Phase transition studies of Na₃Bi system under uniaxial strain.

We investigated the electronic properties and phase transitions of Na3Bi in four structural phases (space groups P63/mmc, P3(_)c1, Fm3(_)m and Cmcm) under constant-volume uniaxial strain using first-principles method. For P63/mmc and P3(_)c1-Na3Bi, an important phase transition from a topological Dirac semimetal (TDS) to a topological insulator (TI) appears under compression strain around 4.5%. The insulating gap increases with the increasing compressive strain and up to around 0.1eV at strain 10%. However, both P63/mmc and P3(_)c1-Na3Bi still keep the properties of TDS within tensile strain 0~10%, although the Dirac points move away from the  point along -A in reciprocal space as tensile strain increases. The Na3Bi with space group Fm3(_)m is identified as a topological semimetal (TS) with the inverted bands between Na-3s and Bi-6p and parabolic dispersion in the vicinity of  point. Interestingly, for Fm3(_)m-Na3Bi, both compression and tensile strain leads to a TDS which are identified by calculating surface Fermi arcs and topological invariants at time-reversal planes (kz=0 and kz=/c) in reciprocal space. Additionally, we confirmed the high pressure phase Cmcm-Na3Bi is an ordinary insulator with a gap about 0.62eV. It is noteworthy that its gap almost keeps a constant around 0.60eV within compression strain 0~10%. In contrast, a remarkable phase transition from an insulator to a metal phase appears under tensile strain. Moreover, this phase transition is highly sensitive to tensile strain and takes place only strain 1.0%. These strain-induced electronic structures and phase transitions of Na3Bi system in various phase are important due to their possible applications under high pressure in future electronic devices.