  Inspired by the synthesis of the high-pressure Fm-3m LaH10 superconducting
superhydride, systematic density functional theory (DFT) calculations are
performed to study ternaries that could be derived from it by replacing two of
the hydrogen atoms with boron or carbon and varying the identity of the
electropositive element. Though many of the resulting alkali-metal and
alkaline-earth MC2H8 phases are predicted to be dynamically stable at mild
pressures, their superconducting critical temperatures (Tcs) are low because
their metallicity results from the filling of an electride-like band.
Substitution with a trivalent element leads to phases with substantial metal
d-character at the Fermi level whose Tcs are typically above 40 K. Among the
MB2H8 phases examined, KB2H8, RbB2H8 and CsB2H8 are predicted to be dynamically
stable at very mild pressures, and their stability is rationalized by a
DFT-Chemical Pressure analysis that elucidates the role of the M atom size.
Quantum anharmonic effects strongly affect the properties of KB2H8, the highest
predicted Tc compound, near 10 GPa, but molecular dynamics simulations reveal
it would decompose below its Tc at this pressure. Nonetheless, at ca. 50 GPa
KB2H8 is predicted to be thermally stable with a superconducting figure of
merit surpassing that of the recently synthesized LaBeH8.
