EMF-STUDY OF THE THERMODYNAMICS OF LIQUID K-X(KI)(1-X) AND CS-X(CSI)(1-X) MELTS

We report EMF measurements of the metal activity a(K) and a(Cs) in K-x (KI)(1-x) (x(K)less than or equal to 0.0136) and Cs-x(CsI)(1-x) (x(Cs) less than or equal to 0.25) at T=800 degrees C. The data have been obt ained simultaneously with optical absorption spectra. The composition has been controlled in situ via coulometric titration. The data of K-x (KI)(1-x) are discussed in terms of a thermodynamic defect model, taki ng into account different electronic defect species in the melt: mobil e electrons, monomer F-center like and bipolaronic states. The results of the analysis of the thermodynamic data are in good agreement with the results of the quantitative analysis of the optical absorption spe ctra. For Cs-x(CsI)(1-x) we report data for the partial molar excess-f ree enthalpies and the integral-free enthalpy. As expected the Cs-x(Cs I)(1-x) system shows only little deviation from ideal mixing behaviour in comparison to other M-x(MX)(1-x) melts, a fact which is also estab lished in the phase diagram by the absence of a miscibility gap in the caesium-caesium halide mixtures.