2484.     NOYES, A. A., AND SMITH, L. B.  Dissociation Pressures of Iron Nitrides.  Jour. Am. Chem. Soc., vol. 43, 1921, pp. 475-481; Chem. Abs., vol. 15, 1921, p. 1245.

        Equilibrium conditions of the reaction between Fe, NH₃, solid Fe nitrides, and H₂ at 460° are determined.  The equilibrium constant given by the expression (p_NH2)²/((p_H2)³ was found to vary with the proportion of N present in the solid phase.  Thus with increasing N content this expression remained constant at the value 1.0 until the atomic ratio N:Fe became 1:9; later increased rapidly, namely to 5.2 for N:Fe=1:7.7 and to 30 for N:Fe=1:4.9; and finally assumed the much smaller value 2.1 for N:Fe=1:2.6.  These results indicate the formation:  1^st, of a nitride of small N content, perhaps Fe₈N; then either of a metastable nitride, such as Fe₄N in solid solution with the 1^st one, or of 2 separate metastable nitrides, such as Fe₆N and Fe₄N as separate solid phases; and, finally, of a stable nitride, probably the well known Fe₂N.  By combining this equilibrium constant with the constant (pHN₃)²/p_N2(p_H2)³ for the formation of ammonia, the dissociation pressures pN₂ of the nitrides in the various solid phases were computed to be 20,000, 102,000, 590,000, and 41,000 atm., respectively.