Computation of formation enthalpies and molar volumes of halides

Abstract

The Macroscopic Atom Model (MAM) is extended with the halogens to obtain formation enthalpies and molar volumes of halides. Molar volumes and electron densities were obtained with ab initio methods assuming a hypothetical metallic state. The electronegativity parameter and transformation enthalpy were obtained by linear extrapolation and regression analysis. Negative transformation enthalpies were introduced to obtain reliable values for the formation enthalpy of halides. Formation enthalpies of halides are computed within 25 kJ/mol·at and their molar volumes are underestimated by 20%. The MAM provides better estimates than models using Pauling electronegativity or Born-Haber cycle calculations assuming an ionic lattice of the Born-Haber type. The inclusion of the halogens in the MAM scheme has extended its application to both alloys and ionic compounds. This model can now predict their formation enthalpies and molar volumes, which is not the case in all other electronegativity schemes.