Thermodynamic properties and phase equilibria for Pt-Rh alloys

Abstract

The activity of rhodium in solid Pt-Rh alloys is measured in the temperature range from 900 to 1300 K using the solid-state cell $$Pt - Rh,Rh + Rh_2 O_3 /(Y_2 O_3 )ZrO_2 /Pt_{1 - x} Rh_x + Rh_2 O_3 ,Pt - Rh$$ The activity of platinum and the free energy, enthalpy, and entropy of mixing are derived. Activities exhibit moderate negative deviation from Raoult’s law. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of functional dependence on composition as the enthalpy of mixing, $$\begin{gathered} \Delta H = X_{Rh} (1 - X_{Rh} )[ - 10,970 + 45X_{Rh} ] {J \mathord{\left/ {\vphantom {J {mol}}} \right. \kern-\nulldelimiterspace} {mol}} \hfill \\ \Delta S^E = X_{Rh} (1 - X_{Rh} )[ - 3.80 + 1.55 \times 10^{ - 2} X_{Rh} ] {J \mathord{\left/ {\vphantom {J {mol}}} \right. \kern-\nulldelimiterspace} {mol}} \cdot K \hfill \\ \end{gathered} $$ The negative enthalpy of mixing obtained in this study is in qualitative agreement with predictions of semiempirical models of Miedema and co-workers and Colinet et al. The results of this study do not support the solid-state miscibility gap suggested in the literature, but are consistent with liquidus data within experimental uncertainty limits.