EMF-Study of the thermodynamics of liquid Kx(KI)1-x and Csx(CsI)1-x melts

We report EMF measurements of the metal activity aK and aCs in Kx(KI)1‐x(xK≤0.0136) and Csx(CsI)1‐x (xCs≤0.25) at T=800°C. The data have been obtained simultaneously with optical absorption spectra. The composition has been controlled in situ via coulometric titration. The data of Kx(KI)1‐x are discussed in terms of a thermodynamic defect model, taking into account different electronic defect species in the melt: mobile 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 spectra. For Csx(CsI)1‐x we report data for the partial molar excess‐free enthalpies and the integral‐free enthalpy. As expected the Csx(CsI)1‐x system shows only little deviation from ideal mixing behaviour in comparison to other Mx(MX)1‐x melts, a fact which is also established in the phase diagram by the absence of a miscibility gap in the caesium‐caesium halide mixtures.