We present a thermodynamic two-state model based on energy-band results at T=0 K and on a Debye approximation of the phonon contribution to the free energy in order to calculate various Invar anomalies. The band results for some Fe-based systems suggest that the nonmagnetic and ferromagnetic total energies are almost degenerate and that Invar behavior occurs when the average number of electrons per atom is near 8.5. Our model invokes fluctuations between the ferromagnetic and nonmagnetic states at equal volume and does not invoke large volume fluctuations.
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Moroni et al. (1990) studied this question.
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