Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides...

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal Fd-3 m-Ge₃N₄ is induced by the binding effect of electronic clouds originated from the Ge_ p orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N₂ dimer in Pa-3-GeN₂ phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (N≡N) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g^-1, with a similar order of magnitude than polymeric LiN₅ (nonmolecular compound, 2.72 kJ·g^-1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the sp² hybridizations that exist in N₄ units are responsible for the stability of the R-3 c-GeN₄ structure and restrict the delocalization of electrons, exhibiting nonmetallic properties.