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 ZnN 3 phases with the same space group P 1 (low-pressure phase LP-ZnN 3 and high-pressure phase HP-ZnN 3 ), Cmm 2-ZnN 5 and Pcc 2-ZnN 6 , 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-ZnN 3 (54-72 GPa) and HP-ZnN 3 (above 72 GPa) have the lowest enthalpies in all known ZnN 3 phases, and the N 6 chains in LP-ZnN 3 polymerize into infinite nitrogen chains in HP-ZnN 3 at 72 GPa, showing a narrow-band-gap-semiconductor to metallic phase transition. Interestingly, P 1-ZnN 3 has a superconducting transition temperature of 6.2 K at 50 GPa and 16.3 K at 100 GPa. In Cmm 2-ZnN 5 and Pcc 2-ZnN 6 , 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.