Cobalt–Nitrogen Compounds at High Pressure

Abstract

The high-pressure phase diagram of Co-N compounds is enriched by proposing five stable phases (<i>Pnnm</i>-Co₂N, <i>Pmn</i>2₁-Co₂N, <i>Pmna</i>-CoN, <i>Pnnm</i>-CoN₂, and <i>P</i>1̅-CoN₄) and two metastable phases (<i>P</i>3̅1<i>c</i>-CoN₈ and <i>P</i>1̅-CoN₁₀). A systematic study has been performed for revealing the novel polymeric nitrogen structure and the outstanding properties of predicted polynitrides, such as structural characterization, energy analysis, stability analysis, and electronic analysis. <i>P</i>3̅1<i>c</i>-CoN₈ with the novel layer-shaped N-structure and <i>P</i>1̅-CoN₁₀ with the novel band-shaped N-structure are first reported in this work. Moreover, <i>P</i>3̅1<i>c</i>-CoN₈ (6.14 kJ/g) and <i>P</i>1̅-CoN₁₀ (5.18 kJ/g) with high energy density can be quenched down to ambient conditions. The proposed seven high-pressure phases are all metallic phases. A weak ionic bond interaction is observed between the Co and N atoms, while a strong N-N covalent bond interaction is observed in the <i>Pnnm</i>-CoN₂, <i>P</i>1̅-CoN₄, <i>P</i>3̅1<i>c</i>-CoN₈, and <i>P</i>1̅-CoN₁₀ phases. The N atoms in the polynitrides hybridize in the sp² state, for which the hybrid orbitals are constructed by the σ bond or lone electronic pair. The charge transfer between the Co and N atoms plays an important role to the structural stability. Moreover, the vibrational analysis of <i>P</i>3̅1<i>c</i>-CoN₈ and <i>P</i>1̅-CoN₁₀ phases is performed to guide the future experimental study.