Clathrate-Like Alkali and Alkaline-Earth Metal Borides: A New Family of Superconductors with Superior Hardness

Conventional hard and superhard materials, such as diamond and cubic boron nitride, are attractive for fundamental material science and practical industrial application, but severely limited by their poor electrical conductivity. Therefore, it is desirable to design and fabricate novel materials for superior hardness and conductivity. Herein, a class of hard superconductors in alkali or alkaline-earth metal (AM) borides, namely AMB(7), constituted by a B-23 cage with one centered metal atom (Li, Na, K, Mg, Ca, and Sr) is reported, which is the first stable clathrate structure in AMB systems. The theoretical calculations demonstrate that all these pressure-stabilized clathrate structures can be quenched down to ambient conditions, which provides an essential prerequisite for experimental synthesis at moderate pressures. Among them, the highest hardness and maximum superconducting transition temperature (T-c) value are achieved in SrB7 (25.1 GPa) and MgB7 (29.3 K), respectively. Interestingly, the results show that KB7 simultaneously behaves high hardness (22.5 GPa) and superconducting transition temperature (T-c approximate to 26.2 K). This study opens up a new way to search and design novel superconductors with favorable mechanical properties under high pressure and high-temperature conditions.