The successive phase transformations in MgO-doped BaTiO 3 were studied. Upon MgO doping, dielectric anomalies corresponding to lower phase transformations were broadened and depressed, while an anomaly for a cubic-tetragonal transformation remained and shifted to a lower temperature. XRD peak splitting upon tetragonality of BaTiO 3 was decreased, and the peaks exhibited abnormally broadened profiles which are different from the one for cubic BaTiO 3 above T c . Raman spectroscopy revealed the existence of orthorhombic phase at room temperature for the solid solution with 0.5 mol% or more MgO. The temperature dependence of the Raman spectrum showed that orthorhombic and rhombohedral phases in MgO-doped BaTiO 3 were stabilized at higher temperatures than pure BaTiO 3 .

Since the early study by Galasso and Pyle, the structure of complex perovskite compounds with 1:2 ordering of B-site ions, ordered perovskite compounds, has been thought to be Ba(Sr1/3Ta2/3)O3-type hexagonal. Recently, a structural phase transformation has been found in A-site substituted ordered perovskite compounds, (Ba,Sr)(B'1/3B"2/3)O3. The structural change due to the phase transformation has been connected with the tilting of oxygen octahedra in the perovskite cell. However, the structure of the transformed phase has not been clarified because there has not been any model to treat the crystal structure of ordered perovskite compounds having tilted oxygen octahedra. In this paper, Glazer's model for structural changes in cubic perovskite due to the tilting of oxygen octahedra has been expanded to the 1:2 ordered perovskite. Based on the model, electron diffraction and X-ray diffraction of (Ba1-x Sr x )(Mg1/3Ta2/3)O3 are discussed. A monoclinic unit cell in space group C 2h 6 with an angle γ > 90 °, which is derived from antiphase tilting of oxygen octahedra, successfully explains the experimental results.

High-toughness and high-strength lead zirconate titanate (PZT) composites that contain fine silver particles were successfully fabricated at low sintering temperatures. Addition of silver to a PZT matrix did not result in unwanted reaction phases; however, some silver diffused toward the perovskite crystal structure. A small quantity of silver accelerated the sinterability of the PZT composites. The formation of oxygen vacancies due to the partial substitution of silver appeared to enhance the sinterability of the PZT. Fracture toughness depended on the size and degree of sphericity of the silver particles, and SEM observations on crack propagation suggested that the toughening mechanism in the PZT/Ag composites involves crack bridging resulting from the ductile behavior of silver particles. It is proposed that high fracture strength in PZT/1 to 5 vol% Ag composites results from the relaxation of transformationinduced internal stress by the silver particles.

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A characteristic change in the dielectric properties with A-site substitution is known in complex perovskite compounds, (Ba, Sr)(B', B")O3, which are expected to be applied to dielectric resonators. Recently, a phase transformation upon tilting of oxygen octahedra has been found in some of the compounds. However the oxygen tilting transformation has not been considered to affect the dielectric constant explicitly. To clarify the relationship between the dielectric constant and the phase transformation, dielectric properties of a solid-solution system, (Ba1-x Sr x )(Mg1/3Ta2/3)O3, have been studied. The origin of the dielectric constant, the polar mode of the lattice vibration, has been investigated using infrared reflectivity spectroscopy and the Kramers-Kronig analysis. It was found that the phase transformation due to the tilting of the oxygen octahedra clearly affects polar phonon modes of the compound. The origin of the change in the dielectric properties with Sr concentration is discussed referring to the behavior of polar phonon modes.