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

The puzzling properties of the first order phase transition in YbInCu$_4$ 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 Ce in Megagauss fields. We speculate, that in case of YbInCu$_4$ 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.