This diploma thesis is focused on a theoretical study of the phase stability of solid solutions in selected aluminium and silver alloys. The ab initio calculations were performed using projected augmented waves method and the thermal dependencies of thermodynamic quantities were acquired using phonon calculations. The main focus of this work is the of aluminium-germanium alloy, while the other examined alloys (Al-Pb, Ag-Ge and Ag-Pb) serve the purpose of comparison of the solid solubility prediction and its reliability in systems with different composition. The temperatures, at which the solid solutions become stable, were evaluated using the energy difference between possible states and this evaluation was carried out for different contributions to the total energy. Also the electronic and phonon densities of states were calculated for all pure elements and solid solutions. The temperatures of solubility were compared to the experimental ones provided by the CALPHAD method and the individual contributions to the total energy were determined and depicted. The results obtained in this work tend to underestimate temperatures of solubility of individual solid solutions by hundreds of Kelvin.

This Master‘s thesis is focused on theoretical study of twinning in magnetic shape memory alloys based on Ni2MnGa using ab initio calculations of electronic structure within the projector augmented wave method. In particular, the effect of increasing concentration of manganese at the expense of gallium was studied on total energy and stress profiles along different deformation paths in the (10-1)[101] shear system of non-modulated martensite. Further, this work deals with the effect of the concentration of manganese on the energy of planar fault caused by presence of partial dislocation due to motion of twin boundary. The results show that the shear modulus in studied shear system increases with the increasing concentration of manganese as well as energy barrier and deformation characteristics along shear deformation paths increases, which makes the shear more difficult in Mn-rich alloys. Increasing concentration of manganese also leads to rising the planar fault energy. All these effects can be responsible for lower mobility of twin boundaries in alloys with higher concentration of manganese.

This Master’s thesis is focused on theoretical study of the high entropy alloy CoCrNi using ab initio calculations. The focus was on the effect of short range order on the relative stability of FCC and HCP structures and the value of stacking fault energy.The results show increase of stability in both types of structures wtih decreasing number of Cr-Cr nearest neighbours. The effect of the number of Cr-Cr nearest neighbours on the stacking fault energy previously shown in literature was not observed. However the strong dependency was found on the change of short range order caused by the shift of (1 1 1) planes after the transformation from the FCC to HCP structure. The effect of interstitial atoms C a N was also studied. Both these interstitials stabilise FCC structure and thus cause the increase of stacking fault energy. Both interstitials prefer octahedral positions with higher amount of Cr in their nearest neighbour shell.

Pomocí molekulového modelování je možné předpovídat chování nových látek a napomáhá při jinak obtížné interpretaci experimentálních dat. Cílem práce byla predikce vybraných vlastností polymeračních katalyzátorů na bázi karbenů, predikce jejich struktur a spektrálních charakteristik a studie mechanismu polymerace za otevření kruhu laktidu. K ověření chování karbenů a jejich prekurzorů ve formě chloridů byly studovány vybrané charakteristiky molekuly. Byl proveden výpočet vybraných molekulových orbitalů a elektrostatických map. Následně pomocí počítačových programů byly získány teoretické vazebné délky a úhly vybraných imidazolových a imidazolinových sloučenin, karbenů a jejich možných produktů hydrolýzy. Data strukturně podobných, již charakterizovaných sloučenin, byla získána z CCDC (Cambridge Crystallographic Data Centre) a následně byla konfrontována s vypočítanými daty. Byla změřena infračervená a Ramanova spektra imidazolové soli a infračervené spektrum příslušného karbenu. Tato spektra byla konfrontována s napredikovanými. Pro lepší interpretaci spekter byla spočítána spektra možných produktů hydrolýzy. Následně byl studován mechanismus polymerace za otevření kruhu laktidu. Na základě spočítaných energií stacionárních bodů byl navržen nový mechanismus polymerace.

This work deals with study of stability of solid crystals under isotropic loading. Ab initio methods were used for this purpose. Crystals of four fcc metals (Al, Cu, Ir, Au) and diamond were subjected to simulated isotropic tensile deformation and maximum value of isotropic stress was evaluated for them. Consequently, phonon spectra were calculated for several strain values in order to assess crystal stability. Phonon instabilities in dispersion curves of diamond, Al, Ir and Au appeared at strains lower than those corresponding to their decohesion. This appearance of instability determinates the value of ideal strength. However, significant reduction (by about 20%) was found only in the cases of Au and Ir.

Le travail porte sur les propriétés vibrationnelles de semi-conducteurs innovants, notamment les composés Cu2ZnSnSe4, Cu2ZnSnS4 de type kesterite, qui, dans le domaine du photovoltaïque, soutiennent la comparaison avec les matériaux leaders de type chalcopyrite, Cu(In,Ga)Se2. Un intérêt pratique immédiat pressenti est que les spectres de vibration devraient permettre de distinguer entre différentes phases structurales possibles à une composition donnée. Les modes de vibration sont abordés en utilisant une approche théorique premiers principes. Le manuscrit est divisé en cinq chapitres, dont le contenu est le suivant : (1) Une brève introduction sur le principe et les enjeux du photovoltaïque, les cellules solaires à base de composés semi-conducteur multinaires; le chapitre se termine par une description du diagramme de phase du système Cu-Zn-Sn-Se. (2) L’exposé de la théorie de la fonctionnelle de la densité (DFT) et la mise en œuvre de simulations numériques via le logiciel SIESTA. (3) Les propriétés vibrationnelles de Cu2ZnSnSe4 dans ses deux phases kesterite et stannite, étudiées par une méthode premiers principes, en faisant une comparaison détaillée et une analyse minutieuse mode par mode. (4) Les propriétés vibrationnelles de la phase secondaire Cu2SnSe3, une concurrente habituelle de la phase Cu2ZnSnSe4 lors de la croissance des échantillons. (5) Le calcul des phonons dans la structure Cu2ZnSnS4 contenant des défauts intrinsèques (lacunes; antisites), avec comme objectif l'évaluation des contributions vibrationnelles à l'entropie, et l'élaboration du diagramme de phases composition – température dans ce système multinaire. La conclusion générale récapitule les résultats, qui sont publiés dans 5 articles / The works addresses vibrational properties of novel semiconductors, specifically the Cu2ZnSnSe4 and Cu2ZnSnS4 compounds of the kesterite structure, which, in the domain of photovoltaics, become competitive with leading materials of chalcopyrite type, notably Cu(In,Ga)Se2. The anticipated immediate practical interest of such study is that the vibration spectra are likely to make possible a distinction between different structural phases, possible for a given composition. The vibration modes are accessed by using a first-principle theory approach. The manuscript is divided into five chapters, with the following contents: (1) A brief introduction into the work principle and the problematics of photovoltaics, specifically of the solar cells based on multinary semiconductors; the chapter closes by the description of the phase diagram of the Cu-Zn-Sn-Se system. (2) An overview of the density functional theory (DFT) and of the technics of numerical simulations using the SIESTA code. (3) The vibrational properties of Cu2ZnSnSe4 in its two phases, kesterite and stannite, as studied by first-principles method, with a detailed comparison being done along with a thorough mode-by-mode analysis. (4) Vibrational properties of a secondary phase Cu2SnSe3, which often competes with the Cu2ZnSnSe4 phase in the process of sample growth. (5) Calculation of phonons in the Cu2ZnSnS4 structure containing intrinsic defects (vacances; anti sites), with the objective of estimating vibrational contributions to entropy and the correction of the composition - temperature phase diagram in this multi nary system. The general conclusion summarises the results which are published in 5 articles

The thesis aims at the study of conformation of a dimethyl tartrate molecule using the methods of vibrational optical activity (VOA), namely vibrational circular dichroism (VCD) and Raman optical activity (ROA). Based on the theoretical background of both VOA methods and current state of research of the studied molecule there was a sample of dimethyl tartrate dissolved in different solvents and its properties were measured using VCD and ROA spectrometers. In addition to the experiment, ab initio calculations were carried out in order to compare calculated and experimental spectra. Based on the comparison, the possible conformations present in water solution of the studied molecule are described in detail.

Nanoelectronics and spintronics are concerned with writing, transporting, and reading information stored in electronic charge and spin degrees of freedom at the nanoscale. Past few years have shown that two spintronics effects discovered in the 19th century, namely anisotropic magnetoresistance and anomalous Hall effect, can be used also for sensing antiferromagnetism which opened the field of antiferromagnetic spintronics. The more than a century of controversial studies of these effects have shown their relativistic spin-orbit coupling and spin-polarisation symmetry breaking origin. However, a complete understanding of these effects and a fully predictive theory capable of identifying novel suitable antiferromagnetic materials are still lacking. Here, by extending modern symmetry and topology concepts in condensed matter physics, we have further developed the theory of anisotropic magnetoresistance and spontaneous Hall effect. Our approach is based on magnetic symmetry and topology analysis of antiferromagnetic energy bands, Bloch spectral functions, and Berry curvatures calculated from the state-of-the- art first-principle theory. This guided us to the prediction of two novel, previously unanticipated effects: relativistic metal-insulator transition from antiferromagnetic Dirac fermions, and crystal Hall...

Les dernières décennies ont donné lieu à un développement rapide des théories ab initio visant à décrire les propriétés des noyaux à partir de l'interaction nucléonique. Un tel développement a été rendu possible à la fois par la très importante croissance de la puissance de calcul et de nouveaux développements formels. Le présent travail se consacre au développement de la théorie de perturbation à N corps de Bogolioubov récemment proposée, qui repose sur l'usage d'un état de référence brisant la symétrie associée au nombre de particules pour permettre une description des noyaux à simple couche ouverte. Le formalisme est tout d'abord décrit en détails, son lien avec la théorie de perturbation à N corps standard est établi, tout comme sa connexion avec la théorie de cluster couplés de Bogolioubov. L'extension du formalisme à des ordres plus élevés à partir de méthodes de théorie des graphes est ensuite présentée ainsi que le programme ADG qui génère et évalue les diagrammes BMBPT à un ordre quelconque. Les implications de ce développement formel dépassent le cadre du présent travail, les méthodes développées pouvant être appliqués à d’autres méthodes à N corps. Pour terminer, de premiers résultats numériques pour les isotopes de l'oxygène, du calcium et du nickel sont présentés. Ces résultats établissent la théorie de perturbation à N corps de Bogolioubov comme une méthode de premier intérêt pour des calculs à grande échelle sur les chaînes isotopiques et isotoniques de masse moyenne. / The last few decades in nuclear structure theory have seen a rapid expansion of ab initio theories, aiming at describing the properties of nuclei starting from the inter-nucleonic interaction. Such an expansion relied both on the tremendous growth of computing power and novel formal developments. This work focuses on the development of the recently proposed Bogoliubov Many-Body Perturbation Theory that relies on a particle-number-breaking reference state to tackle singly open-shell nuclei. The formalism is first described in details, and diagrammatic and algebraic contributions are derived up to second order. Its link to standard Many-Body Perturbation Theory is made explicit, as well as its connexion to Bogoliubov Coupled-Cluster theory. An automated extension to higher orders based on graph theory methods is then detailed, and the ADG numerical program generating and evaluating BMBPT diagrams at arbitrary order is introduced. Such a formal development carries implications that are not restricted to the present work, as the developed methods can be applied to other many-body methods. Finally, first numerical results obtained for oxygen, calcium and nickel isotopes are presented. They establish BMBPT as a method of interest for large-scale computations of isotopic or isotonic chains in the mid-mass sector of the nuclear chart.

Le PbZr₁₋ₓTiₓO₃ (PZT) est une pérovskite mixte possédant de nombreuses propriétés, dont certaines sont déjà utilisées dans l’industrie, ce qui en fait un matériau encore très étudié à l’heure actuelle, malgré la toxicité du plomb et de ses oxydes. Au cours de cette thèse, nous nous sommes intéressés à la spectroscopie d’absorption IR de ce composé, tant au niveau expérimental que théorique. Nous avons donc réalisé des modélisations, via le code de calcul CRYSTAL basé sur les méthodes de Combinaison Linéaire d’Orbitales Atomiques et de la Théorie de la Fonctionnelle de la Densité (LCAO-DFT) périodique, afin d’aider à l’interprétation des spectres expérimentaux réalisés sur la ligne AILES du synchrotron SOLEIL. Dans ce but, nous avons commencé par modéliser les composés de base du PZT : le PbTiO₃ (PT) et le PbZrO₃ (PZ). Nos résultats reproduisant très bien les données de la littérature sur ces deux composés, nous avons pu faire une analyse fine de leur spectre d’absorption IR. D’autre part, leur modélisation nous a permis de déterminer des paramètres de calcul transférables (base et fonctionnelle notamment) et de les appliquer sur le PZT en utilisant la méthode de la supermaille, couplée à une analyse statistique. Les résultats obtenus sont prometteurs pour l’interprétation, car tout à fait comparables aux spectres expérimentaux. Afin de nous rapprocher au mieux du cristal réel de PT, nous nous sommes intéressés à la modélisation de couches ultraminces et de lacunes d’oxygène, dans le but de voir leur effet sur le spectre d’absorption IR du PT. / PbZr₁₋ₓTiₓO₃ (PZT) is a complex perovskite that has many properties, some of which are already used industrially. Thus, in spite of the toxicity of lead and its oxides, this material is still under extensive investigation. In this thesis, we are interested of both experimental and theoretical IR absorption spectroscopy of this compound. To do so, we used the CRYSTAL code, based on the Linear Combination of Atomic Orbitals method and periodic Density Functional Theory (LCAO-DFT) in order to facilitate the interpretation of experimental spectra, recorded on the AILES beamline of synchrotron SOLEIL. In this goal, we first studied the two building blocks of PZT: PbTiO₃ (PT) and PbZrO₃ (PZ). Our results are in very good agreement with what has already been done in the literature. We, thus, could carry out a precise interpretation of their absorbance spectra. Moreover, transferable parameters (in particular the basis set and the functional) have been determined and used to study PZT. The supercell method, coupled with a statistical analysis, provided promising results, comparable with experimental data and, thus, helpful for their interpretation. In order to make a step towards the real PT crystal, we started the simulation of ultrathin films and oxygen vacancies to investigate their effects on the IR absorption spectrum.