The lead zirconate titanate PbZr_1‑xTi_xO₃ (PZT) solid solutions are the basis of the most-commonly used piezoelectric ceramic materials. The parent cubic perovskite structure presents two intrinsic instabilities: ferroelectric displacements and polyhedral tilting, which result in complex phase diagrams. In particular, a common feature of these phase diagrams is the presence of a monoclinic phase between rhombohedral and tetragonal phases, where highest properties are obtained: the so-called morphotropic phase boundary (MPB, x~0.48 for PZT). In this monoclinic phase, the polar axis can lie anywhere between the pseudocubic [111] and [001] directions, thus providing a possible explanation for this high piezoelectric response. The dielectric and piezoelectric properties of ferroelectrics are known to be highly sensitive to stress induced by external elastic or electric fields and in this respect it is also very important to understand the effect of hydrostatic pressure on the phase diagrams of these materials. Therefore, our group has begun a detailed study of the phase diagram of PZT as a function of pressure, temperature and composition. Recently, several additionnal low-symmetry intermediate structures have been observed close to the MPB and close to the antiferroelectric-ferroelectric phase boundary, resulting in a decrease in the stability field of the rhombohedral forms. We decided thus to re-investigated the stability field of the rhombohedral phases by studying Zr-rich PbZr_0.6Ti_0.4O₃ as a function of temperature and pressure using neutron and X-ray diffraction, resonance Raman spectroscopy and dielectric measurements.

Careful Rietveld refinements of the neutron data yield significantly better agreeement factors using monoclinic models for the long-range structure of PbZr_0.6Ti_0.4O₃ rather than the widely accepted rhombohedral symmetry. The spontaneous polarization was found to lie along the pseudocubic [112] direction instead of along the [111] direction. This polarization direction is shown to be adopted for a wide variety of compositions, temperatures and pressures and has major implications for the domain structure of these materials. The transition to a double unit cell structure, observed at low temperature or moderate pressure, arises from an antiferrodistorsive tilting of the oxygen octehedra and can be easily detected by: the appearance of superlattice reflections in the neutron profiles, changing in slope of the dielectric susceptibility as a function of pressure or temperature, a volume change or an increase in compressibility measured by X-ray diffraction, and finally by a dramatic decrease in the intensity of the Raman spectra with increasing pressure resulting from a change of the resonance behaviour which is apparently structure-dependent. Additionnally, at high pressure, a ferroelectric-paraelectric phase transition is observed. Some particular reflections strongly decrease in intensity at the ferroelectric-paraelectric phase transition, which can be observed in neutron diffraction but not in X-ray diffraction. The high-pressure behaviour of these reflections which are strongly linked to polar displacements is shown to be exactly the opposite of that of superlattice reflections. The two intrinsic instabilities of the perovskite structure, namely ferroelectric displacements and polyhedral tilting, can be thus directly probed by neutron diffraction following the intensities of these two types of reflections. Finally, based on further investigations on Ti‑rich compositions (x=0.48-0.60), the pseudocubic unit cell observed just before the ferroelectric-paraelectric phase transition, whatever the composition, is explained by the competition between polar displacements and antiphase oxygen rotations.

The existence of an intrinsic short range dynamical disorder over nearly the entire PZT solid solutions has been reported in the literature. In order to better understand the role of disorder in these materials at high pressure and the resulting implications for ferroelectric properties we have investigated the morphotropic PbZr_0.52Ti_0.48O₃ composition by means of combined high-pressure, high-temperature XAFS(Zr K edge)/X-ray diffraction experiments (BM29 beamline, ESRF). Data have been collected each 1-2GPa along three isotherms (300K, 450K and 650K) in order to determine both P-T effects. This original technique enables us to follow the corresponding changes in short-range-long-range order and the Zr‑environnment in this material. We present here the results obtained for the isotherm at 300K. The pressure at which the ferroelectric paraelectric phase transition occurs, obtained by X-ray diffraction, is in good agreement with that previously determined by neutron diffraction and Raman spectroscopy. The high-pressure behaviour of the first coordination sphere (ZrO₆) is deduced from the EXAFS spectra. The Debye‑Waller factor is found to decrease abruptly at the ferroelectric paraelectric phase transition. The value obtained at atmospheric pressure after extrapolation of the Debye-Waller factors of the paraelectric phase is found to be very close to that of BaZrO₃. As the latter adopts a typical cubic perovskite structure at atmospheric pressure with no static disorder, the abrupt decrease of the Debye‑Waller factor at the ferroelectric paraelectric phase transition is thus a consequence of the suppression of this static disorder. Therefore, high-pressure induces a displacement of the Zr atom towards the centrosymmetric position, as was predicted theoretically by Samara et al., Phys. Rev. Lett. 35, 1767 - 1769 (1975)