  In the present paper, the dielectric behaviour of the
PbLixNb3xZr0.51Ti0.49-4xO3 ( PLNZT) system is analyzed. It is shown that the
decrease in the dielectric constant with respect to an increase of the impurity
content may be due to an aggregation trend of the interacting off-centre
dipoles rather than due to cooperative phenomena which renormalizes the dipole
moment of the off-centre impurity.
