Cobalt-Nitrogen Compounds at High Pressure.

The high-pressure phase diagram of Co-N compounds is enriched by proposing five stable phases (-CoN, 2-CoN, -CoN, -CoN, and 1̅-CoN) and two metastable phases (3̅1-CoN and 1̅-CoN). 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. 3̅1-CoN with the novel layer-shaped N-structure and 1̅-CoN with the novel band-shaped N-structure are first reported in this work. Moreover, 3̅1-CoN (6.14 kJ/g) and 1̅-CoN (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 -CoN, 1̅-CoN, 3̅1-CoN, and 1̅-CoN phases. The N atoms in the polynitrides hybridize in the sp 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 3̅1-CoN and 1̅-CoN phases is performed to guide the future experimental study.