  We review some of the current research on two of the ultra-high piezoelectric
relaxors Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN). The discovery of a
monoclinic phase in Pb(Zr1-xTix)O3 (PZT) in 1999 by Noheda et al. forced a
reassessment of the structural symmetries found near the morphotropic phase
boundary in PZT, as well as in the PbTiO3 (PT) doped systems PZN-PT, and
PMN-PT. All three systems are now known to have nearly identical phase diagrams
that exhibit a rhombohedral-monoclinic-tetragonal structural sequence with
increasing PbTiO3 concentration. The dynamical properties of these relaxors are
controlled by the unique nature of the so-called polar nanoregions (PNR), which
first appear at the Burns temperature Td as evidenced by the onset of diffuse
scattering. That this diffuse scattering persists to temperatures below the
Curie temperatures of PMN (220 K) and PZN (410 K) indicates that the phase
transitions in these relaxors are very different from those observed in
ordinary ferroelectrics. They also differ in this regard from what would be
expected in the often-quoted model based on analogy with the random field
effect.
