RANDOM FIELDS IN RELAXOR FERROELECTRICS - A JUBILEE REVIEW

Substitutional charge disorder as in PbMg1/3Nb2/3O3, structural cation vacancies as in SrxBa1-xNb2O6 and isovalent substitution of off-centered cations as in BaTi1-xSnxO3 and BaTi1-xZrxO3 give rise to quenched electric random-fields (RFs), which we proposed to be at the origin of the peculiar behavior of relaxor ferroelectrics 20 years ago. These are, e.g. a strong frequency dispersion of the dielectric response and an apparent lack of macroscopic symmetry breaking in the low temperature phase. Both are related to mesoscopic RF-driven phase transitions, which give rise to irregularly shaped quasi-stable polar nanoregions below the characteristic temperature T*, but above the global transition temperature T-c. Their co-existence with the paraelectric parent phase can be modeled by time-dependent field equations under the control of quenched RFs and stress-free strain (in the case of order parameter dimension n >= 2). Transitions into global polar order at Tc may occur in uniaxial relaxors as observed on the uniaxial relaxor ferroelectric Sr0.8Ba0.2Nb2O6 and come close to RF Ising model criticality. Re-entrant relaxor transitions as observed in solid solutions of Ba2Pr0.6Nd0.4(FeNb4)O-15 are proposed to evidence the coexistence of distinct normal and relaxor ferroelectric phases within the framework of percolation theory.