  Experimental data obtained for PbFe$_{1 / 2}$Ta$_{1 / 2}$O$_{3}$ (PFT) and
NaNbO$_{3}$: Gd (NaNbGd) single crystals show a diffused dielectric
permittivity peak that is inherent to relaxor ferroelectrics. However some
deviations from the normal relaxor properties were also observed and are under
discussion. One of the key features of the relaxors is the existence of the
Burns temperature, at which the polar regions appear. We found out that PFT
shows this feature but the NaNbGd dielectric behavior is different. We analyse
these properties within a phenomenological theory and propose a microscopic
model.
