Dislocation structures of low-angle boundaries in Nb-doped SrTiO ₃ bicrystals

Dislocation core structures in low-angle boundaries of Nb-doped SrTiO ₃ bicrystals were investigated by high-resolution electron microscopy. Bicrystals with tilt angles of 2°, 4°, 6° and 8° with respect to the [001] zone axis were prepared by joining two single crystals at 1873 K. All of the boundaries consisted of a regular array of dislocations whose spacing gradually decreased with an increase in tilt angle. Except for the 2° tilt-angle boundary, the dislocation cores exhibited a dissociation from a[010] into two partials of a/2[010] on (100). Furthermore, two kinds of dislocation core structures were observed; Sr-Sr atomic columns and Ti-O atomic columns inside the cores. In addition, it was found that the positioning of adjacent cores along the boundary tended to change from a linear form to a zig-zagg shape as the tilt angle was increased from 4° to 8°. In the case of the linear array, dislocation core structures including Sr-Sr columns or Ti-O columns alternately appear. In contrast, only one core structure was observed in the zig-zagged array. On the other hand, the dislocation cores in the 2°-tilt-angle boundary had another type of dissociation with a/2[110] or a/2[111] partials, which included the twist component at a tilt axis of [001].