice longer stacking for $Cmmm$-LaY$_3$H${24}$, the coupling constant increased owing to the chemical pressure of Y, leading to a slightly increased $T_{c}$.