Calculation of enthalpies of formation of actinide nitrides

We report on the results of ab initio electronic structure calculation of total energies of AnN (An = Ac, … , Am), the respective elemental An-metals and the nitrogen molecule using density functional theory (FP APW + lo method and generalized gradient approximation). The obtained energies are further complemented by low temperature heat capacity data and the enthalpies of formation ΔfH2980 are eventually evaluated. While the cohesive energies of AnN reveal an increasing dependence on atomic number from ThN to AmN - a trend similar to that of An-metals - the subtle differences between AnN and An result in enthalpies of formation which show strong negative irregularities at ThN and PaN from a linear downward trend. These are ascribed to substantial covalent contribution of 6d and 5f electrons to chemical bonding.