Screw dislocation in hcp Ti?:?DFT dislocation excess energies and metastable core structures

An extensive DFT search of (meta) stable structures of the a/3 < 11 (2) over bar0 > screw dislocation in hcp-Ti is presented. It reveals that the stable core structures are never basal but always prismatic. This prismatic core dissociates into two partial dislocations in the same or neighboring prismatic planes depending on the initial position of the dislocation line, leading to either a symmetric or an asymmetric core. An alternative way of defining the core region from an electronic structure point of view is also proposed. It evidences clearly the symmetric or asymmetric character of the cores. We then introduce an ansatz for a straightforward and fast calculation of the excess energy, per unit length of dislocation, of a screw dislocation applicable to DFT calculations, in the cluster approach. The method is first validated on calculations of a a/3 < 11 (2) over bar0 > screw dislocation in hcp-Ti, performed with an EAM potential from which exact excess energies can be extracted. Then, it is shown that it does work in a DFT calculation, through its application to the same screw dislocation in hcp-Ti with an accuracy of 8.4 meV/angstrom (1.8% of the excess energy for a cluster of 126 atoms per plane normal to the dislocation line). The comparison of the excess energies of the symmetric and assymmetric cores, calculated with the proposed ansatz, reveals that their energy difference is within the uncertainty of the method, which implies that the potential energy surface is very flat and that there could be many metastable core structures in hcp-Ti.