The atomic structure of a symmetric 27° [001] tilt grain boundary in MgO has been determined by high-resolution Z-contrast imaging using a 300 kV VG HB603U scanning transmission electron microscope with a 1.3 Å probe. The atomic configuration in the grain boundary core is found to be considerably less open than the structures proposed earlier for similar materials.

Fig. 1 shows the Z-contrast image of the grain boundary and the projected structure derived from the image. It is clearly seen that the boundary is a shared plane of atoms with the same atomic column density as a bulk crystal {100} plane. The boundary consists of an array of separated perfect edge dislocation cores with Burgers vector b= a<100>. It is interesting to point out that the spacing between dislocation cores in the boundary is not uniform (as seen in Fig. 2). The arrangement, which follows the Fibonacci sequence, can be accurately predicted.