Effects of Impurities on the Optical Properties of Mixtures

We perform molecular dynamics simulations\footnote{D.A, Horner et. al., Phys. Rev. B {\bf{80}}, 024305 (2009)} of mixtures of CH$_{2}$ and Aluminum (Al) in a plane-wave, finite-temperature density functional theory treatment at the generalized-gradient level with projector-augmented wave pseudopotentials for temperatures between 1eV and 3eV and densities ranging from 1 to 3 g/cm$^{3}$. In this regime, the system generally takes the form of an atomic liquid with the C, H, and Al all dissociated. We vary the concentration of Aluminum from 0\% to 100\% by substituting Al atoms for a CH$_2$ molecules in the initial configuration. Samples consisted of 75 CH$_2$ molecules [C=75; H=150] or their equivalent Al substitution although larger samples were introduced to test size effects. In general, the dc electrical conductivity varied smoothly with Al concentration with a slow increase up to 50\% and a steeper rise after about 70\%. The Rosseland Mean Opacities also behaved smoothly with a factor of two change between the extremes of the Al concentration. However, for a few combinations of density, temperature, and Al concentrations, we did find a ``condensation'' into a large polymer structure and noticeable changes in the system properties.