Linear-mixing model for shock-compressed liquid deuterium

A model has been developed for the equation of state of deuterium that builds in the correct limiting behavior for the molecular fluid at low pressure and extends smoothly through dissociation to the very high-density monatomic-metallic fluid. The key assumption is that the Helmholtz free energy of the dissociating mixture is a function that can be approximated by the composition average of the free energy of the pure molecular and metallic hydrogen equations of state. The composition is determined by minimizing the free energy. In comparison to earlier studies this model leads to an enhancement of molecular dissociation and a lowering of shock temperatures and pressures. Calculations for shock-compressed liquid deuterium are in agreement with experiments to a pressure of 2.1 Mbar. At about 1 Mbar and 20 000 K liquid deuterium is 90% dissociated and is a nearly degenerate metal.The model predicts that molecular dissociation will lead to negative values of (partial derivative P/partial derivative T)(v) in the range 4000 to 10 000 K and volumes below 7 cc/mol, This feature suggests the formation of covalently bonded species in the partially dissociated mixture. [S0163-1829(98)05825-1].