Quasiaperiodic grain boundary phases of Σ5 tilt grain boundaries in refractory metals

We report ground-state structures and phase transitions in Σ5[001] tilt grain boundaries (GBs) in body-centered-cubic (bcc) refractory metals Nb, Ta, Mo, and W. Σ5 tilt GBs have been extensively investigated over the past several decades, with their ground-state structure---composed of kite-shaped structural units---previously thought to be well understood. By performing a rigorous GB structure search that optimizes the number of atoms in the boundary core, we predict different quasiaperiodic ``split kite'' phases analogous to those previously found in GBs in face-centered-cubic metals. Our results suggest that complex aperiodic phases of GBs appear to be a general phenomenon, as validated through density functional theory calculations. Moreover, the atoms in the split kite phase demonstrate distinct collective diffusion dynamics. Phase-contrast image simulations of split kites show better agreement with experimental observations, offering an alternative explanation for previous microscopy results and motivating future atomically resolved imaging of the GB structure.