Structure and dielectric properties of Bi$_{6-x}$Sr$_x$Ti$_{2-x}$Nb$_{2+x}$O$_{18}$ ($x$ = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi$_{6-x}$Sr$_x$Ti$_{2-x}$Nb$_{2+x}$O$_{18}$ ($x$ = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity $\varepsilon/\varepsilon_0(T)$ and dielectric loss tangent $\tan\delta$ have been measured. The dependences of the maximum of the permittivity $\varepsilon/\varepsilon_0$, Curie temperature $T_C$, lattice parameters, and the unit cell volume on $x$ have been obtained. The structural parameter $a$, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in $x$.