Pressure-stabilized hafnium nitrides and their properties

We report hafnium nitrides under pressure using first-principles evolutionary calculations. Metallic $P{6}_{3}/mmc$-HfN (calculated Vickers hardness 23.8 GPa) is found to be more energetically favorable than NaCl-type HfN at zero and high pressure. Moreover, NaCl-type HfN actually undergoes a phase transition to $P{6}_{3}/mmc$-HfN below 670 K at ambient pressure. ${\mathrm{HfN}}_{10}$, which simultaneously has infinite armchairlike polymeric N chains and ${\mathrm{N}}_{2}$ molecules in its crystal structure, is discovered to be stable at moderate pressure above 23 GPa and can be preserved as a metastable phase at ambient pressure. At ambient conditions (298 K, 0 GPa), the gravimetric energy densities and the volumetric energy densities of ${\mathrm{HfN}}_{10}$ are 2.8 kJ/g and 14.1 kJ/${\mathrm{cm}}^{3}$, respectively.