Comparative studies of microstructure and impedance of small-angle symmetrical and asymmetrical grain boundaries in SrTiO3

Several low-angle symmetrical [0 0 1]-tilt grain boundaries and an asymmetrical grain boundary with both tilt and twist components were examined by high-resolution transmission electron microscopy, conventional transmission electron microscopy, and impedance spectroscopy. The symmetrical tilt grain boundaries consist of isolated dislocation cores and the Burgers vectors are translation vectors of the crystal. Strain analysis shows that the strain fields around the dislocation cores overlap with increasing misorientation angle. The a.c. impedance response of the low-angle grain boundaries reveals a marked blocking effect for charge transport across the low-angle symmetrical grain boundaries, which increases with increasing misorientation angle. In the case of the mixed asymmetrical grain boundary, however, the blocking effect is low despite the large misorientation. The resistance and electrical potential of this grain boundary were calculated and compared with those obtained from the symmetrical grain boundaries. A relationship between microstructure and macroscopic electrical property can approximately be arrived by considering anion vacancies to be preferentially situated in highly distorted regions such as dislocation cores.