Bias-field effect on the temperature anomalies of dielectric permittivity in PbMg_(1/3)Nb_(2/3)O_3-PbTiO_3 single crystals

摘要

In contrast to ordinary ferroelectrics where the temperature T_m of the permittivity maximum monotonically increases with bias field E in (1-x)PbMg_(1/3)Nb_(2/3)O_3-(x)PbTiO_3 (0 ≤ x ≤ 0.35) 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 0.06 ≤ x ≤ 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.