A first principles investigation of the electronic, mechanical and optical properties of XPbN2 (X=Mg, Zn)

The electronic, mechanical, optical and conductive properties of MgPbN2 and ZnPbN2 are investigated using hybrid-functional first-principles calculations. Our calculations show that these two compounds preferentially stabilize in the tetragonal chalcopyrite structure. The calculated phonon dispersions show that both compounds are thermodynamically stable. From the calculated elastic constants we infer that the compound are also mechanically stable. As for the electronic properties, MgPbN2 is computed to be a semiconductor with a direct band gap of 1.071 eV at the Gamma. point, while ZnPbN2 is semi-metallic with the valence band and conduction band crossing at the Gamma point. Both MgPbN2 and ZnPbN2 show an enhanced optical absorption and electric conductivity compared to those of the related semiconductors MgSnN2 and ZnSnN2. ZnPbN2 gives a much stronger electric conductivity and a slightly larger light absorption than MgPbN2. In addition the influence of alloying is investigated for Mg1-xZnxPbN2 at different doping amount. When Mg was doped at 25% level, a band gap of 0.22 eV opens up and turns the system to semiconducting. When Sn was doped at Pb site, a similar phenomena was observed. On the other hand, the electric conductivity will increase with a small amount of Zn doping in MgPbN2. With large amount of doping, a conductivity higher than that of pristine ZnPbN2 is achieved. Our investigation widens the knowledge of II-IV-N-2 family of ternary nitrides, and may hence help to boost their applications.