Dielectric properties and phase transitions of Bi3Nb1−xTaxO7 fluorite-type dielectrics

A study of Bi3Nb1−xTaxO7 ceramics revealed that these solid solutions undergo an incommensurate-to-commensurate, Fm3m⇔I4¯m2, phase transition. An analysis of the microwave dielectric properties showed that an increase of x for the Bi3Nb1−xTaxO7 results in a slight decrease in the permittivity and an increase in the Q × f values for both crystal modifications. However, the order–disorder phase transition at temperatures below 900 °C reduces the dielectric losses: the Q × f values for the commensurate-tetragonal Bi3Nb1−xTaxO7 samples are in the range 300–400 GHz higher than the values of the incommensurate-cubic analogues. In addition, the phase transition induces positive values of the temperature coefficient of resonant frequency (τf) for the commensurate-tetragonal Bi3Nb1−xTaxO7 fluorite-type dielectrics.