High-<i>T</i>_c Superconducting Hydrides Formed by LaH₂₄ and YH₂₄ Cage Structures as Basic Blocks

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 structural stabilities of (LaH$_6$)(YH$_6$)$_y$ series as high-$T_c$ hydride compositions using genetic algorithm $\it ab$ initio$ calculations. Our evaluation showed that $Pm\bar{3}m$-LaYH$_{12}$ reported in 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$_{12}$ $Cmmm$-LaY$_3$H$_{24}$ ($y=3$), $R\bar{3}$-LaY$_3$H$_{24}$ showing stability against such decomposition. While $R\bar{3}c$ ($y=1$) $R\bar{3}$ ($y=3$) did not exhibit superconductivity owing to extremely low density states at Fermi level, $Cmmm$ phases exhibited $T_{c}$ approximately 140~K around 200~GPa high electron--phonon coupling constant ($\lambda$ = 1.876 for LaYH$_{12}$). By twice longer stacking $Cmmm$-LaY$_3$H$_{24}$, increased chemical pressure Y, leading slightly $T_{c}$.