Topological semimetal to insulator quantum phase transition in the Zintl compounds B a2X (X =Si ,Ge )

By first-principles calculations, we find that B a₂X (X =Si ,Ge ) hosts a topological semimetal phase with one nodal ring in the k_x=0 plane, which is protected by the glide mirror symmetry when spin-orbit coupling (SOC) is ignored. The corresponding drumheadlike surface flat band appears on the (100) surface in surface Green's function calculation. Furthermore, a topological-semimetal-to-insulator transition is found. The nodal line semimetal would evolve into a topological insulator as SOC is turned on. The topologically protected metallic surface states emerge around the Γ ¯ point, which could be tuned into the topologically trivial insulator state by more than 3% hydrostatic strain. These results reveal a category of materials showing quantum phase transition between topological semimetal and insulator, and tunability through elastic strain engineering.