  The temperature dependence of the superlattice reflections: a) F spots and b)
alpha spots in a lead magnesium niobate (PMN) single crystals containing 0% and
6% of PbTiO3 (PT) has been studied using synchrotron x-ray scattering
techniques. (No superlattice reflections were found in PMN doped with 32% PT).
Analysis of the temperature dependence of the alpha spots suggests the
existence of the correlated anti-parallel atomic displacements that form
nanoregions different from the chemical nanodomains. While the correlation
length is temperature independent, the magnitude of these displacements
increases on cooling below the freezing temperature Tf. Intensities of the
alpha spots above this temperature become indistinguishable from the
background. Our results show that value of Tf for each composition is very
close to the one obtained from a Vogel-Fulcher fit to the frequency dependence
of the dielectric constant maximum Tm. The relation of these correlated
anti-ferrodistortive fluctuations to polar ferroelectric nanodomains and
relaxor behavior needs further study.
