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

High-pressure superhydrides have attracted much attention due to their high superconducting critical temperatures (Tcs). Our density functional theory (DFT) calculations, focusing on YCaHn (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 Tcs 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 Tcs 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 Tc 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.