  In contrast to ordinary ferroelectrics where the temperature, Tm, of the
permittivity maximum monotonically increases with bias field, E, in
(1-x)PbMg1/3Nb2/3O3-(x)PbTiO3 (0<x<0.35) single crystals, Tm was found to
remain constant or decrease with E up to a certain threshold field, Et, above
which Tm starts increasing. Et decreases with x and almost disappears at about
x=0.4. We explain this field dependence by the quenched random fields and
coupling between the orientable dipoles with average polarization. For crystals
with 0.06<x<0.13, the T-E phase diagrams are constructed, which include the
relaxor, ferroelectric, glass and mixed ferroelectric-glass phases. The
boundary between the ferroelectric and relaxor phases corresponds to a
first-order phase transition and is characterized by thermal hysteresis with a
large curvature of the line obtained in the field cooling mode, and by the
presence of the end-point above which there is no difference between the
ferroelectric and relaxor phases. The glass phase has a nearly
field-independent boundary in the vicinity of the Vogel-Fulcher temperature
TVF. The coupling between the glass order parameter and polarization changes
the ferroelectric boundary at temperatures lower than TV
