Investigation of grain boundaries in abnormal grain growth structure of TiO2-excess BaTiO3 by TEM and EELS analysis

BaTiO3 with 0.2 mol% excess TiO2 shows abnormal grain growth (AGG) behavior when sintered in air at 1250degreesC, below the eutectic temperature (1332degreesC). Abnormal grains have polyhedral shape and their grain boundaries are flat and parallel to the {111} planes of the abnormal grains. Many grain boundaries between matrix grains are faceted with small steps and long segments parallel to the {111} planes of one of the grains. It is shown through HRTEM that such a polyhedral shape is correlated with an ordered grain boundary phase between the abnormal grains and neighboring matrix grains. This is in contradiction to a generally accepted interpretation where some residual liquid phase induces the polyhedral abnormal grains. By EELS, the chemistry and the change in the atomic bonding of both the flat boundaries of the abnormal grains and the faceted grain boundaries of the matrix grains are studied. A correlation between faceting, AGG and the formation of the ordered layer at the grain boundaries of the abnormal grains is proposed. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.