Optimising bismuth magnesium niobate pyrochlores as potential ceramic dielectrics: Reaction progression, phase equilibria and impedance properties

The phase-pure Bi₂O₃-MgO-M₂O₅ (BMN) cubic pyrochlores formed across a broad compositional range, with bismuth content varying from 41.88 to 44.50 mol %. The quadrilateral subsolidus region is defined by two substitution mechanisms: (i) Mg²⁺ substitution by Nb⁵⁺, balanced by oxygen stoichiometry variations and (ii) one-to-one Bi³⁺ replacement by Nb⁵⁺, with charge compensation by O²⁻. This yields a general formula of Bi_3.36+xMg_1.92-yNb_2.72-x + yO_{13.76-x+(3/2)y}, involving two variables with the corresponding subsolidus solution limits of 0.10 ≤ x ≤ 0.20 and 0.00 ≤ y ≤ 0.16, respectively. Notably, these BMN pyrochlores exhibit high dielectric constants, ε′ in the range of 167–204 at 30 °C, 1 MHz and low dielectric losses, tan δ in the order of 10⁻⁴ - 10⁻³. The elevated dielectric constant is likely due to the enhanced polarisability of Nb₂O₅ within the crucial BO₆ octahedra of the pyrochlore structure. Furthermore, these pyrochlores display negative temperature coefficients of permittivity, TCε' ranging from −528 to −742 ppm/°C at 30–300 °C and 1 MHz. These thermally stable and highly insulating BMN pyrochlores with high activation energies, Ea >1.0 eV, are promising candidate for potential application in ceramic capacitors.