Activities and immiscibility in the system Cu-Rh

The thermodynamic activity of rhodium in solid Cu-Rh alloys is measured by the electromotive force method in the temperature range from 1050 to 1325 K with a solid-state cell: Rh, Cu+Cu₂O∥(Y₂O₃)ZrO₂∥Cu-Rh+Cu₂O, Rh. The activity of copper and the Gibbs energy, enthalpy, and entropy of mixing of the solid solution are derived. Activities exhibit large positive deviation from Raoult's law. The mixing properties can be represented by a pseudo-subregular solution model in which excess entropy has the same type of functional dependence on composition as the enthalpy of mixing: ΔH = X_Rh(1-X_Rh)[41,340+12,670 X_Rh] J mol^-1 ΔS^E = X_Rh(1-X_Rh)[15.46+4.72 X_Rh] J mol^-1 K^-1. The results predict a solid-state miscibility gap with T_c = 1408 (±5) K at X_Rh = 0.59 (±0.02). The computed critical temperature is 15 K lower than that reported in the literature. The coherent spinodal, calculated using Cahn's model, is characterized by T′_c = 983 (±5) K and X′_Rh = 0.43 (±0.02). The large difference between the coherent and chemical spinodal is caused by the relatively large difference in lattice parameters of Cu and Rh and high Young's modulus of the alloy. The positive enthalpy of mixing obtained in this study contrasts with the negative values predicted by Miedema's model. Thermodynamic properties of liquid alloys are estimated from the data for solid alloys obtained in this study and the approximate location of the liquidus available in the literature.