  Neutron scattering investigations were carried out in
PbMg$_{1/3}$Ta$_{2/3}$O$_3$ and BaMg$_{1/3}$Ta$_{2/3}$O$_3$ complex
perovskites. The crystal structure of both compounds does not show any phase
transition in the temperature range 1.5 -- 730 K. Whereas the temperature
dependence of the lattice parameter of BaMg$_{1/3}$Ta$_{2/3}$O$_3$ follows the
classical expectations, the lattice parameter of relaxor ferroelectric
PbMg$_{1/3}$Ta$_{2/3}$O$_3$ exhibits anomalies. One of these anomalies is
observed in the same temperature range as the peak in the dielectric
susceptibility. We find that in PbMg$_{1/3}$Ta$_{2/3}$O$_3$, lead ions are
displaced from the ideal positions in the perovskite structure at all
temperatures. Consequently short-range order is present. This induces strong
diffuse scattering with an anisotropic shape in wavevector space. The
temperature dependences of the diffuse neutron scattering intensity and of the
amplitude of the lead displacements are similar.
