  Using neutron elastic and inelastic scattering and high-energy x-ray
diffraction, we present a comparison of 40% Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-60%
PbTiO$_{3}$ (PMN-60PT) with pure Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$ (PMN) and
PbTiO$_{3}$ (PT). We measure the structural properties of PMN-60PT to be
identical to pure PT, however, the lattice dynamics are exactly that previously
found in relaxors PMN and PZN. PMN-60PT displays a well-defined macroscopic
structural transition from a cubic to tetragonal unit cell at 550 K. The
diffuse scattering is shown to be weak indicating that the structural
distortion is long-range in PMN-60PT and short-range polar correlations (polar
nanoregions) are not present. Even though polar nanoregions are absent, the
soft optic mode is short-lived for wavevectors near the zone-centre. Therefore,
PMN-60PT displays the same waterfall effect as prototypical relaxors PMN and
PZN. We conclude that it is random fields resulting from the intrinsic chemical
disorder which is the reason for the broad transverse optic mode observed in
PMN and PMN-60PT near the zone centre and not due to the formation of
short-ranged polar correlations. Through our comparison of PMN, PMN-60PT, and
pure PT, we interpret the dynamic and static properties of the PMN-xPT system
in terms of a random field model in which the cubic anisotropy term dominates
with increasing doping of PbTiO$_{3}$.
