Crystal chemistry and dielectric properties of chemically substituted (Bi1.5Zn1.0Nb1.5)O7 and Bi2(Zn2/3Nb4/3)O7 pyrochlores

The effect of the chemical substitution of Ti, Zr, and Gd cations on the structure and properties of bismuth-based pyrochlores was investigated for (Bi1.5Zn1.0Nb1.5)O-7 and Bi-2(Zn2/3Nb4/3)O-7. Broad ranges of solid solutions based on cubic (Bi1.5Zn1.0Nb1.5)O-7 were observed for Ti and Zr cations: (Bi1.5Zn1-(x/3)Nb1.5-(2x/3)Zrx)O-7 (0.0 less than or equal to x less than or equal to 1.5) and (Bi1.5Zn1-(x/3)Nb1.5-(2x/3)Tix)O-7 (0.0 less than or equal to x less than or equal to 2.25). For Gd cations, the range of solid solution was quite limited in (Bi1.5-xGdxZn1.0Nb1.5)O-7 (0.0 less than or equal to x less than or equal to 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi1.5-xGdxZn0.5Ti1.5Nb0.5)O-7 (0.0 less than or equal to x less than or equal to 0.6)). The stability fields of the solid solutions could be interpreted by using the ratio of the ionic sizes of the A- and B-site ions in the pyrochlore structure (R-A/R-B). The substituted phases exhibited permittivities (kappa') in the range of similar to 100-200 and relatively tow dielectric losses (tan delta < 7 x 10(-4)) at 1 MHz. The temperature coefficients of permittivity (tau(kappa)) varied in accordance with the R-A/R-B value and could be tuned from -88 ppm/K to -1300 ppm/K. For the orthorhombic Bi-2(Zn2/3Nb4/3)O-7 pyrochlore, the ranges of solid solution were very limited and the dielectric properties remained similar to those of the undoped phase (kappa' = 90, tau(kappa) = 150 ppm/K).