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.

The microwave dielectric properties of the Ba6-3 x Sm8+2 x Ti18O54 (0.3≤x≤0.7) solid solutions are studied in relation to the crystal structure. Q·f of suitable materials obtained by optimizing the sintering condition is in inverse proportion to the dielectric constant (ε r ). With increase of x, ε r decreased and Q·f increased. Good-quality microwave resonators with ε r =80 and Q·f=9720 GHz can be obtained in the composition range of x=0.5 to 0.7 in the solid-solution region. Substitution of 2Sm3+ for 3Ba2+ in the large cation sites results in the creation of vacancies and crystal distortion, showing that the shrinkage of the TiO6 octahedral site plays an important role for improvement of the microwave dielectric properties.

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Microwave dielectric ceramics with high quality have received attention due to the rapid progress in microwave telecommunication and satellite broadcasting. The desirable properties in microwave dielectric resonators are a high dielectric constant (εr), low dielectric loss (tanδ) (i.e. high Q (Q =1/tanδ)), and low temperature coefficient of the resonant frequency(τf). Properties of dielectric ceramics in the TiO2-rich region of the BaO-Nd2O3-TiO2 ternary system (Fig. 1) were reported to have excellent dielectric constants, low dielectric losses and low temperature coefficients of capacitance by Kolar et al. (1978).