Pressure-induced novel nitrogen-rich aluminum nitrides: AlN₆, Al₂N₇and AlN₇with polymeric nitrogen chains and rings

Pressure-induced non-molecular phases of polymeric nitrogen have potential applications in the field of energetic materials. Here, through a structural search method combined with first-principles calculations, we have predicted four novel nitrogen-rich aluminum nitrides C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7. Nitrogen atoms polymerize into infinite chains in C2/m-AlN6, C2-Al2N7 and P1-AlN7 structures and form N3 chains and N4 rings in Cm-Al2N7. C2/m-AlN6 is stable in the pressure range from 30 to 80 GPa and Cm-Al2N7, C2-Al2N7 and P1-AlN7 are metastable in the pressure ranges of 35-65, 65-80 and 41-80 GPa, respectively. The present study predicts that C2/m-AlN6 has a superconducting transition temperature of 18.9 K at 0 GPa and can be quenched and recovered under ambient conditions. The energy densities of C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7 compounds are expected to be 4.80, 4.59, 5.46 and 5.59 kJ g-1, respectively, due to their high nitrogen content, indicating that they have potential to be high-energy density materials. The calculated Vickers hardness of C2/m-AlN6, Cm-Al2N7, Cm-Al2N7 and P1-AlN7 is 43.86, 39.32, 63.96 and 33.58 GPa, respectively, showing that the novel nitrogen-rich aluminum nitrides are potential superhard materials as well. This study may encourage further experimental exploration of high N content superhard or high-energy density nitrides.