Metal Atom (Dis)Order and Superconductivity in YCaH_n (n = 8–20) High-Pressure Superhydrides

High-pressure superhydrides have attracted much attention due to their high superconducting critical temperatures (T_cs). Our density functional theory (DFT) calculations, focusing on YCaH_n (n = 8–20) compositions, found a number of nearly isoenthalpic YCaH₈ phases, differing only in the arrangement of the metal atoms, suggesting the potential stability of metal alloy superhydrides. The computed T_cs of the considered YCaH₈ phases were higher than those of the isostructural I4/mmm MH₄ parent compounds. DFT enthalpies suggested that YCaH₁₂ could also be disordered; however, the T_cs 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 Im3̅m MH₆ parents. For YCaH₁₈ and YCaH₂₀, only a single dynamically stable ordered superhydride was found, which we attribute to the differences in the structures of the most stable MH₉ and MH₁₀ binary hydride parents.