Dielectric characteristics of the relaxor state of the perovskite ceramics 0.9(Na$_{1-x}$K$_x$Bi)$_{1/2}$TiO$_3$–0.1Bi(ZnTi)$_{1/2}$O$_3$ near the morphotropic phase boundary

X-ray diffraction investigations have revealed that, in the system of solid solutions 0.9(Na$_{1-x}$K$_x$Bi)$_{1/2}$TiO$_3$–0.1Bi(ZnTi)$_{1/2}$O$_3$, there is a morphotropic phase boundary in the potassium concentration range of $x\approx$ 0.25, which separates the regions of compositions with rhombohedral $(R3c)$ and tetragonal $(P4mm)$ structures. It has been shown that, in the vicinity of this boundary, ceramic samples of the studied system exhibit properties of relaxor ferroelectrics. The results of the investigation into the dielectric properties of relaxor ceramics of the composition $x$ = 0.3 with the use of the impedance spectra measured in the frequency range from 25 to 10$^6$ Hz at temperatures from 100 to 900 K have been presented. It has been found that, in the temperature region of the existence of the relaxor state lying below the temperature corresponding to the maximum of the real part of the permittivity ($T'_m$ = 550 K), the dielectric polarization is determined by the sum of the contributions from the matrix and dipole clusters. The temperature dependence of the contribution from the clusters, which is determined by the kinetics of their formation and freezing, is characterized by a curve with the maximum at approximately 400 K. The process of freezing of dipole clusters occurs over an extended temperature range of more than 200 K.