Relevance of magnetic instabilities to the properties of interstitial solid solutions and compounds of Fe

At the 10 at% limit of C and N interstitial solubility in gamma -Fe the atomic volume crosses the critical volume separating the low-spin and high-spin states. This suggests that the lower energy state of the FCC solid solutions should be low-spin. This is confirmed by the temperature dependence of the thermal expansion coefficient of gamma -Fe-C in the temperature range of stability of the FCC phase. gamma -Fe-C exhibits anti-Invar behavior, which is an indication for the presence of moment-volume instabilities with the low-spin state of lower energy. Above the solubility limit the compounds have FM ground states. They show Invar behavior provided that their valence electron concentration, reduced to the metallic element concentration, lies in the range 8.66 less than or equal to (e/a)(m) less than or equal to9.2, as in Fe3C. The high solubility of C and N interstitials, particularly in FCC Fe, is discussed in relation to the magnetic properties. (C) 2001 Elsevier Science B.V. All rights reserved.