Phase diagrams of solid solutions of relaxor ferroelectrics from the dielectric spectroscopy data

The dielectric spectra of Pb$_{(1-z)}$Ba$_{z}$ (Mg$_{1/3}$Nb$_{2/3})_{m}$ (Zn$_{1/3}$Nb$_{2/3})_{y}$ (Ni$_{1/3}$Nb$_{2/3})_{n}$Ti$_{x}$O$_{3}$ ($x$ = 0.25–0.4, $y$ = 0.1130–0.0842, $m$ = 0.4844–0.1298, $n$ = 0.1266–0.4726, $z$ = 0–0.15) ceramics with substitution in both A and B crystallographic positions of the perovskite structure are studied. The system demonstrates a transition from the relaxor state to the normal ferroelectric state in both cases: when the concentration of lead titanate grows and the concentration of barium is reduced. On the basis of experimental results, the $x$–$T$ and $z$–$T$ phase diagrams are plotted. Despite different crystal chemical reasons of the relaxor state emergence in the investigated solid solutions, their diagrams demonstrate an evident similarity. We have revealed the disappearance of the temperature hysteresis at the transition to the relaxor state in both cases, which has allowed us to make an assumption of the existence of tricritical points on the corresponding diagrams.