Semiconductors with a chiral crystal structure in group IVB transition metal pernitrides

Group IVB transition metal (TM) nitrides rarely exhibit the semiconductor phase, except for TM₃N₄ (TM = Ti, Zr, and Hf) compounds. In this study, using the ab initio calculations based on density functional theory, we report two chiral crystal structures, namely P3₁21 and P3₂21, of TMN₂, which are dynamically stable at ambient pressure. Unlike conventional metal phases of transition metal dinitrides, the P3₁21 and P3₂21 configurations exhibit intriguing semiconductor properties (with bandgaps of 1.076 eV, 1.341 eV, and 1.838 eV for TiN₂, ZrN₂, and HfN₂, respectively). The mechanism of metal-to-semiconductor transition from the I4/mcm to P3₁21 phase is deeply explored by investigating their crystal structure and electronic structures. When hydrostatic pressure is applied from 0 GPa to 200 GPa, the bandgaps of the P3₁21 phase of TiN₂, ZrN₂, and HfN₂ exhibit a different response, which is related to the orbital contribution at the conduction band minimum (CBM) and valence band maximum (VBM) and the lattice constants. Furthermore, according to the calculated mechanical properties, P3₁21 and P3₂21 phases exhibit higher bulk and shear moduli than the semiconductor phases of c-Zr₃N₄ and c-Hf₃N₄ in the corresponding systems.