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Theses

Modélisation du champ électrique interne d'un solide:
application à la détermination des propriétés dérivées de la polarisation. Calcul du spectre de phonon.

Abstract : This thesis deals with modeling internal electric field in solids and the study of related properties (spontaneous polarisation, piezoelectricity, dynamical Born charges,...). For most of the studied compounds, the strain applied for the determination of piezoelectric constants is also used for the calculation of the full set of elastic constants. Analysis of the dynamical behaviour via the phonon spectrum at the center of the Brillouin zone simulation and calculation of the OH stretching mode anharmonicity of some alkali and alkali-earth metals hydroxides complete this work. Nowadays, ab initio methods are helpful for a microscopic scale interpretation of experimental observations and may even bridge gaps in or lead to correction of experimental data. Ab initio modeling of polarisation related properties with the « modern » polarisation theory of King-Smith, Vanderbilt and Resta1, using the Berry phase approach, allows one to evaluate such properties as piezoelectricity and spontaneous polarisation. One of the first goals of my thesis is the study of that kind of properties on a quartz isotype material, namely, the berlinite aluminophosphate AlPO4. Indeed, research of new compounds going forward quartz limitation, as ultrasonic generator or acousto-electronic materials for the realisation of some electronic filter with large band gap, has recently been boosted. One of the possibilities is the same quartz crystalline structure compounds in which silicon is substituted, by aluminium in berlinite for exemple. To our knowledge, this is the first ab initio study that determinates the optimal conditions to evaluate elastic, piezoelectric and vibrational properties. Moreover, berlinite being the compact form of some zeolithes like AlPO4-5 (AFI), the structural as well as the vibrational analysis, constitute an interesting first approach for the current study of AlPO4-5. This work has been done in collaboration with Ross BROWN, research director in our laboratory, by using ab initio as well as molecular dynamics methods. We also focus on ferroelectric semiconductor materials, such as nitrides semiconductor compounds. Those nitrides, as well as their alloys, are remarkable semiconductors when used in optoelectronic features. Before the provided study of AlxGa1-xN type heterostructures or alloys, some basic analysis were necessary, in order to clearly establish the optimal conditions in order to better predict the polarisation related properties. The second part of my thesis deals with the structural, electronic, elastic, and spectroscopic properties of a technologically important family of materials : the matlockites (MFCl, M = Ca, Sr, Ba).Indeed, those materials, when doped, can be used for X-ray imaging.The ab initio approach used in this work to compute elastic properties and calculate the phonon spectrum at the center of the Brillouin zone constitutes the first attempt to evaluate this type of properties for those systems. Last but not least, the harmonic asumption used for the calculation of the phonon spectrum at the center of the Brillouin zone being no longer sufficient to compute those frequencies with acccucary, the calculation of the OH stretching mode anharmonicity of some alkali and alkali-earth metals hydroxides complete this work. A detailed analysis of the conditions of this study, the influence of the differents Hamiltonians used, as well as the presence of hydrogen bonds, is also made. All ab initio calculations were made using the development version of the periodic CRYSTAL program, graciously placed at my disposal by Professor Roberto DOVESI (University of Torino, Italy) with whom I have actively collaborated.
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Pierre Labeguerie. Modélisation du champ électrique interne d'un solide:
application à la détermination des propriétés dérivées de la polarisation. Calcul du spectre de phonon.. Matériaux. Université de Pau et des Pays de l'Adour, 2005. Français. ⟨tel-00264922⟩

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