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 Δ f H 298 0 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.