Ordering of nitrogen in nickel nitride Ni(3)N determined by neutron diffraction

The metallic interstitial nitride Ni(3)N was prepared from Ni(NH(3))(6)Cl(2) and NaNH(2) in supercritical ammonia (p(NH(3)) approximately 2 kbar) at 523 K. Its previously reported crystal structure, as determined from X-ray powder data, was confirmed by neutron powder diffraction: Ni(3)N crystallizes in the hexagonal epsilon-Fe(3)N-type structure (P6(3)22, Z = 2, a = 4.6224 A and c = 4.3059 A 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 approximately 600 K. This behavior is in marked contrast to that of the isotypic iron nitride, epsilon-Fe(3)N, which shows reversible partial disordering within the same range of temperatures. Possible reasons for the different behaviors of the two nitrides epsilon-Fe(3)N and Ni(3)N are discussed.