Dielectric spectroscopy of Pb$_{1-x}$Ba$_{x}$(Mg$_{1/3}$Nb$_{2/3}$)$_{m}$ (Zn$_{1/3}$Nb$_{2/3}$)$_{y}$(Ni$_{1/3}$Nb$_{2/3}$)$_{n}$ Ti$_{z}$O$_{3}$ solid solutions in a wide temperature interval

The dielectric spectra of Pb$_{1-x}$Ba$_{x}$(Mg$_{1/3}$Nb$_{2/3}$)$_{m}$ (Zn$_{1/3}$Nb$_{2/3}$)$_{y}$(Ni$_{1/3}$Nb$_{2/3}$)$_{n}$ Ti$_{z}$O$_{3}$ ($x$ = 0–0.15, $m$ = 0.4541, $y$ = 0.0982, $n$ = 0.1477, and $z$ = 0.3) ceramic samples have been studied in wide intervals of temperature (10–873 K) and frequency of the measurement electric field (0.1–1000 kHz). It has been found that an increase in the Ba$^{2+}(x)$ content leads to a reduction in the phase transition temperature (from 418 K at $x$ = 0 to 256 K at $x$ = 0.15), to the transition from the normal ferroelectric state to the relaxor ferroelectric one (at $x\ge$ 0.025), and to the disappearance of temperature hysteresis of dependences of the relative dielectric permittivity. It has been hypothesized that a tricritical point is present near $x\sim$ 0.125 in the $x$–$T$ phase diagram of the studied solid solutions.