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Doctoral thesis (Dissertations and theses)
First- and Second-Principles Studies of Perovskites
Schmitt, Michaël 
2020
 

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Keywords :
first-principles calculations; Metal-to-insulator transitions; effective potentials
Abstract :
[en] The Metal-Insulator transition (MIT) upon cooling at elevated temperatures is a fascinating effect that is observed in some ABX 3 perovskite compounds with specific electronic configurations on the transition metal B cation d states. These compounds behave completely oppositional to ordinary metals who become better conductors upon cooling. At the MIT temperature these specific perovskite compounds show electron localization accompanied by cooperative lattice distortions that deform the BX 6 corner shared octahedral network. Since the discovery of these MITs, theoretical concepts about its origin have been proposed and debated. In this work we study from ab initio density functional theory (DFT) calculations LaMnO 3 and the alkaline earth ferrite series AFeO 3 who all have a formal d 4 occupation. As a basis to our discussion we introduce canonical notations, that were missing until now, for lattice distortions in perovskites that distort the X anion octahedra and are connected to MITs. While LaMnO 3 shows electron localization through orbital ordering - the appearance of a static order of Mn d orbital occupations-, CaFeO 3 shows electron localization through charge ordering - the appearance of a static order of formal Fe charges-. From DFT calculations we can show that both mechanisms are compatible with the concept of a Peierls transition and are enabled by octahedral rotations. From Monte-Carlo simulations we show furthermore that spin disorder in the paramagnetic phase is a key ingredient to explain the high MIT temperatures. Last we study the influence of external epitaxial strain on these compounds. Here, our canonical notations help to discriminate internal relaxations and external constraints. Our calculations show that in LaMnO 3 epitaxial strain alone can provoke a change from an antiferromagnetic to an ferromagnetic order without the necessity of Oxygen vacancies as has been often speculated. In CaFeO 3 epitaxial strain can provoke a change from the bulk charge ordering MIT to an orbital ordering MIT, which explains the experimental finding of a strongly elevated MIT in CaFeO 3 thin films. In the second part of this work, we present methodological developments for generating effective lattice potentials by a polynomial expansion that describe electronic potential energy surfaces in which atomic nuclei move. This is the so-called second-principles approach. The aim thereby is to replace the computational intensive self consistent DFT procedure by an lightweight evaluation of a polynomial depending on nuclear positions. If successful, this approach provides a mean to achieve a scale-transfer retaining the accuracy of ab initio calculations applicable to large scale systems with many thousand atoms and statistical simulations. In a proof of concept study we apply this approach to the perovskite CaTiO 3 . The retained effective potential reproduces with good accuracy a number of ab initio quantities. Moreover, in the low to average temperature range (T<=1000K) the lattice dilatation of CaTiO 3 is well described. In the highest temperature range the effective potential deviates from the experimentally measured lattice dilatation and proposed phase transition sequence that is itself, however, debated. We conclude that the lattice dilatation properties can be refined by extending the lattice expansion and that a reexamination of the high temperature phase sequence of CaTiO 3 due the the information of our effective potential might be meaningful. Finally, we also highlight that there exists a strongly ferroelectric low energetic phase of CaTiO 3 whose stabilization through external constraints is part of ongoing research.
Research center :
CESAM - Complex and Entangled Systems from Atoms to Materials - ULiège
Disciplines :
Physics
Author, co-author :
Schmitt, Michaël ;  Université de Liège - ULiège > Département de physique > Département de physique
Language :
English
Title :
First- and Second-Principles Studies of Perovskites
Defense date :
15 September 2020
Institution :
ULiège - Université de Liège
Degree :
Doctor in Science
Promotor :
Ghosez, Philippe  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
President :
Raty, Jean-Yves  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Secretary :
Bousquet, Eric  ;  Université de Liège - ULiège > Complex and Entangled Systems from Atoms to Materials (CESAM)
Jury member :
Dupé, Bertrand ;  Université de Liège - ULiège > Département de physique > Physique des matériaux et nanostructures
Gibert, Marta
Garcia-Fernandez, Pablo
Funders :
Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture (Communauté française de Belgique) - FRIA

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