RbB₃Si₃: An Alkali Metal Borosilicide that is Metastable and Superconducting at 1 atm

The stability, electronic structure, and potential superconductivity in AB3Si3 (A = Na, K, Rb, and Cs) compounds that assume a clathrate-based sodalite structure whose framework consists of covalent B-Si bonds are investigated via first-principles calculations. This structure type has recently been predicted in a number of high-temperature superconducting hydrides, but these are only stable under megabar pressures. Herein, we predict a novel superconducting phase, RbB3Si3, that could be synthesized under pressures that are smaller by a factor of 10, ∼10 GPa, and quenched to atmospheric conditions. Electron-phonon coupling calculations predict that RbB3Si3 possesses a superconducting critical temperature, Tc, of 14 K at 1 atm. The dynamic stability of RbB3Si3 and CsB3Si3 at ambient pressure can be explained by considering the chemical pressure exerted on the B-Si framework that is caused by the size effect of the alkali metal atom.