Heat capacity of some liquid Zintl compounds: Equiatomic alkali--lead alloys

Enthalpies for equiatomic APb (A = Na, Rb, and Cs) have been measured by high-temperature drop calorimetry over a temperature range covering solid and liquid phases. Thermodynamic information on the solid state transitions and the melting process is derived. With the exception of NaPb, the heat capacities of the liquid alloys show a remarkably strong negative temperature dependence above the melting point. It is suggested that this behavior is a consequence of the dissociation of molecular entities present in the liquid state related to the Zintl structures detected in the corresponding solid compounds. Thermodynamic calculations based on the presence of polyvalently charged anions yield valuable information on the temperature dependence of the concentration of these species