Evidence of decoupled lattice distortion and ferroelectric polarization in the relaxor system PMN-xPT

We report high q-resolution neutron scattering data on $\mathrm{PMN}\ensuremath{-}x\mathrm{PT}$ single crystals with $x=20%$ and $27%.$ No rhombohedral distortion occurs in the 20PT sample for temperatures as low as 50 K. On the other hand, the 27PT sample transforms into a rhombohedral phase below ${T}_{C}\ensuremath{\sim}375\mathrm{K}.$ Our data provide conclusive evidence that a phase with an average cubic lattice is present in the bulk of this relaxor system at low PT concentration, in which the ferroelectric polarization and lattice distortion are decoupled. The rhombohedral distortion is limited to the outermost tens of microns of the crystal.