Structure and Dynamics of the Ferroelectric Relaxors Pb(Mg1/3Nb2/3)O3   and Pb(Zn1/3Nb2/3)O3
  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. 
G. Shirane and P.M. Gehring