Metal Atom (Dis)Order and Superconductivity in YCaH _<i>n</i> ( <i>n</i> = 8–20) High-Pressure Superhydrides

High-pressure superhydrides have attracted much attention due to their high superconducting critical temperatures ( T c s). Our density functional theory (DFT) calculations, focusing on YCaH n ( n = 8–20) compositions, found a number of nearly isoenthalpic YCaH 8 phases, differing only in the arrangement of the metal atoms, suggesting the potential stability of metal alloy superhydrides. The computed T c s of the considered YCaH 8 phases were higher than those of the isostructural I 4/ mmm MH 4 parent compounds. DFT enthalpies suggested that YCaH 12 could also be disordered; however, the T c s of the ordered variants spanned a wide range from 105 to 253 K at 200 GPa, showing that alloying could either mildly enhance or drastically reduce T c from that of the Im 3̅ m MH 6 parents. For YCaH 18 and YCaH 20, only a single dynamically stable ordered superhydride was found, which we attribute to the differences in the structures of the most stable MH 9 and MH 10 binary hydride parents.