Structure and dielectric properties of solid solutions Bi$_7$Ti$_{4+x}$W$_x$Nb$_{1-2x}$O$_{21}$ ($x$ = 0–0.5)

The structural and electrophysical characteristics of a series of solid solutions of layered perovskite-type oxides Bi$_7$Ti$_{4+x}$W$_x$Nb$_{1-2x}$O$_{21}$ ($x$ = 0–0.5) have been investigated. According to X-ray powder diffraction data, all the studied compounds are single-phase and have the structure of Aurivillius phases ($m$ = 2.5) with an orthorhombic crystal lattice (space group I2cm, $Z$ = 2). The changes in the tetragonal and orthorhombic distortions of perovskite-like layers in the compounds have been considered depending on their chemical composition. The temperature dependences of the relative permittivity $\varepsilon(T)$ have been measured. It has been shown that the Curie temperature TC of the perovskite-type oxides Bi$_7$Ti$_{4+x}$W$_x$Nb$_{1-2x}$O$_{21}$ ($x$ = 0–0.5) decreases linearly with an increase in the parameter $x$. The activation energies of charge carriers have been obtained in different temperature ranges. It has been found that there are three temperature regions with very different activation energies due to different natures of charge carriers in the studied compounds.