Etude des corrélations entre propriétés structurales et physiques dans la famille de matériaux piézoélectriques type quartz par diffraction de rayons x

Abstract : The understanding of the correlations between the structure and physical proprieties of crystals is important for the optimisation and innovation of new materials for technological applications. The response of a crystal to an applied external electric fields is particularly interesting for pyro-, piezo-, ferroelectric's materials. From high resolution X-rays diffraction data, we can obtain the atomic structure and the electronic density distribution. This information concerns the ground state of the crystal. Similar experiments on crystals under electric filed give knowledge about relationships between structure and physical proprieties. The first effect observed on a piezoelectric crystal is a change of the Bragg angle due to the converse piezoelectric effect. The second effect, from this experiment, is the modification of the diffracted intensity due to the changes of atomic positions and/or polarisation of electronic density. A dedicated apparatus was build for these studies in our laboratory. It allows us to measured diffraction profiles for a crystal under electric fields. It can be interfaced to different diffractometers. Alpha quartz and its isotypes are chosen because they are used in numerous technical applications. The enhancement of the electro mechanic coupling is request by new technologies. Our studies allow to explain the structural modifications induced by an electric field. We have determined, using synchrotron radiation, the piezoelectric tensor coefficients of quartz and GaPO4. The values are in good agreement with those determined by other methods. For the structural modifications, the two experiments give intensity variations of the order of a percent. Refinements lead to perturbed structures where the distances between heavy atoms and oxygen are little modified. These results have made it possible to propose an explanation of the origin of the modifications under an electric field. For the compounds, the polarization induced by displacing charged atoms have been calculated. In both cases, the values obtained are in qualitative agreement with what one gets from the dielectrics constants.
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Régis Guillot. Etude des corrélations entre propriétés structurales et physiques dans la famille de matériaux piézoélectriques type quartz par diffraction de rayons x. Autre [cond-mat.other]. Université Henri Poincaré - Nancy 1, 2002. Français. ⟨NNT : 2002NAN10223⟩. ⟨tel-01754391⟩

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