Self-interstitial defects in hexagonal close packed metals revisited: Evidence for low-symmetry configurations in Ti, Zr, and Hf

In addition to the eight conventional high-symmetry configurations for self-interstitials in the hexagonal close packed (hcp) structure, we show that four other configurations, obtained by breaking the symmetry of some of the original ones, may be low-energy local minima or saddle points. The first two, BC' and C', consist of the basal crowd ion and the crowd ion buckled perpendicular to their axes in the pyramidal plane, respectively. The two others, PS and P2S, are obtained by rotating the c-axis split dumbbell in the prismatic plane of first and second type, respectively. Using first-principles density functional theory calculations we show that BC', C', and PS are within 0.4 eV of the lowest-energy conventional structure, BO, in Ti, Zr, and Hf. BC' could even be the lowest-energy configuration in hcp-Zr and its symmetry and possible reorientation mechanisms are compatible with internal friction measurements at variance with the conventional structures. The PS and C' configurations exhibit a helicoidal easy glide motion of the dumbbell-crowd ion type in the c-axis direction. These configurations therefore constitute an important element to take into account when predicting the microstructural evolution of zirconium-based materials under irradiation.