Hidden thermodynamic ground state of calcium diazenide

Abstract

A new member to the family of alkaline earth diazenides, calcium diazenide (CaN2), was recently synthesized by a controlled decomposition of calcium azide at high-pressure/high-temperature conditions (Schneider et al. Inorg. Chem. 2012, 51, 2366-2373). Isotypic to tetragonal calcium carbide, the synthesized CaN2 phase adopts a tI6 structure in which Ca 2+ cations and [N2]2- anions are arranged in a distorted rocksalt structure. Using an unbiased structure searching method, we investigate the energy landscape of CaN2 at the first principles level and suggest that the tI6 structure is a metastable form of CaN 2. A new orthorhombic oP12 structure, formed by a distorted bcc lattice of Ca2+ cations interpenetrated by [N2] 2- anions, is predicted to be the thermodynamic ground state of CaN2. The energy of the oP12 structure is considerably lower than that of the synthesized phase, by ∼80 meV/CaN2, while it is also lower than the energies of the candidate structures previously considered. The oP12 structure remains as the thermodynamic ground state of CaN2 at high pressures up to 2.9 GPa, at where it undergoes a first-order phase transition to a higher density oP12′ structure. Above 2.9 GPa, the synthesized tI6 phase becomes the thermodynamic ground state of CaN2. The formation of the oP12 structure is predicted to be exothermic which suggests that this structure may be synthesized in future experiments with proper experimental conditions. © 2013 American Chemical Society.