  Hydrogen-based superconductors provide a route to the long-sought goal of
room-temperature superconductivity, but the high pressures required to
metallize these materials limit their immediate application. For example,
carbonaceous sulfur hydride, the first room-temperature superconductor, can
reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267
GPa. The next recognized challenge is the realization of room-temperature
superconductivity at significantly lower pressures. Here, we propose a strategy
for the rational design of high-temperature superconductors at low pressures by
alloying small-radius elements and hydrogen to form ternary hydride
superconductors with alloy backbones. We identify a hitherto unknown
fluorite-type backbone in compositions of the form AXH8, which exhibit high
temperature superconductivity at moderate pressures. The Fm-3m phase of LaBeH8,
with a fluorite-type H-Be alloy backbone, is predicted to be metastable and
superconducting with a Tc ~ 191 K at 50 GPa; a substantially lower pressure
than that required by the geometrically similar clathrate hydride LaH10 (170
GPa). Our approach paves the way for finding high-Tc ternary hydride
superconductors at conditions close to ambient pressures.
