Prediction of superconductivity in pressure-induced new silicon boride phases

The crystal structures and properties of boron-silicon (B-Si) compounds under pressure have been systematically explored using particle swarm optimization structure prediction method in combination with first-principles calculations. Three new stoichiometries, ${\mathrm{B}}₂\mathrm{Si}$, BSi, and ${\mathrm{BSi}}₂$, are predicted to be stable gradually under pressure, where increasing pressure favors the formation of silicon rich B-Si compounds. In the boron-rich compounds, the network of boron atoms changes from ${\mathrm{B}}₁₂$ icosahedron in the ambient phases to the similar buckled graphenelike layers in the high-pressure phases, which crystalize in the same $P\overline{3}m1$ symmetry but with different numbers of boron layers between adjacent silicon layers. Phonon calculations show that these structures might be retained to ambient conditions as metastable phases. Further electron-phonon coupling calculations indicate that the high-pressure phases of boron-rich compounds might superconduct at 1 atm, with the highest ${T}_c$ value of 21 K from the Allen-Dynes equation in $P\overline{3}m1$ ${\mathrm{B}}₂\mathrm{Si}$, which is much higher than the one observed in boron doped diamond-type silicon. Moreover, further fully anisotropic Migdal-Eliashberg calculations indicate that ${\mathrm{B}}₂\mathrm{Si}$ is a two-gap anisotropic superconductor and the estimated ${T}_c$ might reach up to 30 K at 1 atm. On the silicon-rich side, ${\mathrm{BSi}}₂$ is predicted to be stable in the ${\mathrm{CuAl}}₂$-type structure. Our current results significantly enrich the phase diagram of the B-Si system and will stimulate further experimental study.