Nonideal Mixing Effects in the Warm Dense Matter*

Shock compression experiments enable us to probe the physical properties of warm dense matter (WDM) and to check the validity of predictions derived from first-principle computer simulations. At sufficiently high temperatures, WDM becomes fully ionized and its properties closely resemble an ideal mixture. As temperature decreases, systems become strongly coupled, making WDM characterization more challenging. A detailed knowledge of WDM allows us to constrain the interior of stars and planets and also enables us to make predictions for inertial confinement fusion experiments.

Here, we study how well the ideal mixing approximation works in the WDM regime for BN, MgO, and MgSiO₃. For each material, we build an equation of state (EOS) table from first-principles simulations for the fully interacting mixture. Starting from EOS table for the individual elements, we invoke the linear mixing approximation to compare the prediction for shock Hugoniot curves with those derived from the fully interacting EOS. Our results show that the linear mixing approximation works remarkably well over a wide range of temperatures and pressures. We identify conditions where nonideal effects are relevant.