Kondo Volume Collapse and the $\gamma \to \alpha$ Transition in Cerium

The free energy stabilizing the Kondo singlet state is shown to be important in the total-energy and stability conditions for cerium and related solids. Explicit calculations are given for the simplest spin-½ Kondo model, using the relation to the Anderson Hamiltonian, which leads to a semiquantitative description of the $\gamma \to \alpha$ phase transition in cerium. The temperature dependence of the free energy has a universal form which can lead to a phase boundary terminating in two critical points.