Ferroelectric to Relaxor Crossover in Perovskite-type Systems

The ferroelectric to relaxor crossover of A(B1-xB '(x))O-3-type perovskites with increasing x is discussed from the order-disorder point of view for Ba(Ti1-xSnx)O-3 (BTS-x) as an example. The polar short-range order of the disordered off-centred Ti ions in BaTiO3 increases with decreasing temperature, leads to nucleation and, finally, to the long-range ordered ferroelectric phase. This process becomes impeded with increasing content of Sn ions x that make the nucleation and the transition more and more incomplete and finally impossible (relaxor). The composition BTS-0.1 corresponds to PMN-PT and PZN-PT systems near the morphotropic phase boundary where the temperature T-m of maximum permittivity is mainly determined by nucleation rather than by a transition to tetragonal long-range order. This is concluded from different electromechanical properties and confirmed by additional aging experiments on BTS-0.1. Other reasons for incomplete transitions to long-range order are not discussed in this paper.