A family of superconducting boron crystals made of stacked bilayer borophenes

 Monolayer borophenes tend to be easily oxidized, while thicker borophenes have stronger antioxidation  properties. Herein, we proposed four novel metallic boron crystals by stacking the experimentally syn-  thesized borophenes, and one of the crystals has been reported in our previous experiments. Bilayer units  tend to act as blocks for crystals as determined by bonding analyses. Their kinetic, thermodynamic and  mechanical stabilities are confirmed by our calculated phonon spectra, molecular dynamics and elastic  constants. Our proposed allotropes are more stable than the boron α-Ga phase below 1000 K at ambient  pressure. Some of them become more stable than the α-rh or γ-B28 phases at appropriate external  pressure. More importantly, our calculations show that three of the proposed crystals are phonon-  mediated superconductors with critical temperatures of about 5–10 K, higher than those of most super-  conducting elemental solids, in contrast to typical boron crystals with significant band gaps. Our study  indicates a novel preparation method for metallic and superconducting boron crystals dispensing with  high pressure.