Cohesive Energies and Enthalpies: Complexities, Confusions, and Corrections

The cohesive or atomization energy of an ionic solid is the energy required to decompose the solid into its constituent independent gaseous atoms at 0 K, while its lattice energy, Upot, is the energy required to decompose the solid into its constituent independent gaseous ions at 0 K. These energies may be converted into enthalpies at a given temperature by the addition of the small energies corresponding to integration of the heat capacity of each of the constituents. While cohesive energies/enthalpies are readily calculated by thermodynamic summing of the formation energies/enthalpies of the constituents, they are also currently intensively studied by computational procedures for the resulting insight on the interactions within the solid. In supporting confirmation of their computational results, authors generally quote "experimental" cohesive energies which are, in fact, simply the thermodynamic sums. However, these "experimental" cohesive energies are quoted in many different units, atom-based or calorimetric, and on different bases such as per atom, per formula unit, per oxide ion, and so forth. This makes comparisons among materials very awkward. Additionally, some of the quoted values are, in fact, lattice energies which are distinctly different from cohesive energies. We list large numbers of reported cohesive energies for binary halides, chalcogenides, pnictogenides, and Laves phase compounds which we bring to the same basis, and identify a number as incorrectly reported lattice energies. We also propose that cohesive energies of higher-order ionic solids may also be estimated as thermodynamic enthalpy sums.