  Boron-based clathrate materials, typically with three-dimensional networks of
B atoms, have tunable properties through substitution of guest atoms, but the
tuning of B cages themselves has not yet been developed. By combining crystal
structural search with the laser-heated diamond anvil cell technique, we
successfully synthesized a new B-based clathrate boride, LaB8, at ~108 GPa and
~2100 K. The novel structure has a B-richest cage, with 26 B atoms
encapsulating a single La atom. LaB8 demonstrates phonon-mediated
superconductivity with an estimated transition temperature of 14 K at ambient
pressure, mainly originating from electron-phonon coupling of B cage. This work
creates a prototype platform for subsequent investigation on tunable electronic
properties through the choice of captured atoms.
