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, P [Formula: see text] c1, Fm [Formula: see text] m and Cmcm) under constant-volume uniaxial strain using the first-principles method. For P6₃/mmc and P [Formula: see text] c1-Na₃Bi, an important phase transition from a topological Dirac semimetal (TDS) to a topological insulator appears under compression strain around 4.5%. The insulating gap increases with the increasing compressive strain and up to around 0.1 eV at a strain of 10%. However, both P6₃/mmc and P [Formula: see text] c1-Na₃Bi still keep the properties of a TDS within a tensile strain of 0-10%, although the Dirac points move away from the Γ point along Γ-A in reciprocal space as the tensile strain increases. The Na₃Bi with space group Fm [Formula: see text] m is identified as a topological semimetal with the inverted bands between Na-3s and Bi-6p and a parabolic dispersion in the vicinity of Γ point. Interestingly, for Fm [Formula: see text] m-Na₃Bi, both compression and tensile strain lead to a TDS which is identified by calculating surface Fermi arcs and topological invariants at time-reversal planes (k _z   =  0 and k _z   =  π/c) in reciprocal space. Additionally, we confirmed the high pressure phase Cmcm-Na₃Bi is an ordinary insulator with a gap of about 0.62 eV. It is noteworthy that its gap almost keeps constant around 0.60 eV within a compression strain of 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 at a strain 1.0%. These strain-induced electronic structures and phase transitions of the Na₃Bi system in various phases are important due to their possible applications under high pressure in future electronic devices.