Novel polymerization of nitrogen in zinc nitrides at high pressures

Nitrogen-rich compounds containing polynitrogen are attractive candidates for high-energy-density materials. In this work, using first-principles calculations and a particle swarm optimization structural search method, four novel nitrogen-rich structures are predicted at high pressures, i.e., two ZnN₃ phases with the same space group P1 (low-pressure phase LP-ZnN₃ and high-pressure phase HP-ZnN₃), Cmm2-ZnN₅ and Pcc2-ZnN₆, the energy density are estimated to be 1.41 kJ g⁻¹, 1.88 kJ g⁻¹, 4.07 kJ g⁻¹, and 2.60 kJ g⁻¹, respectively. LP-ZnN₃ (54–72 GPa) and HP-ZnN₃ (above 72 GPa) have the lowest enthalpies in all known ZnN₃ phases, and the N₆ chains in LP-ZnN₃ polymerize into infinite nitrogen chains in HP-ZnN₃ at 72 GPa, showing a narrow-band-gap-semiconductor to metallic phase transition. Interestingly, P1-ZnN₃ has a superconducting transition temperature of 6.2 K at 50 GPa and 16.3 K at 100 GPa. In Cmm2-ZnN₅ and Pcc2-ZnN₆, nitrogen atoms polymerize into three-dimensional network structures and network layers under high pressures. Those predicted structures may enrich the phase diagram of high-pressure zinc nitrides, and provide clues for synthesis and exploration of novel stable polymeric nitrogen.