The structure and electronic properties of Zr and Hf nitrides and oxynitrides

The atomic structure, stability and electronic properties of various phases of zirconium and hafnium nitrides in both metallic MN and insulator M3N4 forms, and oxynitrides M2N2O (M = Zr, Hf) were studied using first-principles plane-wave DFT calculations. The orthorhombic Pnam structure of M3N4, which was experimentally observed for zirconium nitride, was found to be the most stable with regard to the rock-salt-type structure often offered for such compounds. The total energy calculations showed that the nitridation of zirconium and hafnium oxides is thermodynamically unfavorable, and the formation of nitrogen vacancies in M3N4 converts it into the metallic MN phase. Calculations of the electronic density of states showed that the rock-salt type structure of zirconium and hafnium nitrides leads to their metallic properties in both the MN and M3N4 phases, while the orthorhombic structure of the M3N4 phase reveals its insulating nature in agreement with the experimental observations. Calculations of the electronic structure of zirconium and hafnium oxynitrides with the cubic Bixbyite-type crystal structure found in a recent experimental study demonstrate that both Zr2N2O and Hf2N2O are insulators with large energy band gaps.