Résumé:

Perovskite (ABO3) ferroelectric materials exhibit a Ferroelectric-Paraelectric (Ferro-Para) phase transition at a critical temperature TC (and a critical pressure PC), which can be defined as a phase transition from a non-centrosymmetric to a centrosymmetric structure due to the disappearance of the spontaneous polarization (PS) characterizing these materials. Neutron diffraction is particularly useful to investigate this Ferro-Para phase transition in pressure-temperature space due to the relatively strong scattering of oxygen compared to XRD. It enables 1) to calculate PS with the dipole equation moment and also 2) to detect phase transitions involving octahedral tilting. In this study, high-temperature and high-pressure neutron powder diffraction patterns of typical perovskites PbTiO3 and PbZr0.52Ti0.48O3 contain specific reflections, (h, k, 2l or h, 2k, 2l), strongly linked to polar shifts. The progressive disappearance of these reflections is due to the anti-phase scattering of A (Pb), B (Ti, Zr) and O atoms. In other words, this disappearance is related to the decrease in the imaginary part of the form factor corresponding to these reflections which becomes zero at the Ferro-Para phase transition. In addition, the temperature- and pressure- dependence of the calculated PS is different from the behavior of the c/a ratio which should be proportion to √(P_C-P) in the case of purely displacive transition.