Computational Defect Study to Incorporate Fission Products into Zirconium Nitride*

We present a computational study of defects in Zirconium Nitride (ZrN). Zirconium was chosen as a surrogate for Uranium due to our experimental collaborator’s transition to synthesize ZrN over Uranium Nitride (UN). Our first-principles density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP). A ZrN supercell was used to create various defect structures including vacancies, di-vacancies, Schottky, Frenkel and interstitials. Two sets of reference energies were used for Zr and N with one intermediate set to account for Zr-rich and N-rich conditions. The Schottky defect was the most favorable followed by the N tetrahedral interstitial, and individual Zr and N vacancies. Analysis of the density of states (DOS) and charge density for each defect reveals that the loss of strong Zr-N covalent bonds changes the cell volume and charge distribution, with Zr vacancies causing cell contraction and N vacancies causing expansion. These findings suggest that the ZrN crystal will favorably accommodate Molybdenum (Mo) at vacancy sites over interstitial sites due to the atomic size similarity of Mo and Zr. We found the Mo inside a Schottky defect to be the most stable structure, where it took the place of the Zr vacancy. The Mo substitution of a Zr atom was the next favorable site while Mo interstitials and Mo substitution of N atoms were found to be unfavorable. The high formation energies of Zr interstitials and size similarity of Zr and Mo, indicate that the behavior of fission products in ZrN will be dependent on their atomic size.