Concentration fluctuations and partial structure factors of compound-forming binary molten alloys

The paper discusses some characteristic features of the concentration fluctuations $S_{\mathrm{CC}}$ and hence of the partial structure factors (in the zero-wave-vector limit) of compound forming solutions. The discussion is based on a phenomenological model: It is assumed that the binary mixture consists of $A$ and $B$ atoms and their chemical associations $A_{\mu }{B}_{\nu }$ ($\mu$, $\nu$ small integers) in chemical equilibrium. The formulation is given in general terms, but the actual calculations are made here by assuming that (a) only one type of chemical association (one pair of $\mu$ and $\nu$) are formed and (b) the mixture of $A$, $B$ and $A_{\mu } B_{\nu }$ can be considered to be ideal. The theoretical results for $S_{\mathrm{CC}}$ for $(\mu ,\nu )=(3,2),(3,1), \mathrm{and} (2,1)$ are compared with those determined from the measured thermodynamic activity data for the Mg-Bi, Ag-Al, and Hg-K systems, respectively. It is concluded that the agreement between the calculated and experimental values is not unsatisfactory, although for full quantitative agreement the simplifying assumptions (a) and (b) above are, in general, too crude. Finally it is shown that the concentration dependence of the partial structure factors in these solutions depends characteristically on the values of ($\mu ,\nu$).