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 TM 3 N 4 (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 P 3 1 21 and P 3 2 21, of TMN 2 , which are dynamically stable at ambient pressure. Unlike conventional metal phases of transition metal dinitrides, the P 3 1 21 and P 3 2 21 configurations exhibit intriguing semiconductor properties (with bandgaps of 1.076 eV, 1.341 eV, and 1.838 eV for TiN 2 , ZrN 2 , and HfN 2 , respectively). The mechanism of metal-to-semiconductor transition from the I 4/ mcm to P 3 1 21 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 P 3 1 21 phase of TiN 2 , ZrN 2 , and HfN 2 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, P 3 1 21 and P 3 2 21 phases exhibit higher bulk and shear moduli than the semiconductor phases of c-Zr 3 N 4 and c-Hf 3 N 4 in the corresponding systems.