Cobalt-Nitrogen Compounds At High Pressure

The high-pressure phase diagram of Co-N compounds is enriched by proposing five stable phases (Pnnm-Co2N, Pmn2(1)-Co2N, Pmna-CoN, Pnnm-CoN2, and P (1) over bar -CoN4) and two metastable phases (P (3) over bar -CoN8 and P (1) over bar -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. P (3) over bar 1c-CoN8 with the novel layer-shaped N-structure and P (1) over bar -CoN10 with the novel band-shaped N-structure are first reported in this work. Moreover, P (3) over bar 1c-CoN8 (6.14 kJ/g) and P (1) over bar -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 Pnnm-CoN2, P (1) over bar -CoN4, P (3) over bar 1c-CoN8, and P (1) over bar -CoN10 phases. The N atoms in the polynitrides hybridize in the sp(2) state, for which the hybrid orbitals are constructed by the sigma 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 P (1) over bar 1c-CoN8 and P (1) over bar -CoN10 phases is performed to guide the future experimental study.