Synthesis of hexagonal Ni(3)N using high pressures and temperatures.

The only known bulk ambient pressure nickel nitride phase is hexagonal Ni(3)N (space group P6(3)22). Multianvil synthesis experiments at 20 GPa and 2000 K using nickel (Ni) and sodium azide (NaN(3)) starting materials, and ex situ analysis using transmission electron microscopy and scanning electron microscopy measurements show that this phase can be recovered at ambient pressure (space group P6(3)22, a = 4.62 Å, c = 4.30 Å, Z = 2). Formation of this phase is correlated with the repulsive interactions between closely spaced nitrogen ions and with the extent of thermal stability of nickel nitride at ambient and at high densities. These two factors are also important in relating the high temperature and pressure behaviour of nickel nitride to those of several other interstitial nitrides recovered from similar pressures after heating. Further, we report formation of a sodium rhenium nitride phase by reaction of the azide with the rhenium capsule in which the reactants were contained.