The invar phenomenon (very small thermal expansion in some iron alloys or compounds) is usually explained by the thermally-induced transitions between different spin states of Fe, having different atomic volumes. We consider these processes taking into account elastic interaction between Fe atoms in different spin states. Inclusion of these interactions explains why thermal expansion may be close to zero in a broad temperature interval and thus gives rise to the invar effect.
This is a pre-print. It is also available at: http://arxiv.org/abs/cond-mat/0402374.