Thermal stability of ionic solids: A melting points survey

Abstract

While predictive correlations of the melting points of organic solids are well-established and reliable, the melting points of ionic solids are less understood. We provide a comprehensive survey of the related literature for ionic solids which shows that the primary factors to consider are the ambient temperature values of the formation enthalpies and the distances among the ions as enshrined in their crystal structures. These conclusions from standard thermodynamic analyses are supported by an independent current machine-learning program. This result contrasts with the common belief that lattice energies are a leading factor in melting. This basic idea is confounded by the irrelevant (but generally overwhelming) inclusion of the formation energy of gaseous ions in the evaluation of lattice energies through thermochemical cycles. The melting points of simple metal halides are correlated with their formation enthalpies per halide ion and distances among ions as determined by their crystal structures rather than their lattice energies. As the formula units of ionic solids increase in chemical and structural complexity the relations also become more complex.