Refined phase portrait of the rhombohedral region of the $x$–$T$ diagram of the Pb(Zr$_{1-x}$Ti$_x$)O$_3$ system and singularities of dielectric spectra of its solid solutions

The boundaries of the phase states of solid solutions in the rhombohedral region of the Pb(Zr$_{1-x}$Ti$_x$)O$_3$ (0.00 $\le x\le$ 1.00) system and the curve of the $R3c\to R3m$ phase transition have been refined on the basis of detailed (with a small concentration step) high-temperature (25$^\circ$C $\le T\le$ 700$^\circ$C) studies of the structural characteristics and frequencies of the piezoelectric resonance of the radial oscillation mode of polarized ceramic samples. It has been shown that, in the range 0.07 $\le x\le$ 0.20, the transition curve practically coincides with the generally accepted one and, in the interval 0.20 $\le x\le$ 0.36, it differs from the latter by a nonmonotonic behavior and agrees in the boundaries and regions of the phase states. The curve of the transition from the region of coexistence of the $R$- and $Rh_1$-phases in the clusterized state to a region with single $Rh_1$-phase in the interval of concentrations 0.08 $\le x\le$ 0.12 and temperatures 200–250$^\circ$C has been constructed. The full $x$–$T$ phase diagram of the Pb(Zr$_{1-x}$Ti$_x$)O$_3$ system has been presented. Dispersion spectra of solid solutions on variation in the frequency of the measuring electric field in the interval of 25–106 Hz have been studied, and their correlation with singularities of the phase portrait has been established.