La structure pérovskite, largement étudiée depuis plusieurs décennies, présente un vaste champ d’application. Ces matériaux peuvent notamment être utilisés en tant que condensateurs, matériaux pour les piles à combustible, catalyseurs, analogues à la pérovskite mantellique (Bridgemanite), cellules photovoltaïques ou encore comme matériaux de confinement des déchets nucléaires de haute activité. La structure pérovskite est dite simple (ABO3) ou complexe (A’A’’BO6, AB’B’’O6, A’A’’B’B’’O6, …) en fonction de la composition chimique et de l’arrangement atomique des éléments dans la structure. L’incorporation d’éléments trivalents dans les pérovskites simples A2+B4+O3 induit des modifications structurales et des problèmes de compensation de charges. L’étude cristallochimique de ces pérovskites substituées est nécessaire à la compréhension des propriétés physiques de ces matériaux. Dans le cadre de cette thèse, l’incorporation de terres rares trivalentes, simulant potentiellement des actinides, dans des pérovskites orthorhombiques calciques a été étudiée. Pour ce faire, nous avons incorporé des terres rares trivalentes par frittage à haute température (1550 °C) dans des pérovskites CaSnO3 et CaTiO3.Dans un premier temps, l’étude de l’incorporation de néodyme dans la pérovskite CaSnO3 a été menée par microsonde électronique, MEB, diffraction des rayons X (poudres, monocristaux), spectroscopie µ-Raman, et MET. Il a été montré qu’environ 17 % at de Nd pouvait être incorporé dans une pérovskite CaSnO3 préformée. Les études par diffraction des rayons X sur monocristaux, sur les échantillons les plus concentrés (≈ 14 et 17% at Nd), ont montré que l’incorporation du Nd s’effectue en site A avec une compensation de charge par migration du calcium en site B. A notre connaissance, ce type de mécanisme n’a jamais été reporté dans la littérature. Les études par microsonde électronique, diffraction sur poudres et spectroscopie µ-Raman ont permis d’établir que l’incorporation du Nd dans la pérovskite CaSnO3 s’opérait selon le même schéma, sur toute la gamme de composition.Dans un second temps, au vu des résultats obtenus, nous avons étudié l’incorporation d’une série de terres rares trivalentes (La, Pr, Nd, Sm, Gd, Dy, Er, Yb) dans une pérovskite CaTiO3 préformée, suivant un protocole similaire. L’objectif consistait à évaluer l’effet du rayon ionique de la terre rare sur le mécanisme d’incorporation de cette dernière. Cette étude par microsonde électronique, MEB, diffraction des rayons X sur poudres et spectroscopie Raman a révélé qu’il était possible d’incorporer les différentes terres rares selon un schéma similaire à celui observé pour le système (1-x) CaSnO3 – x Nd2O3, indépendamment du rayon ionique de celles-ci.Ce travail expérimental a mis en évidence un nouveau mécanisme d’incorporation de terres rares dans deux pérovskites calciques (CaSnO3, CaTiO3) et pourrait contribuer à moduler les propriétés physico-chimiques de ces matériaux en fonction des conditions de synthèse / The perovskite structure is widely studied since several decades and a myriad of materials presenting this structure are used in numerous applications. These materials are notably used as capacitors, anodes for solid oxide fuel cells, catalysts, mantellic perovskite analogues, solar cells or matrix for the nuclear waste storage. The perovskite structure can be described as simple (ABO3) or complex (A’A’’BO6, AB’B’’O6, A’A’’B’B’’O6, …) depending on its chemical composition and its structural features. The incorporation of trivalent elements in A2+B4+O3 perovskites induces structural modifications and charge compensation problems that can be accommodated by several ways. The crystallochemistry of these materials must be understood in order to better apprehend their physical properties. During this thesis, the incorporation of trivalent rare earths (used as potential actinide analogues) in orthorhombic calcic perovskites was studied. Thus, trivalent rare earths were incorporated at high temperature (up to 1550°C) in CaSnO3 and CaTiO3 perovskites.In the first part of this study, the incorporation of Nd in CaSnO3 was studied by means of electronic microprobe (EMPA), scanning electron microscopy (SEM), powder X-ray diffraction, single crystal X-ray diffraction, µ-Raman spectroscopy and transmission electronic microscopy (TEM). It was shown that approximately 17 at % of Nd can be incorporated in a preformed CaSnO3 perovskite. The single crystal X-ray diffraction on the most concentrated samples (Nd ≈ 14 at % and Nd ≈17 at %) showed that Nd is replacing Ca in the A site of the perovskite with charge compensation by Ca migration into the perovskite B site (instead of Sn). This mechanism can be written as:(1-x) CaSnO3 + x Nd2O3  (Ca1-2xNd2x)(Sn1-xCax)O3To our knowledge, this mechanism was never reported in the literature. The chemical studies (EMPA), coupled with the structural studies (powder X-ray diffraction, µ-Raman spectroscopy) on the less Nd concentrated samples showed that this substitution mechanism occurs in the whole range of composition.In the second part of this study and regarding the previous substitution mechanism, we studied the incorporation of eight rare earths (La, Pr, Nd, Sm, Gd, Dy, Er, Yb), presenting decreasing ionic radii, in a preformed calcium titanate perovskite, CaTiO3, following a similar protocol. This crystallochemical study by means of EMPA, powder x-ray diffraction and µ-Raman spectroscopy strongly suggests that a substitution mechanism similar to the one observed in the (1-x) CaSnO3 – x Nd2O3 system was possible for the (1-x) CaTiO3 –x Ln2O3 (Ln = La, Pr, Nd, Sm, Gd, Dy, Er, Yb) perovskite systems.This experimental study evidenced a new substitution mechanism of trivalent rare earths element in two calcic perovskites (CaSnO3, CaTiO3). This result could be in turn used for crystal engineering purposes

Submitted by Maike Costa (maiksebas@gmail.com) on 2017-06-19T14:00:05Z No. of bitstreams: 1 arquivototal.pdf: 7342318 bytes, checksum: dc68cd00ebae607f5275182f51548710 (MD5) / Made available in DSpace on 2017-06-19T14:00:05Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 7342318 bytes, checksum: dc68cd00ebae607f5275182f51548710 (MD5) Previous issue date: 2016-12-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / This dissertation has as its central point the characterization through the density functional theory (DFT) of the orthorhombic perovskite CaSnO3 belonging to the space group Pbnm, using Gaussian type orbitals. At first, the bulk of the material was explored with the most diverse solids simulation techniques, focusing on the structural evaluation under pressure and frequency calculations, as well as its intensities (Raman and Infrared spectra) evaluated by the newly implemented (In Crystal program) Coupled-Perturbed Hartree-Fock/Kohn- Sham (CPHF/KS). Following, the bulk was again exploited under pressure, including various exchange-correlation type formulations within the DFT theory, as well as the use of routines recently implemented in the Crystal program as: Elastic constants under pressure, directional seismic velocity analysis and quasi-harmonic approximation (QHA). The latter is of great physical, mineralogical and geophysical interest due to its description of a material under conditions of high temperatures and pressures simultaneously. To further explore the possible catalytic properties of the material, the studies the defects, including oxygen vacancies and doping with copper, was carried out focusing on the energy differences and electronic charge analysis, the last, taking into account the Mulliken technique and Hirshfeld-I (available only to developers at present). Those calculation where performed at PBE and PBE0 level (for the doping with Cu only PBE0), with RHF and UHF formulations for the open shell and spin polarized cases. The used supercell (2x2x2) presented an adequate size to work with. The oxygen vacancy formed has the tendency to reduce the Sn neighbors, that form a mid gap close to the Valence Band, and presenting as the most stable formulation the RHF and Singlet (UHF). The oxygen vacancy is here characterized as been a neutral one. In the doping cases, where the tin atom is substituted by a coper one, the most stable case was when the Cu was near the vacancy (first neighbor). To finish the studies on this perovskite, the surface (001) is studied. The surface terminated in -CaO is a bit more stable than the -SnO2 one, however, the first one, using the applied methodology, presented some problems when adsorbing gases, hence been discarded in favor of the -SnO2 one. The CO and NH3 gases are adsorbed over the -SnO2 terminated surface, to evaluate it, charge density maps, density of states (DOS) and Raman intensity where used. The results showed that the CO adsorption is weak, but has a visible response in the Raman spectra. In the NH3 case the adsorption is strong and can be assigned as a chemisorption. In the last, an intense Raman peak appear and it is assigned to a bond between the hydrogen and the surface oxygen and it appear about 3098 cm−1. / Esta tese tem como ponto central a caracterização através da teoria do funcional da densidade (DFT) da perovskita ortorrômbica CaSnO3 pertencente ao grupo espacial Pbnm, pela ótica de orbitais Gaussianos. No primeiro momento, o bulk do material foi explorado com as mais diversas técnicas de simulação de sólidos, com enfoque na avaliação estrutural sob pressão e a frequência, bem como suas intensidades (espectros Raman e no Infravermelho) avaliadas pelo recém implementado (no programa Crystal) Coupled- Perturbed Hartree-Fock/Kohn-Sham (CPHF/KS). Em seguida, o bulk foi novamente explorado sob pressão, incluindo diversas formulações do tipo troca-correlação dentro da teoria do funcional da densidade (DFT), bem como o uso das rotinas recentemente implementadas no programa Crystal como: constantes elásticas sob pressão, análise da velocidade sísmica direcional e a aproximação quase-harmônica (QHA). Esta última é de grande interesse físico, mineralógico e geofísico pela descrição de um material em condições de elevadas temperaturas e pressões simultaneamente. Para explorar as possíveis propriedades catalíticas do material o estudo de defeitos incluindo vacâncias de oxigênio e dopagens com cobre foram incluídas. As análises focam-se nas diferenças energéticas e análise de carga eletrônica, a última, levando em conta a técnica de Mulliken e a de Hirshfeld-I (disponível apenas para desenvolvedores, no presente). Os cálculos foram realizados em nível PBE e PBE0 (para o caso da dopagem apenas PBE0), nas versões RHF e UHF para os casos de camada aberta e polarização de spin. A supercélula utilizada (2x2x2) apresentou tamanho adequado para o trabalho com os defeitos. A vacância de oxigênio quando formada tende a reduzir os Sn vizinhos, que por ventura formam um mid gap próximo à banda de valência (VB), sendo a configuração mais estável a RHF ou Singleto (UHF). A vacância de oxigênio foi caracterizada como uma vacância neutra. Nos cálculos de dopagem em que o estanho é substituído pelo cobre, a situação mais estável é aquela em que o cobre encontra-se como um primeiro vizinho da vacância de oxigênio. Para conclusão do estudo dessa perovskita, a superfície 001 foi avaliada. A superfície terminada em -CaO é um pouco mais estável que a superfície -SnO2, no entanto a primeira - através da metodologia utilizada - teve problemas para adsorção dos gases, dessa forma sendo descartada para os estudos de adsorção aqui realizados. Os gases CO e NH3 estudados sobre a superfície terminada em -SnO2, na avaliação foram utilizados mapas de densidade de carga, densidade de estados (DOS) e espectros Raman. Os resultados indicaram que a adsorção com o CO é fraca, mas possui uma resposta visível no espectro Raman. Já no caso do NH3 a adsorção forte podendo configurar-se como quimissorção. Na última, há o aparecimento de um pico intenso referente à ligação hidrogênio e oxigênio da superfície em torno de 3098 cm−1.