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

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 $ab$ $initio$ 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 $T_c$.

13 pages

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