  Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_{3}$ is one of the simplest representatives of the
lead ferroelectric relaxors. Its solid solution with PbTiO$_{3}$, at
concentrations near the morphotropic phase boundary, is especially important.
In this paper, we apply the tested earlier on Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$
approach, to analyze the light scattering from both, nominally pure
Pb(Zn$_{1/3}$Nb$_{2/3} $)O$_{3}$ crystal and a crystal containing 4.5% of
PbTiO$_{3}$, measured in a broad temperature range from 1000 to 100 K. We
propose a comprehensive picture of the temperature evolution of the lattice
dynamics in these crystals and associated structural transformations. We show,
that in PZN, like in PMN, short-lived dynamic lattice distortions exist even at
the highest measured temperatures. With cooling down, these distortions develop
in Fm$\bar{3}$m clusters and R3m polar nanoregions that are still capable of
reorientational motion. From the Burns temperature T$_{d}$, this motion becomes
progressively restricted. The freezing process starts at the temperature
T$^{*}$ and continues down to T$_{d0}$. The major Raman lines in all these
materials behave very similarly. However, the temperature behavior of a weak
line E in each case is essentially different. Its splitting can serve as a
measure of the order parameter of the system.
