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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.

We have investigated the formation and the microwave dielectric properties of the Ba4(Nd28/3-y  R y)Ti18O54 (R = Eu, Dy, Ho, Er and Yb) solid solutions in which R ions are substituted for Nd ions in Ba6-3x Nd8+2x Ti18O54 solid solutions with x=2/3. Based on the change of the volume of the unit cell, it was inferred that Eu, Dy, Ho and Er ions substituted into rhombic A1-sites, but Yb ions did smaller sites such as B-sites and C-sites after substituting A1-sites. The quality factor (Q·f) improved as the ionic radius of substituted R ions increased. In the Eu series, the best Q·f value of 10405 GHz was obtained. The temperature coefficients of the reasonant frequency (τ f ) in the Dy, Ho and Er series were improved by more than 50%, but the dielectric constant (ε r ) showed only a nominal change of about 3–4 in the vicinity of 80.