Dielectric Properties of the Fluorite‐like Bi 2 O 3 –Nb 2 O 5 Solid Solution and the Tetragonal Bi 3 NbO 7

Our analysis of the microwave dielectric properties of the δ‐Bi 2 O 3 –Nb 2 O 5 solid solution (δ‐BN ss ) showed a continuous increase in permittivity and dielectric losses with an increasing concentration of Nb 2 O 5 . The only discontinuity was found for the temperature coefficient of resonant frequency, which is negative throughout the entire homogeneity range but reaches a minimum value for the sample with 20 mol% Nb 2 O 5 . At the same composition there is a discontinuity in the grain size of the δ‐BN ss ceramics. For the sample containing 25 mol% Nb 2 O 5 two structural modifications were observed. A single‐phase tetragonal Bi 3 NbO 7 , in the literature referred to as a Type‐III phase, is formed in a very narrow temperature range from 850° to 880°C. A synthesis performed below or above this temperature range resulted in the formation of the end member of the δ‐BN ss homogeneity range. Compared with the δ‐BN ss the Bi 3 NbO 7 ceramics exhibit lower microwave dielectric losses, an increased conductivity, and a positive temperature coefficient of resonant frequency.