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At the 10 at% limit of C and N interstitial solubility in γ-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 γ-Fe–C in the temperature range of stability of the FCC phase. γ-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 8.6⩽( e / a ) m ⩽9.2, as in Fe 3 C. The high solubility of C and N interstitials, particularly in FCC Fe, is discussed in relation to the magnetic properties. |
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