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The experimental conditions for {111} twin formation in BaTiO3 were investigated. When BaTiO3 compacts without excess TiO2 were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N2–5H2), no {111} twins formed within the BaTiO3 grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess‐TiO2‐containing BaTiO3 samples sintered in air, while no twins were observed in the excess‐TiO2‐containing samples sintered in 95N2–5H2. X‐ray diffraction analysis showed that excess TiO2 forms a Ba6Ti17O40 phase during sintering with the space group A2/a in air and a Ba6Ti17O40−x phase with the space group C in 95N2–5H2. It appears therefore that excess TiO2 and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO3. These results may also indicate that the interface structure between BaTiO3 and Ba6Ti17O40 influences the twin formation.

Two series of experiments were performed to study the experimental conditions for the formation of {111} twins and related microstructures in barium strontium titanate ((Ba, Sr)TiO3). In the first series, the phase equilibria in the BaTiO3–SrTiO3–TiO2 system were determined. XRD and WDS analysis, done in the BaTiO3‐rich region, of 45(Ba,Sr)TiO3–10TiO2 samples annealed at 1250°C for 200 h in air showed that (Ba,Sr)TiO3 was in equilibrium with Ba6Ti17O40 (B6T17) and Ba4Ti13O30 phases with strontium solubility (Sr/(Ba + Sr)) of ∼0.02 and 0.20, respectively. In the second series the microstructures of samples consisting of a mixture of (Ba,Sr)TiO3 and 2.0 mol% TiO2, were observed after sintering at 1250°C for 100 h in air. {111} twins formed only in the samples with faceted B6T17 second phase particles, similar to the case of BaTiO3. In these samples, abnormal grain growth occurred in the presence of the {111} twins. In contrast, no {111} twins formed and no abnormal grain growth occurred in the samples containing second phase particles other than B6T17. With an increased substitution of strontium for barium, the aspect ratio of abnormal grains containing {111} twin lamellae was reduced. This result was attributed to a reduction in the relative stability of the {111} planes with the strontium substitution.

A structural transition of Ba6Ti17O40/BaTiO3 interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin‐assisted abnormal growth of faceted BaTiO3 grains was also obtained.

When sintered 85A1,0,-15Fe20, (in wt %) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO, and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe,O, from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation-dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.

A core-shell structure was observed in SrTiO 3 doped with 1.2 mol% of Nb 2 O 5 , after sintering in a reducing atmosphere (5H 2 -95N 2 ) and then in an oxidizing atmosphere (air). In undoped and Al 2 O 3 -doped SrTiO 3 specimens, no core-shell structure formed after the same sintering treatments as those for SrTiO 3 doped with 1.2 mol% of Nb 2 O 5 . The measured chemical compositions of the core and shell regions of 1.2-mol%-Nb 2 O 5 -doped SrTiO 3 grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was ∼1% less than that of core regions grown in 5H 2 -95N 2 , which was in good agreement with a value predicted by available defect equations. Therefore, the observed coreshell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.

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