Merging of the polar and tilt instability lines near the respective morphotropic phase boundaries of PbZr1-xTixO3

We present the results of anelastic and dielectric spectroscopy measurements performed on large-grain, ceramic PbZr1-xTi xO3 (PZT) compositions located near the two morphotropic phase boundaries (MPBs) that separate the ferroelectric (FE) rhombohedral (R) phase from the Zr-rich antiferroelectric and Ti-rich FE tetragonal phases. Additional evidence is provided of the existence of a temperature T IT, which we interpret as the onset of the tilt instability in the R phase, where tilting is initially frustrated by lattice disorder but achieves long-range order at the lower temperature TT. The TITx line is observed for x<0.17, beginning at the TTx line at the point where TTx drops abruptly and then continuing up to the T Cx line. If TITx indeed signals the onset of short-range ordered tilting, then it follows that the tilt instability lines should tend to be attracted and merge with those of the polar instabilities. Not only does the TITx line bend toward and then merge with the TCx line but, in our series of samples, the temperature TMPB defined by the dielectric and anelastic maxima at the rhombohedral/tetragonal MPB does not cross the TTx line. Instead it gradually bends downward to become parallel to, and possibly merges with, the TTxline. An abrupt change is found in the shape of the anelastic anomaly at TT when x passes from 0.465 to 0.48, possibly indicative of a rhombohedral/monoclinic boundary.