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

Abstract

The $\zeta $-iron nitride phase (Fe$_{2}$N), the existence of which is confirmed, is prepared by passing ammonia over iron at temperatures not exceeding 450 degrees C and under such conditions that the partial pressure of hydrogen is negligible. The positions of the nitrogen atoms in $\gamma ^{\prime}$(Fe$_{4}$N) and in $\zeta $ are determined. $\gamma ^{\prime}$ is a 'normal' 12a6 interstitial structure, the cubic unit cell of which contains four iron atomsat 0 0 0, $\frac{1}{2}\frac{1}{2}\,0$, $\frac{1}{2}\, 0\,\frac{1}{2}$, $0\,\frac{1}{2}\frac{1}{2}$, and one nitrogen atom at $\frac{1}{2}\,\frac{1}{2}\,\frac{1}{2}$; at 6$\cdot $1 weight% N, a=3$\cdot $787kX. $\zeta $ has a distorted 12b6 structure. The unit cell, with dimensions a, 5$\cdot $512; b, 4$\cdot $820; c, 4$\cdot $416 kX at 11$\cdot $3 weight% N, contains eight iron atoms at 0 0 0, $\frac{1}{2}\, 0\,0$, $\frac{1}{4}\,\frac{1}{2}\,0$, $\frac{3}{4}\,\frac{1}{2}\,0$, $0\,\frac{1}{3}\,\frac{1}{2}$, $\frac{1}{2}\,\frac{1}{3}\,\frac{1}{2}$, $\frac{1}{4}\,\frac{5}{6}\,\frac{1}{2}$, $\frac{3}{4}\,\frac{5}{6}\,\frac{1}{2}$, and four nitrogen atoms at $\frac{1}{4}\,\frac{1}{6}\,\frac{1}{4}$, $\frac{1}{2}\,\frac{2}{3}\,\frac{1}{4}$, $\frac{3}{4}\,\frac{1}{6}\,\frac{3}{4}$, $0\,\frac{2}{3}\,\frac{3}{4}$. Both $\gamma ^{\prime}$ and $\zeta $ are fully ordered interstitial alloys.