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-xTixO3 (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 T-T. The T-IT (x) line is observed for x < 0.17, beginning at the T-T (x) line at the point where T-T (x) drops abruptly and then continuing up to the T-C (x) line. If T-IT (x) 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 T-IT (x) line bend toward and then merge with the T-C (x) line but, in our series of samples, the temperature T-MPB defined by the dielectric and anelastic maxima at the rhombohedral/tetragonal MPB does not cross the T-T (x) line. Instead it gradually bends downward to become parallel to, and possibly merges with, the T-T (x) line. 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. DOI: 10.1103/PhysRevB.87.094108