Crystal Chemistry and Dielectric Properties of Chemically Substituted (Bi1.5Zn1.0Nb1.5)O7 and Bi2(Zn2/3Nb4/3)O7 Pyrochlores

Abstract

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: (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Zrx)O7 (0.0 ≤x≤ 1.5) and (Bi1.5Zn1−(x/3)Nb1.5−(2 x/3)Tix)O7 (0.0 ≤x≤ 2.25). For Gd cations, the range of solid solution was quite limited in (Bi1.5−xGdxZn1.0Nb1.5)O7 (0.0 ≤x≤ 0.2); however, the range of solid solution is more extensive in the system that contains Ti and Gd cations ((Bi1.5−xGdxZn0.5Ti1.5Nb0.5)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 A‐ and B‐site ions in the pyrochlore structure (RA/RB). The substituted phases exhibited permittivities (κ′) in the range of ∼100–200 and relatively low dielectric losses (tan δ < 7 × 10−4) at 1 MHz. The temperature coefficients of permittivity (τκ) varied in accordance with the RA/RB value and could be tuned from −88 ppm/K to −1300 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 (κ′= 90, τκ= 150 ppm/K).