Formation free energies of point defects and thermal expansion of bcc U and Mo

gamma-U is a high temperature body-centred cubic (bcc) phase of uranium which is mechanically unstable at T = 0 K. The point defect properties in pure bcc uranium are not sufficiently well studied. In this work we use classical molecular dynamics simulations with the thermodynamic integration approach to calculate the formation free energies of vacancies and interstitials in bcc uranium and, for comparison, in bcc molybdenum. Contrary to the majority of other metals where the formation free energy is (much) higher for interstitials than for vacancies, our results show that in gamma-uranium interstitials are the dominating type of defects in thermal equilibrium. We discuss the possible implications of this finding in the context of the thermal expansion data for gamma-U that provide a certain supporting evidence.