The order–disorder transition in Bi2O3–Nb2O5 fluorite-like dielectrics

An analysis of the microwave dielectric properties showed that the cubic fluorite solid solutions (1 − x)Bi2O3·xNb2O5 (0.23 ≤ x ≤ 0.26), the so-called δ-BNSS Type II, exhibit a high permittivity of 100, but also high dielectric losses (Q × f = 300 GHz) and a high temperature dependence of resonant frequency (τf = −200 ppm/K). Investigations using X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed that the cubic incommensurate phase undergoes an order–disorder phase transition into a commensurate tetragonal phase at temperatures below 900 °C (for x = 0.25). The higher degree of ordering in the tetragonal phase reduces the dielectric losses (Q × f = 1000 GHz) and changes the sign of the temperature coefficient of resonant frequency (τf = 100 × 10−6/K).