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The formation mechanism of the core-shell structure of a BaTiO3(BT)–MgO–Ho2O3-based system was studied. Mg reacted with BT at low temperatures and showed poor diffusivity compared with Ho. The core-shell structure was considered to be formed by the suppression of the diffusion of Ho into the core region by Mg. Also, replacement modes of Mg and Ho in perovskite were investigated. Lattice parameters were determined at temperatures higher than the Curie temperature in order to avoid crystal structure change. It was confirmed that Mg dissolved in Ti site, and Ho dissolved in both Ba and Ti sites. This indicates that Mg ions act as acceptors and Ho ions act as both donors and acceptors in the shell phase.

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The formation of solid solutions of microwave dielectric compounds having compositions in the vicinity of Ba3.75-R 9.5Ti18O54 and/or BaO·R 2O3·4TiO2 (R=rare earth) has been investigated. 3BaO·2R 2O3·9TiO2 (Ba6 R 8Ti18O54) solid solutions with the ideal formulae of Ba6-3x R 8+2x Ti18O54 (0.0≤x≤1.0) have been derived on the tie line with the ratio Ti:O=1:3 between perovskite and R 2Ti3O9 based on the new tungsten bronze-type crystal structure with 2×2 perovskitelike unit cells. The type of solid solutions is substitutional with following the relation 3Ba2+\rightleftarrows2R 3++\Box. Lattice parameters as a function of composition have been determined by the WPPD (whole-powder-pattern decomposition) method.17) The formation range is located at 0.3≤x≤0.7 compositions in the case of R=Sm and 0.0≤x≤0.7 in R=Nd. In the latter case, two kinds of substitutions of the solid solutions are considered.