  The results are presented of anelastic and dielectric spectroscopy
measurements on large grain ceramic PZT with compositions near the two
morphotropic phase boundaries (MPBs) that the ferroelectric (FE) rhombohedral
phase has with the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases.
These results are discussed together with similar data from previous series of
samples, and reveal new features of the phase diagram of PZT, mainly connected
with octahedral tilting and its coupling with the polar modes. Additional
evidence is provided of what we interpret as the onset of the tilt instability,
when is initially frustrated by lattice disorder, and the long range order is
achieved at lower temperature. Its temperature T_IT(x) prosecutes the long
range tilt instability line T_T(x) up to T_C, when T_T. It is proposed that the
difficulty of seeing the expected 1/2<111> modulations in diffraction
experiments is due to the large correlation volume associated with that type of
tilt fluctuations combined with strong lattice disorder. It is shown that the
lines of the tilt instabilities tend to be attracted and merge with those of
polar instabilities. Not only T_IT bends toward T_C and then merges with it,
but in our series of samples the temperature T_MPB of the dielectric and
anelastic maxima at the rhombohedral/tetragonal MPB does not cross T_T, but
deviates remaining parallel or possibly merging with T_T. These features,
together with a similar one in NBT-BT, are discussed in terms of cooperative
coupling between tilt and FE instabilities, which may trigger a common phase
transition. An analogy is found with recent simulations of the tilt and FE
transitions in multiferroic BiFeO3. An abrupt change is found in the shape of
the anelastic anomaly at T_T when x passes from 0.465 to 0.48, possibly
indicative of a rhombohedral/monoclinic boundary.
