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)O7 and Bi2(Zn2/3Nb4/3)O7. Broad ranges of solid solutions based on cubic (Bi1.5Zn1.0Nb1.5)O7 were observed for Ti and Zr cations: (Bi1.5Zn12(x/3)Nb1.52(2x/3)Zrx)O7 (0.0 < x < 1.5) and (Bi1.5Zn12(x/3)Nb1.52(2x/3)Tix)O7 (0.0 < x < 2.25). For Gd cations, the range of solid solution was quite limited in (Bi1.52xGdxZn1.0Nb1.5)O7 (0.0 < x < 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi1.52xGdxZn0.5Ti1.5Nb0.5)O7 (0.0 < x < 0.6)). The stability fields of the solid solutions could be interpreted by using the ratio of the ionic sizes of the Aand B-site ions in the pyrochlore structure (RA/RB). The substituted phases exhibited permittivities (k*) in the range of ;100–200 and relatively low dielectric losses (tan d < 7 3 10) at 1 MHz. The temperature coefficients of permittivity (tk) varied in accordance with the RA/RB value and could be tuned from 288 ppm/K to 21300 ppm/K. For the orthorhombic Bi2(Zn2/3Nb4/3)O7 pyrochlore, the ranges of solid solution were very limited and the dielectric properties remained similar to those of the undoped phase (k* 5 90, tk 5 150 ppm/K).