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C-195similar formal procedure can be applied to form layers 2, 4, 6, and 8. In that case, however, one additional TiOd octahedron has to be added in order to fulfill the stoichiometry requirement.The most Ti06-rich layers 1 and 5 can be formed similarly by removing 2 h of the Ba atoms and filling the empty space by an appropriate number of TiOh octahedra.A model of a topotactic boundary between the two phases can now be constructed simply by adjoining layer 7 of Ba6Ti17040, i.e., that one with the smallest surplus of Ti02, to the (111) layer of BaTiO, so that Ba atoms of layer 7 enter the sites B of layer (111). This particular sequence of layers enables (i) continuity of natural stacking of layers in both phases across the boundary, and (ii) continuity of natural decrease of Ti02 surplus in successive layers of Ba,Ti17040. This decrease can be described by the ratio of numbers on the left side of Fig. 3(a) as 6:2.67: 1.5: 1.2: 1, where 1.2 corresponds to the Ti/Ba ratio in the last (111) layer of "BaTi03" which shares missing Ba atoms with the first layer 7 of Ba6Ti17040 and 1 corresponds to the Ti/Ba ratio in pure BaTiO,. CONCLUSIONThe results presented above show that topotaxy between BaTiO, and Ba,Ti ,7040 (111) twins are found frequently in the microstructure of BaTi03 ceramics prepared by the mixed-oxide technique. The processing steps by which the (111) twins are formed are in question. A BaC03-Ti02 powder mixture is milled and then calcined at 11OO"Cfor 2 h , where BaTi03 is formed by the release of C 0 2 . After the powder is calcined, it is analyzed by TEM. Image and diffraction pattern data are combined to prove the presence of ( I l l ) twins in powder particles; the formation o f ( l l 1 ) twins occurred frequently during calcination. The formation mechanism of such twins and their role during anomalous grain growth are discussed.