Binary and ternary interstitial alloys I. The iron-nitrogen system: the structures of Fe 4 N and Fe 2 N

Abstract

The ς-iron nitride phase (Fe 2 N), the existence of which is confirmed, is prepared by passing ammonia over iron at temperatures not exceeding 450° C and under such conditions that the partial pressure of hydrogen is negligible. The positions of the nitrogen atoms in y'(Fe 4 N) and in ς are determined. y' is a ‘normal’ 12 a 6 interstitial structure, the cubic unit cell of which contains four iron atomsat 0 0 0, 1/2 1/2 0, 1/2 0 1/2, 0 1/2 1/2, and one nitrogen atom at 1/2 1/2 1/2; at 6.1 weight % N, a = 3.787 kX. ς has a distorted 12 b 6 structure. The unit cell, with dimensions a , 5.512; b , 4.820; c , 4.416 kX at 11.3 weight % N, contains eight iron atoms at 0 0 0, 1/2 0 0, 1/4 1/2 0, 3/4 1/2 0, 0 1/3 1/2, 1/2 1/3 1/2, 1/4 5/6 1/2, 3/4 5/6 1/2, and four nitrogen atoms at 1/4 1/6 1/4, 1/2 2/3 1/4, 3/4 1/6 3/4, 0 2/3 3/4. Both y' and ς are fully ordered interstitial alloys.