Self-Trapped Interstitial-Type Defects in Iron D. A. Terentyev, 1 T. P. C. Klaver, 2 P. Olsson, 3 M.-C. Marinica, 4 F. Willaime, 4 C. Domain, 3 and L. Malerba 1, * 1 Nuclear Materials Science Institute, SCK-CEN, Boeretang 200, B-2400, Mol, Belgium 2 School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, Northern Ireland, United Kingdom 3 De ´partement MMC, EDF R&D, Les Renardie `res, 77250 Moret-sur-Loing, France 4 Service de Recherches de Me ´tallurgie Physique, CEA/Saclay, 91191 Gif-sur-Yvette Cedex, France (Received 14 September 2007; published 9 April 2008) Small interstitial-type defects in iron with complex structures and very low mobilities are revealed by molecular dynamics simulations. The stability of these defect clusters formed by nonparallel h110i dumbbells is confirmed by density functional theory calculations, and it is shown to increase with increasing temperature due to large vibrational formation entropies. This new family of defects provides an explanation for the low mobility of clusters needed to account for experimental observations of microstructure evolution under irradiation at variance with the fast migration obtained from previous atomistic simulations for conventional self-interstitial clusters. DOI: 10.1103/PhysRevLett.100.145503 PACS numbers: 61.72.J, 61.72.Bb, 61.80.Az, 61.82.Bg The safety of existing and future nuclear power plants largely depends on the radiation resistance of the chosen structural materials, determined by their microstructure evolution under irradiation. With a view to developing reliable numerical multiscale models of the microstructural changes produced by irradiation, one of the key issues, especially in body-centered cubic (bcc) metals, is that of the migration properties of self-interstitial atoms (SIA) and their clusters [1–7]. The case of iron, the base material for steels, is especially important because steels are widely used in nuclear reactors, and the properties of interstitial defects in this metal are peculiar and their physics is not yet completely understood. Experimentally observed SIA loops in iron may have both 1 2 h111i and h100i Burgers vectors [6,8]. For the single SIA, experiments [9] and density functional theory (DFT) calculations [2,4] agree on a h110i dumbbell configuration, at variance with the h111i crowdion found in other bcc metals [3]. Initially it was believed that small SIA clusters in iron had to be collections of parallel h110i dumbbells. The formation of the observed 1 2 h111i and h100i loops was explained as the unfaulting of h110i clusters above a critical size [10]. However, the use of many-body potentials (MBPs) in molecular dynamics (MD) studies suggested these clusters to be collections of parallel h111i crowdions instead, ca- pable of gliding one-dimensionally (1D) with very small migration energy (tens of meV) [5,7,11]. This is qualita- tively consistent with recent in situ transmission electron microscopy (TEM) experiments, showing that not only 1 2  h111i SIA loops, but also h100i loops, can move in ultra pure Fe above 450 K [6]. However, while accepted microstructure-evolution models succeeded in describing void swelling assuming high 1D mobility of at least part of the SIA clusters [12], other models showed that, in iron, agreement with experimental observations can only be obtained if the MD calculated high mobility of SIA clusters is artificially reduced, by either introducing traps [13] or even postulating their immobility [2]. Recently, DFT calculations showed that clusters of up to four SIAs are more stable as collections of parallel h110i dumbbells than h111i crowdions, with accordingly larger migration energy [14]. This is due to a higher energy difference between the h111i crowdion and the h110i dumbbell than was found with early MBPs [4] (  0:7 eV versus less than 0.3 eV). Using this informa- tion, more reliable MBPs for iron have been developed [15,16]. However, the revisited stability and higher migra- tion energy of small h110i clusters does not remove the need for traps for the fast migrating clusters in microstruc- tural evolution models [13]. MD simulations of cascades in iron using an early MBP showed that SIA-cluster configurations made of nonparal- lel dumbbells form spontaneously [17]. MD studies with a more recent MBP [16] confirmed the formation of SIA clusters in nonparallel configurations (NPCs), not only in cascades [18], but also during cluster migration at high temperature [7], thereby suggesting that they are thermally stable. The specificity of these self-trapped con- figurations is that no simple mechanism exists whereby they can migrate, as they need first to unfault to parallel configurations. In this Letter, the stability of some NPCs is studied by combining MD and lattice dynamics, using the MBP ap- plied in Refs. [7,18] and DFT methods. DFT provides the ground states at 0 K, while with the MBP the study is extended to finite temperatures, looking tentatively at un- faulting mechanisms, too. We show that the thermal stabil- ity of these NPCs may provide an explanation for many of the still unclear issues briefly outlined above. DFT calculations were performed with the SIESTA code in the Perdew-Burke-Enzerhof generalized gradient ap- proximation (GGA), with a real-space grid spacing equal to 0.067 A ˚ using a norm-conserving pseudopotential. Details on the localized basis set and its validation can be found in Ref. [14]. For comparison, calculations were also carried out with VASP [19], a plane-wave code employ- PRL 100, 145503 (2008) PHYSICAL REVIEW LETTERS week ending 11 APRIL 2008 0031-9007= 08=100(14)=145503(4) 145503-1  2008 The American Physical Society