TEM observations of singular grain boundaries and their roughening transition in Ti0(2)-excess BaTi0(3)

Abstract

When BaTiO3 powder compacts with 0.2 mol% excess TiO2 are sintered in air at 1250 degreesC, below the eutectic temperature, some grains grow abnormally to very large sizes with double twins in the <1 1 1> directions at their centers. The abnormal grains, elongated in the <1 1 1> directions, form mostly straight grain boundaries, lying on their {1 1 1} planes with the fine matrix grains. Some grain boundaries between the nearly (1 1 1) face of an abnormal grain and the matrix grains are faceted with some facet planes lying on either the (1 1 1) plane of the abnormal grain or (210) plane of the matrix grain. These flat grain boundaries, as observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), must be singular, corresponding to cusps in the polar plots of the grain boundary energy against the boundary normal. A schematic analysis with the capillarity vectors shows that the straight {1 1 1} grain boundaries across the triple junctions with the matrix grains are consistent with their singular characteristics. When the specimen is heat-treated further in hydrogen atmosphere, the flat {1 1 1} grain boundaries and the faceted grain boundaries between the abnormal grain and the matrix grains become curved when observed under TEM and HRTEM. The singular Grain boundaries thus become rough in hydrogen. The results represent a direct observation of the grain boundary roughening transition, which appears to be analogous to the roughening transitions of crystal surfaces.