AB₃Si₃ (A= Na, K, Rb, Cs), the alkali metal borosilicide in sodalite structure with superconductivity under high pressure

Prediction of high-temperature superconductivity in clathrate-like hydrogen cage under high pressure has generated an irresistible interest of searches for clathrate-like superconductors. However, when the pressure is released, atomic hydrogens will decompose into H₂, accompanying the disappearance of superconductivity. We herein report the structural and physical properties of AB₃Si₃ (A= Na, K, Rb and Cs) in clathrate-based sodalite structure, whose frameworks consist of covalent B-Si bonds, by first-principles calculations. Our results show that KB₃Si₃ and RbB₃Si₃ are thermally stable within pressure ranges of 6.9-37.0 GPa and 7.3-34.2 GPa, respectively. Phonon calculations confirm that RbB₃Si₃ is dynamically stable at both ambient and high pressures, while KB₃Si₃ is not at ambient pressure. Electron-phonon calculations predict that RbB₃Si₃ possess a T_c of 14.5 K at ambient pressure. Moreover, RbB₃Si₃ is mechanically stable, and its estimated Vickers hardness values are between 11.8 – 15.2 GPa, which are in good agreement with the ideal strength results.