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 ato mic volume crosses the critical volume separating the low-spin and high-spi n states. This suggests that the lower energy state of the FCC solid soluti ons should be low-spin. This is confirmed by the temperature dependence of the thermal expansion coefficient of gamma -Fe-C in the temperature range o f stability of the FCC phase. gamma -Fe-C exhibits anti-Invar behavior, whi ch is an indication for the presence of moment-volume instabilities with th e 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, lie s 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 Else vier Science B.V. All rights reserved.