New Cadmium–Nitrogen Compounds at High Pressures

Abstract

A systematic high-pressure study of the CdN<i>_x</i> (<i>x</i> = 2, 3, 4, 5, and 6) system is performed by using the first-principles calculation method in combination with the particle swarm optimization algorithm. We proposed four stable high-pressure phases (<i>P</i>4<i>mbm</i>-CdN₂, <i>Cmmm</i>-CdN₄, <i>I</i>4̅2<i>d</i>-CdN₄, and <i>C</i>2/<i>c</i>-CdN₅) and one metastable high-pressure phase (<i>C</i>2/<i>m</i>-CdN₆), for which the structural frames are composed of a diatomic quasi-molecule N₂, standard armchair N-chain, S-type bent armchair N-chain, zigzag-antizigzag N-chain, and N₁₄ network structure. Among them, the novel zigzag-antizigzag N-chain and N₁₄ network structure are reported for the first time. More importantly, <i>Cmmm</i>-CdN₄ and <i>C</i>2/<i>m</i>-CdN₆ possess high stability under ambient conditions, which may be quenched to ambient conditions once they are synthesized at high-pressure conditions. The high decomposition energy barrier (1.14 eV) results in a high decomposition temperature (2500 K) of <i>Cmmm</i>-CdN₄, while a low decomposition energy barrier (0.19 eV) results in a mild decomposition temperature (500 K) of <i>C</i>2/<i>m</i>-CdN₆. The high energy density and outstanding explosive performance make <i>Cmmm</i>-CdN₄, <i>I</i>4̅2<i>d</i>-CdN₄, <i>C</i>2/<i>c</i>-CdN₅, and <i>C</i>2/<i>m</i>-CdN₆ potential high-energy materials. The electronic structure analyses show that these predicted high-pressure structures are all metallic phases, and the N-N and Cd-N bonds are the strong covalent and ionic bond interactions, respectively. The charge transfer from the Cd atom plays an important role in the stability of the proposed structures.