Bias-field effect on the temperature anomalies of dielectric permittivity in PbMg1 3Nb2 3O3-PbTiO3 single crystals

In contrast to ordinary ferroelectrics where the temperature ${T}_{m}$ of the permittivity maximum monotonically increases with bias field $E$ in $(1\ensuremath{-}x)\mathrm{Pb}{\mathrm{Mg}}_{1∕3}{\mathrm{Nb}}_{2∕3}{\mathrm{O}}_{3}\text{\ensuremath{-}}(x)\mathrm{Pb}\mathrm{Ti}{\mathrm{O}}_{3}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.35)$ 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 $0.06\ensuremath{\leqslant}x\ensuremath{\leqslant}0.13$, the $E\text{\ensuremath{-}}T$ 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.