First-order valence transition in YbInCu4 in the (B, T)-plane

The puzzling properties of the first-order phase transition in YbInCu4 and its alloys in the wide range of magnetic fields and temperatures are perfectly described in terms of a simple entropy transition for free Yb ions. In particular, it turns out that the transition line in the (B, T)-plane is very close to the elliptic shape, as it has been observed experimentally. Similar calculations are done, and the experiments are proposed for the (gamma-alpha) phase transition in Cein Megagauss fields. We speculate, that in the case of YbInCu4 the first-order transition is a Mott transition between a higher temperature phase in which localized moments are stabilized by the entropy terms in the free energy, and a band-like non-magnetic ground state of the f-electrons.