  A review is given of recent neutron and x-ray scattering studies of the
lead-oxide perovskite relaxor systems Pb(Zn1/3Nb2/3)O3-xPbTiO3 and
Pb(Mg1/3Nb2/3)O3-xPbTiO3. X-ray measurements by Noheda et al. have established
that these two systems exhibit nearly identical phase diagrams in which a
rhombohedral-monoclinic-tetragonal structural sequence takes place with
increasing PbTiO3 concentration. Recent high-energy x-ray and neutron
measurements on single crystals of PZN and PMN-10PT, however, show that the
rhombohedral distortions occur only in the outermost 20 - 40 microns, while the
bulk of each crystal transforms into a new phase X, which has a nearly cubic
unit cell. This situation is very similar to the structural behavior of pure
PMN at Tc = 220 K. A simple model has been suggested that correlates phase X to
the unique atomic displacements of the polar nanoregions, which are created at
the Burns temperature.
