Hafnium nitrides

The hafnium nitrides are the various salts produced from combining hafnium and nitrogen. The two most important such are hafnium(III) nitride, HfN; and hafnium(IV) nitride, Hf₃N₄. None can be prepared from hafnium oxide, but must instead be prepared from the elemental metal or a different hafnium nitride salt; attempted nitridation of the oxide gives an oxynitride instead.^[1]

HfN is refractory and generally produced as a thin film coating,^[2] although zone annealing gives the bulk material.^[3] HfN adopts the rock-salt crystal structure.^[2] The surplus hafnium electron delocalizes, so that HfN is a metal, conducting at room temperature and superconducting below 8.8 K (−443.83 °F). Its bright gold color is a cheaper alternative to gilding.^[4]

The dark red semiconductor Hf₃N₄ does not form at room temperature, but requires high pressure, high temperature synthesis in a diamond anvil cell. At 18 GPa (180,000 atm) and 2,800 K (4,580 °F), it adopts the cubic crystal structure and repeats according to space group I{{{1}}}3d.^[2] At lower pressures, the cubic structure is believed metastable, decaying to the orthorhombic structure of zirconium(IV) nitride.^[4][5] That structure forms outright at 19 GPa and 2,000 K (3,140 °F), and another metastable tetragonal structure forms at 12 GPa and 1,500 K (2,240 °F). Computational studies suggest that it may catalyze polymerization of nitrogen at very high temperatures, through a catenary anion in HfN₁₀.^[5]

In systems with limited nitrogen, hafnium also forms Hf₃N₂, as well as a solid solution hafnium alloy.^[6]