The stability of interstitial defect and dislocation structures in body-centered cubic Fe as a function of temperature was believed to play a crucial role in determining defect evolution under irradiation conditions. The vibrational properties of defects constitute one contribution to the corresponding energetics and much work was done within the harmonic approximation to determine the vibrational formation free energy and formation entropy of such defects. Defects could however exhibit strong local strain fields that break the cubic symmetry of the body-centered cubic lattice leading to large anharmonicities and a breakdown of the harmonic approximation as an accurate means to calculate vibrational thermodynamic quantities. Moreover, if defect diffusion was active at a time scale comparable to an atomic vibration, strong anharmonicities will always be present at any finite temperature. The current work investigates the vibrational free energy and entropy of the ⟨110⟩ self-interstitial dumbbell defect in body-centered cubic Fe using both harmonic and anharmonic free-energy calculation methods for a range of modern empirical potentials. It was found that depending on the empirical potential and for temperatures where diffusion was limited, the harmonic approximation was justified especially for empirical potentials that were fitted to third-order elastic constants. The unique applicability range of such calculations for body-centered cubic Fe was also discussed given that with rising temperature spin fluctuations become increasingly important ultimately leading to a softening of the 110 shear modulus and to the α-bcc/γ-bcc structural phase transformation

Free Energy of a <110> Dumbbell Interstitial Defect in BCC Fe - Harmonic and Anharmonic Contributions. S.Chiesa, P.M.Derlet, S.L.Dudarev: Physical Review B, 2009, 79[21], 214109