  Metal borides have been considered as potential high-temperature
superconductors since the discovery of record-holding 39 K superconductivity in
bulk MgB2. In this work, we identified a superconducting yet thermodynamically
stable F43m Sr2B5 at 40 GPa with a unique covalent sp3-hybridized boron
framework through extensive first-principles structure searches. Remarkably,
solving the anisotropic Migdal-Eliashberg equations resulted in a high
superconducting critical temperature (Tc) around 100 K, exceeding the boiling
point (77 K) of liquid nitrogen. Our in-depth analysis revealed that the
high-temperature superconductivity mainly originates from the strong coupling
between the metalized {\sigma}-bonded electronic bands and E phonon modes of
boron atoms. Moreover, anharmonic phonon simulations suggest that F43m Sr2B5
might be recovered to ambient pressure. Our current findings provide a
prototype structure with a full {\sigma}-bonded boron framework for the design
of high-Tc superconducting borides that may expand to a broader variety of
lightweight compounds.
