  Ferroelectric domain structure evolution induced by an external electric
field was investigated by means of nematic liquid crystal (NLC) method in two
strontium-barium niobate single crystals of nominal composition:
Sr_{0.70}Ba_{0.30}Nb_{2}O_{6} (SBN:70 - relaxor) and
Sr_{0.26}Ba_{0.74}Nb_{2}O_{6} (SBN:26 - ferroelectric). Our results provide
evidence that the broad phase transition and frequency dispersion that are
exhibited in SBN:70 crystal have a strong link to the configuration of
ferroelectric microdomains. The large leakage current revealed in SBN:26 may
compensate internal charges acting as pinning centers for domain walls, which
gives rise to a less restricted domain growth similar to that observed in
classical ferroelectrics. Microscale studies of a switching process in
conjunction with electrical measurements allowed us to establish a relationship
between local properties of the domain dynamics and macroscopic response i.e.,
polarization hysteresis loop and dielectric properties.
