Pressure induced semiconductor-semimetal-superconductor transition of magnesium hexaborides

A thorough structural exploration was performed for MgB 6 combining the global structure searching method with first-principles calculations. Besides the known Cmcm phase, new phases, i.e. I 4/ mmm , C 2/ m -I, C 2/ m -II and P 2 1 / m , were predicted to be stable in the pressure range of 18-100 GPa. Unexpectedly, Cmcm -MgB 6 was found to be a semiconductor with an indirect band gap of 0.38 eV with the HSE06 functional, in good agreement with the experimental finding. I 4/ mmm -MgB 6 stabilized above 18 GPa exhibits semimetallic behaviour with a topological node-line near the Fermi level. Consequently, C 2/ m -I MgB 6 with a sandwich structure similar to MgB 2 is predicted to be a superconductor with a critical temperature ( T c ) of 9.5 K. By analysing the electronic structure, the intriguing semiconductor-semimetal-superconductor transition may be ascribed to the delocalization of more B-p electrons in the boron sublattice. The novel functions uncovered for MgB 6 may inspire more efforts to discover materials with intriguing properties. The semiconductor → semimetal → superconductor transition of MgB 6 induced by the B-p electron delocalization.