  We suggest considering the Burns temperature in lead containing relaxor
ferroelectrics as the temperature, at which Pb becomes off-center and nearly
randomly occupies positions around the center on a sphere with the radius,
which increases with the temperature decrease. The significance of the Burns
temperature, in this approach, is the crossover between the soft-mode and
order-disorder dynamics. The lattice parameter, birefringence and neutron
scattering experiments are analyzed in the light of this model.
