Ionic-pair equal-time displacement correlations in relaxor lead magnesium niobate, $\mathrm{Pb}\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right){\mathrm{O}}_3,$ have been investigated at 300, 270, and 240 K in terms of an x-ray diffuse scattering technique. Functions of the distinct correlations have been determined quantitatively. The results show the significantly strong rhombohedral-polar correlations regarding Pb-O, Mg/Nb-O, and O-O’ pairs. Their spatial distribution at 300 K forms an ellipse or a sphere with the diameters of 30–80 Å, and the sizes are reduced to 30–40 Å on cooling through local condensation of thermal lattice fluctuations. This observation of local structure in the system proves the presence of the polar microregions in the paraelectric state which leads to the dielectric dispersion.

• Received 8 August 2001

DOI:https://doi.org/10.1103/PhysRevB.64.184112