  The antipolar and antiferrodistortive (AFD) components of the
antiferroelectric (AFE) transition in PbZr1-xTixO3 (x=0.054) can occur
separately and with different kinetics, depending on the sample history, and
are accompanied by elastic softening and stiffening, respectively. Together
with the softening that accompanies octahedral tilting in the fraction of phase
that is not yet transformed into AFE, they give rise to a variety of shapes of
the curves of the elastic compliance versus temperature. All such anomalies
found in samples with x=0.046 and 0.054, in addition to those already studied
at x=0.050, can be fitted consistently with a phenomenological model based on
the simple hypothesis that each of the polar and AFD transitions produces a
step in the elastic modulus, whose position in temperature and width reflect
the progress of each transition. The slowing of the kinetics of the
transformations is correlated with the formation of defect structures during
aging in the ferroelectric or AFE state, which are also responsible for a
progressive softening of the lattice with time and thermal cycling, until
annealing at high temperature recovers the initial conditions.
