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The results are presented of anelastic and dielectric spectroscopymeasurements on large grain ceramic PZT with compositions near the twomorphotropic phase boundaries (MPBs) that the ferroelectric (FE) rhombohedralphase has with the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases.These results are discussed together with similar data from previous series ofsamples, and reveal new features of the phase diagram of PZT, mainly connectedwith octahedral tilting and its coupling with the polar modes. Additionalevidence 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 isachieved at lower temperature. Its temperature T_IT(x) prosecutes the longrange tilt instability line T_T(x) up to T_C, when T_T. It is proposed that thedifficulty of seeing the expected 1/2<111> modulations in diffractionexperiments is due to the large correlation volume associated with that type oftilt fluctuations combined with strong lattice disorder. It is shown that thelines of the tilt instabilities tend to be attracted and merge with those ofpolar 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 andanelastic maxima at the rhombohedral/tetragonal MPB does not cross T_T, butdeviates remaining parallel or possibly merging with T_T. These features,together with a similar one in NBT-BT, are discussed in terms of cooperativecoupling between tilt and FE instabilities, which may trigger a common phasetransition. An analogy is found with recent simulations of the tilt and FEtransitions in multiferroic BiFeO3. An abrupt change is found in the shape ofthe anelastic anomaly at T_T when x passes from 0.465 to 0.48, possiblyindicative of a rhombohedral/monoclinic boundary.

Effects of coupling between octahedral tilting and polar modes on the phase diagram of PbZr1-xTixO3 (PZT)