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.

We show that the phase diagram of the ͑001͒ PbMg 1/3 Nb 2/3 O 3 -xPbTiO 3 ͑PMN-xPT͒ ͑x = 0.00, 0.06, 0.13, and 0.24͒ and ͑111͒ PMN-0.24PT lead-magnesium niobate mixed with lead titanate possesses a quasivertical line in the E electric field -T temperature plot, which hardly depends on the field. The existence of this line has been confirmed by independent studies of single crystals grown in different laboratories, by measuring the dielectric permittivity, compliances, and optical transmission, also in different laboratories. A thermal hysteresis inherent to first order phase transitions complicates the phase diagram. The piezoelectric coefficients of the ͑001͒ PMN-xPT ͑x ϭ 0.06 and 0.13͒ have two peaks versus temperature, at finite fields. The first peak is due to the quasivertical phase boundary. The second is in the vicinity of a turning point of the T m ͑E͒ temperature of the dielectric permittivity diffuse maximum. We show that the second peak prevails at reasonable fields, and the piezoelectric coefficients have comparatively large values at this peak, even at small x.

We study the E-T phase diagram and dynamical pyroelectric coefficient of an (001)-oriented flux-grown 0.8Pb(Mg1/3Nb2/3)O3–0.2PbTiO3 single crystal. We found that the highest pyroelectric coefficient can be achieved at the point having quasicritical features. These data establish a quasicritical behavior of the field-induced pyroelectric coefficients in relaxors.

Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO3:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization.