Strontium-barium niobates Sr_xBa_1-xNb₂O₆ (SBN) are environmental friendly (lead free) polar materials. Their potential applications are based on very attractive pyroelectric, electromechanical, electro-optic, photorefractive, and nonlinear optical properties. By changing the ratio between strontium and barium components one may tune the system from a conventional ferroelectric (x<0.5) to an extreme relaxor (x>0.6) behaviour while maintaining the structure unchanged.

In this contribution we present results of systematic investigations of the dielectric response measured along the polar c-axis of the four single crystalline compounds with nominal x=0.40, 0.50, 0.61 and 0.75, designated hereafter as SBN40, SBN50, SBN61, and SBN75, respectively. Temperature dependences of the linear dielectric susceptibility reveal a gradual crossover from the conventional ferroelectric (SBN40) to extreme relaxor (SBN75) behaviour. Analysis of the data within the framework of the Fisher law shows that the Curie point of ferroelectric SBN40 is a linear extrapolation of the transition temperatures determined for the other SBN crystals. Correspondingly, a change from the "normal" compact domains to the smaller ones with jagged (fractal-like) boundaries was revealed by piezoresponse force microscopy (PFM).

The dielectric spectra of SBN61 and SBN75 disclose two distinct slow dynamics modesThey refer to the borders of mesoscopic polar nanoregions (PNRs) embedded in the neutral paraelectric environment. Due to their very broad size distribution the largest PNRs become visible by PFM.

From the nonlinear dielectric response the nonlinearity coefficient B appearing in the equation of state is determined. It is found to decrease when approaching the phase transition point even in the extreme relaxor crystal. This kind of behaviour shows that the uniaxial SBN relaxor cannot be considered as a dipole glass system.

Financial support by KBN (under grant 4 T08A 007 25) and DFG (SPP "Strukturgradienten in Kristallen") is gratefuly acknowledged.