Abstract
We show that the anomalous thermal expansion behaviour of Pu-Ga alloys can be explained by an Invar effect, which is modelled by assuming the thermal transfer of Pu atoms from a lower-energy higher-volume Δ1, state to a higher-energy lower-volume Δ2 state. The energy separation of these states is found to be 1400 K from a fit to the available neutron diffraction data on the lattice constants of Pu-Ga alloys. The Invar model enables a precise evaluation of the coefficient of thermal expansion and of the Grüneisen constant. The addition of Ga forces a stabilization of the Δ2 state and suppresses the negative thermal expansion.