EQUATIONS OF STATE AND PHASE-DIAGRAMS FOR DENSE MULTI-IONIC MIXTURE PLASMAS

Equations of state for dense binary-ionic mixture (BIM) plasmas with c harge ratios R(Z) less-than-or-equal-to 4.5 are evaluated for both flu id and solid phases on the basis of internal energies calculated by th e Monte Carlo (MC) simulation method. The accuracy of the results has been confirmed through a sum-rule analysis of the free energy with res pect to screening potentials at zero separation in one-component plasm as. Phase diagrams for various BIM's appropriate to white-dwarf interi ors are constructed, including the effects of compressibility of the r elativistically degenerate electrons. Eutectic diagrams, which result in significant chemical separation upon solidification, are predicted for BIM's with R(z) greater than or similar to 1.6. Free energies for dense ternary-ionic mixtures are also derived for both fluid and solid phases through comparison of the corresponding MC internal energies a nd by a sum-rule analysis using BIM screening potentials at zero separ ation. We apply the results to the process of solidification of C-12-O -16-Ne22 mixtures in the interiors of white dwarfs. We find that chemi cal separation does indeed occur at solidification, and we obtain the result that nearly pure Ne cores are formed in such white dwarfs, even though Ne is only a trace element in the mixture.