Ordering of nitrogen in nickel nitride Ni3N determined by neutron diffraction

The metallic interstitial nitride Ni3N was prepared from Ni(NH3)(6)Cl-2 and NaNH2 in supercritical ammonia (p(NH3) approximate to 2 kbar) at 523 K. Its previously reported crystal structure, as determined from X-ray powder data, was confirmed by neutron powder diffraction: Ni3N crystallizes in the hexagonal epsilon -Fe3N-type structure (P6(3)22, Z = 2, a = 4.6224 Angstrom and c = 4.3059 Angstrom at room temperature). The N atoms on the octahedral sites of an hcp arrangement of Ni show virtually complete occupational order at ambient temperatures, which is preserved up to its thermal decomposition at T approximate to 600 K. This behavior is in marked contrast to that of the isotypic iron nitride, epsilon -Fe3N, which shows reversible partial disordering within the same range of temperatures. Possible reasons for the different behaviors of the two nitrides epsilon -Fe3N and Ni3N are discussed.