  Diffuse scattering around the (110) reciprocal lattice point has been
investigated by elastic neutron scattering in the paraelectric and the relaxor
phases of the disordered complex perovskite crystal-Pb(Zn1/3Nb2/3)O3(PZN). The
appearance of a diffuse intensity peak indicates the formation of polar
nanoregions at temperature T*, approximately 40K above Tc=413K. The analysis of
this diffuse scattering indicates that these regions are in the shape of
ellipsoids, more extended in the <111> direction than in the <001> direction.
The quantitative analysis provides an estimate of the correlation length, \xi,
or size of the regions and shows that \xi <111>~1.2\xi < 001>, consistent with
the primary or dominant displacement of Pb leading to the low temperature
rhombohedral phase. Both the appearance of the polar regions at T*and the
structural transition at Tc are marked by kinks in the \xi < 111> curve but not
in the \xi < 001> one, also indicating that the primary changes take place in a
<111> direction at both temperatures.
