Heat capacity and internal motion of molten Bi+BiI3mixtures

Abstract

Measurements of the heat capacities, Cp, of molten Bi+BiI3 mixtures at compositions up to around 50 mol.% Bi were made with an adiabatic calorimeter. The deviation Delta Cp,a from the Dulong-Petit law of the atomic heat capacities was calculated over the whole concentration range, including proviously published data for Cp above 50 mol.% Bi. A drastic increase of Delta Cp,a for the ionic liquid-semiconductor regime between around 20 and 50 mol.% Bi (i.e. it has a maximum value of around 2.7R at 50 mol.% Bi) is satisfactorily explained in terms of the internal motions (i.e. the vibrational and rotational modes) of the bismuth homopolyatomic ions, in addition to a contribution from the bismuth iodide complex ions. Between the onset ( approximately 50 mol.% Bi) and the end ( approximately 90 mol.% Bi) of the miscibility gap, Delta Cp,a decreases to less than half of its maximum value. This is because the metallic micro-regions, which are made up of the aggregation of a few of the polybismuth ions or more with the increase of the Bi content, no longer show any internal motion and do not give rise to the large value of Delta Cp,a. Since the molten mixtures between approximately 50 and approximately 90 mol.% Bi are microscopically inhomogeneous solutions consisting of the ionic liquid-semiconducting and metallic micro-regions, the so-called inhomogeneous cluster model is applied to the Delta Cp,a of these mixtures.