  We have measured the bending-induced polarization of Pb(Mg1/3Nb2/3)O3-PbTiO3
single crystals with compositions at the relaxor-ferroelectric phase boundary.
The crystals display very large flexoelectricity, with flexocoupling
coefficients an order of magnitude bigger than the theoretical upper limit set
by the theories of Kogan and Tagantsev. This enhancement persists in the
paraphrase up to a temperature T* that coincides with the start of elastic
softening in the crystals. Analysis of the temperature dependence and
cross-correlation between flexoelectric, dielectric and elastic properties
indicates that the large bendinginduced polarization of relaxor ferroelectrics
is not caused by intrinsically giant flexoelectricity, but by the reorientation
of polar nanotwins that become ferroelastically active below T*.
