  The relaxor ferroelectric PbMg_1/3Ta_2/3O_3 was studied by single-crystal
neutron and synchrotron x-ray diffraction and its detailed atomic structure has
been modeled in terms of static Pb-displacements that lead to the formation of
polar nanoregions. Similar to the other members of the Pb-based relaxor family
like PbMg_1/3Nb_2/3O_3 or PbZn_1/3Nb_2/3O_3 the diffuse scattering in the
[H,0,0]/[0,K,0] scattering plane has a butterfly-shape around the (h,0,0) Bragg
reflections and is transverse to the scattering vector for (h,h,0) peaks. In
the [H,H,0]/[0,0,L] plane the diffuse scattering is elongated along the <1,1,2>
directions and is transverse to the scattering vector for (h,h,h) reflections.
We find that a model consisting of correlated Pb-displacements along the
<1,1,1>-directions reproduces the main features of the diffuse scattering in
PbMg_1/3Ta_2/3O_3 adequately when the correlation lengths between the Pb-ion
displacement vectors are longest along the <1,1,1> and <1,-1,0> and shortest
along <1,1,-2> directions.
