First-principles calculations on dislocations in MgO

Abstract

While ceramic materials are widely used in our society, their understanding of the plasticity is not fully understood. MgO is one of the prototypical ceramics, extensively investigated experimentally and theoretically. However, there is still controversy over whether edge or screw dislocations glide more easily. In this study, we directly model the atomic structures of the dislocation cores in MgO based on the first-principles calculations and estimate the Peierls stresses. Our results reveal that the screw dislocation on the primary slip system exhibits a smaller Peierls stress than the edge dislocation. The tendency is not consistent with metals, but rather with TiN, suggesting a characteristic inherent to rock-salt type materials.

Keywords: Dislocation; ceramics; first-principles calculation.

Graphical abstract

Figure 1.

Schematic showing the dislocation dipole structure used in this study. The area shaded in light green represents a supercell, and the dotted areas show the repeated cells. The area shaded in dark blue marks a quadrupole structure.

Figure 2.

Atomic core structures and energy profiles during glide of the edge dislocations. Core structures of the (a, b) 1/2<110>{100} and (c, d) 1/2<110>{110} (a, c) before and (b, d) after relaxation. The green and red circles are Mg and O atoms, respectively. Bonds with a length 1.15 times longer than Mg-O (2.10 Å) in the bulk are illustrated throughout this paper. (e, f) energy profiles of the 1/2<110>{100} and 1/2<110>{110} edge dislocations, respectively. The horizonal dotted lines represents zero energy. The initial and final structures are set at 0 and 1 in the x-axis, respectively. The numbers in the figures show the numbers of the atoms in the supercells. Black and white dislocation labels (‘⊥’) are initial and final positions of the dislocation centres. Letters of ‘A’ to ‘D’ in the panels of the energy profiles represent the transition states, whose core structures are illustrated in the right figures.

Figure 3.

Dislocation elastic and core energies as a function of the distance between dislocation dipole. Filled marks are dislocation core energies while open marks are elastic energies.

Figure 4.

Atomic core structures and energy profiles during glide of the screw dislocations. (a, b) core structures (a) before and (b) after relaxation. (c, d) energy profiles of the 1/2<110>{100} and 1/2<110>{110} screw dislocations, respectively. Black arrows are differential displacement maps (see text for details). The length of the arrows is normalized the largest differential displacement. Black and white squares are initial and final positions of the dislocation centres, respectively. The other details are the same as in Figure 2.