Excess Heat Capacity in Liquid Binary Alkali-Fluoride Mixtures

Using drop calorimetry we measured enthalpy increments of the LiF-KF, LiF-RbF and LiFCsF binary systems at temperatures above melting. 10 samples with different compositions (four compositions for LiF-KF, one composition for LiF-RbF and five compositions for LiFCsF) were prepared and measured between 884K and 1382 K. To protect the calorimeter from corrosive fluoride vapor at high temperature an encapsulating technique developed for this purpose was used. The samples were filled in nickel containers which were sealed by laser welding and afterwards used for the measurements. From the obtained results we derived the molar heat capacity functions of the respective samples. The heat capacities of the samples, having different compositions of the same binary system, were compared with the values for ideal behavior and the excess heat capacity function was determined for the whole composition range of the liquid solution. It was found that the excess heat capacities clearly depend on the cation radius and increases in the order LiF-NaF < LiF-KF < LiF-RbF < LiF-CsF.
JRC.E.3-Materials research