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BaTi1-x Mn x O3 solid solutions with 0≤x≤0.04 were synthesized in reducing atmosphere (P O2<10-9 Pa) and in air at 1350°C for 2 h. The effect of Mn ions with different valences on the crystal structure of BaTiO3 was studied by the room-temperature Raman scattering, electron spin resonance (ESR) and X-ray powder diffraction analyses. The crystal structure of the solid solution was analyzed to determine the change of its lattice parameters. It was found that in a reducing atmosphere, Mn2+ ions promoted the tetragonal-to-cubic transformation, as both a contraction of c-axis and an extension of a-axis were observed with an increase in Mn content. In contrast, Mn ions in the air-synthesized samples promoted the tetragonal-to-hexagonal transformation. The formation of the hexagonal phase was accompanied by a strong extension of the c-axis of the tetragonal phase from 4.032 Å to 4.042 Å. This was attributed to Jahn-Teller distortions caused by Mn3+ ions.

The influence of the microstructure evolution on electrical properties as a parameter of the firing temperature was studied for materials in the BaTiO3(BT)–MgO–Ln2O3 (Ln=Ho and Dy) system. The sintering behavior and the formation of the core-shell structure were dependent on the kind of doped rare earth elements. The stability of the core-shell structure and electrical properties of Dy doped specimens against the firing temperature were much lower than those of the Ho doped specimens. Especially, the Dy doped disk specimens fired at more than 1320°C, in which the core-shell grains were destroyed as judged by the differential scanning calorimetry (DSC) measurement and the transmission electron microscopy (TEM) observation, exhibited characteristic electrical properties. The electrical properties of the Ho doped multilayer capacitor (MLC) specimen were superior to those of the Dy doped one. It was found that the microstructure had a definite influence on the electrical properties, such as the temperature dependence of the dielectric constant and the capacitance aging behavior under an unloaded field.

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The effect of re-oxidation treatment on the solubility in dopants and the dielectric properties of Ho–Mg- and Ho–Mn-substituted BaTiO3 was investigated. The lattice parameter and electrical resistivity results indicated that the predominant occupational sites of Ho ions changed from A-sites to B-sites by increasing the doped content in both systems. In the case of Ho–Mn-substituted samples, the decrease in lattice parameter and a shift of the Curie point (T c ) to higher temperatures by re-oxidation treatment were observed in the range in which Ho ions predominantly occupy B-sites (act as acceptors). On the other hand, Ho–Mg-substituted samples showed little change. Electron spin resonance measurements revealed that these changes due to re-oxidation were based on the decrease of the ionic size of Mn due to the oxidation of Mn2+ to Mn3+ or Mn4+. The occupational sites of Ho ions strongly affected the valence state of Mn and the dielectric properties of BaTiO3.