Concentration fluctuations and thermodynamic properties of some compound forming binary molten systems

Abstract

First, the equations for calculating the concentration dependence of the free energy of mixing ${G}_{M}$, the activity ${a}_{A}$, and the concentration fluctuations ${S}_{\mathrm{CC}}(0)$ (in the zero-wave-number limit) are derived by assuming (i) the $A$ and $B$ atoms of a binary mixture may form chemical complexes of the type ${A}_{\ensuremath{\mu}}{B}_{\ensuremath{\nu}}$ ($\ensuremath{\mu}$,$\ensuremath{\nu}$ small integers) and (ii) the components $A$, $B$, and ${A}_{\ensuremath{\mu}}{B}_{\ensuremath{\nu}}$ interact only weakly with one another (the strong bonding interaction between $A$ and $B$ atoms having been taken care of via the formation of the chemical complexes). The ternary mixture is then treated (a) in the conformal solution approximation, which assumes that the differences in volumes between $A$, $B$, and ${A}_{\ensuremath{\mu}}{B}_{\ensuremath{\nu}}$ are small and (b) in Flory's approximation for mixtures of monomers and polymers. Next, using the above equations, explicit expressions for ${S}_{\mathrm{CC}}(0)$ are obtained for dilute mixtures and, for mixture of any concentration in the two limiting cases where the tendency to form chemical complexes is very strong and very weak. Finally, numerical calculations for the concentration dependence of ${G}_{M}$, ${a}_{A}$, and ${S}_{\mathrm{CC}}(0)$ are compared with experiment for the systems Tl-Te, Mg-Bi, Ag-Al, and Cu-Sn, the interaction parameters in the theory being determined from the observed data on ${G}_{M}$ for each case.