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 (pNH2)2/((pH2)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:  1st, of a nitride of small N content, perhaps Fe8N; then either of a metastable nitride, such as Fe4N in solid solution with the 1st 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/pN2(pH2)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.