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21 November 2013
Doctoral thesis (Dissertations and theses)
Etude ab-initio d’oxydes antiferroélectriques de structure pérovskite
Amisi, Safari 
2013
 

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Keywords :
Perovskite oxides; density functional theory; phonons; structural phase transitions; ferroelectric mode; antiferroelectric mode; antiferrodistortive mode
Abstract :
[en] In this thesis we present an original ab-initio study of the evolution of antiferrodistortive (AFD), anti-polar electric (APE), and ferroelectric (FE) instabilities in various ABO3 oxides of perovskite structure, as well as their structural and dynamic properties. The main goal is to understand better the microscopic origin of the antiferroelectricity exhibited in these compounds. Three prototypical compounds are studied in detail : PbZrO3 , NaNbO3 , and SrZrO3. After a general introduction on ABO3 compounds, and the ab-initio techniques, we review the concept of antiferroelectricity in perovskites, highlighting some ambiguities in the usual definition and the necessity of turning to what we call a modern definition of antiferroelectricity. First, we highlight that it is the rigidity of the oxygen cage that tends to favor the FE distortion compared to the APE instability. Although illustrated on BaTiO3 , this argument is general, and confirmed by the inspection of the phonons dispersion curves of the ABO3 compounds in whom the strongest instability of the FE/APE branch is systematically at Γ. We show that the emergence of a stable or meta-stable APE distortion appear naturally through a coupling with other instabilities. The presence of AFD modes turns out to be a concrete way to create mixed FE/AFD and APE/AFD phases, crucial for the emergence of antiferroelectricity (AFE). This clarifies why the known AFE compounds systematically include AFD distortions. In this context, since the FE, APE and AFD instabilities are usually in competition, the coexistence of FE, APE and AFD instabilities of strong amplitudes seems required to create mixed phases combining them. This establishes the context convenient to the development of FE and AFE metastable phases close in energy. Another important element concerns the need of a first order AFE-FE transition under electric field producing a double hysteresis loop, typical of AFE compounds. Here also the AFD modes could play a key-role by allowing the emergence of FE/AFD and APE/AFD phases close in energy and developing distinct tilt patterns. These various elements give a new perspective on AFE and allow us to have a more precise idea of the origin of the AFE behavior in perovskites. We identify some key intrinsic characteristics allowing the prediction of materials with the propensity of developing an AFE behavior.
Research center :
Physique théorique des matériaux
Disciplines :
Physics
Author, co-author :
Amisi, Safari ;  Université de Liège - ULiège > Département de physique > Physique théorique des matériaux
Language :
French
Title :
Etude ab-initio d’oxydes antiferroélectriques de structure pérovskite
Alternative titles :
[en] First-principles study of antiferroelectric perovskite oxide structures
Defense date :
12 November 2013
Number of pages :
XII, 144+15
Institution :
ULiège - Université de Liège
Degree :
Docteur en Sciences
Promotor :
Ghosez, Philippe  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Karume, Katcho
President :
Martin, John  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Secretary :
Verstraete, Matthieu  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Jury member :
Bousquet, Eric  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Dkhil, Brahim
Kreisel, Jens
Funders :
CTB - Coopération Technique Belge
ULiège ARD - Université de Liège. Administration Recherche et Développement
PACODEL - Centre pour le Partenariat et la Coopération au Développement

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