  Synchrotron x-ray scattering studies of anti-ferroelectric nanoregions (AFR)
as fluctuations, which give rise to 1/2(hk0) superlattice reflections (alpha
spots), were systematically performed in Pb(Mg1/3Nb2/3)O3 (PMN) ferroelectric
relaxor. Separation of the alpha spots from underlying diffuse scattering
background allowed studying them as separate entities for the first time. AFR
fluctuations were shown to be different from the chemical nanodomains (CND) and
ferroelectric polar nano-regions (PNR). Instead, they are formed by
anti-parallel short-range correlated Pb^2+ displacements that lead to unit cell
doubling in <110> directions, based upon structure factor calculations.
Correlation length is only on the 30 Ang scale, which defines the effective
average size of the AFR fluctuations. This size remains temperature independent
while the total number of AFRs significantly increases near the Vogel-Fulcher
freezing temperature of 220 K. Presence of local fluctuations produced by
anti-parallel atomic displacements can explain relaxor behavior as a result of
competition between ferroelectric and anti-ferroelectrics type polar ordering
in the glass-like systems.
