In contrast to ordinary ferroelectrics where the temperature T m of the permittivity maximum monotonically increases with bias field E in ͑1−x͒PbMg 1/3 Nb 2/3 O 3 -͑x͒PbTiO 3 ͑0 ഛ x ഛ 0.35͒ single crystals, T m was found to remain constant or to decrease with E up to a certain threshold field E t , above which T m starts increasing. The threshold field E t decreases with increasing x, tending toward zero at approximately x = 0.4. We explain this dependence in the framework of models which take into account quenched random fields and random bonds. For crystals with 0.06ഛ x ഛ 0.13, the E-T phase diagrams are constructed. In contrast to PMN, they exhibit an additional, nearly field-independent boundary, in the vicinity of the Vogel-Fulcher temperature. We believe this boundary to correspond to an additional phase transition and the appearing order parameter is likely to be nonpolar.

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In the present study, we diluted either A- or B- sublattice of perovskite multiferroic PbFe(0.5)Nb(0.5)O(3) (PFN) and studied the changes of the magnetic and ferroelectric phase transition temperatures and dielectric properties caused by such dilution. Dielectric studies of PFN single crystals show that, in contrast to the commonly adopted view, the ferroelectric phase transition in PFN is non-diffused and the relaxor-like behavior usually observed in ceramic samples has an extrinsic nature. A-site substitutions (Ba, Ca) lead to the smearing of the permittivity-temperature maximum, lowering its maximum temperature, T(m) and inducing relaxor behavior. B-site diluting of PFN by Ti increases T(m) and only slightly affects the permittivity maximum diffuseness. Both A-site and B-site substitutions in PFN lead to lowering of its Neel temperature, T(N). However, above a certain compositional threshold, fast lowering of T(N) stops and a new magnetic state with comparatively high (~50K) transition temperature becomes stable in a rather wide compositional range.