As the demand for clean and renewable energy sources continues to grow, much attention has been given to solid oxide fuel cells (SOFCs) due to their efficiency and low operating temperature. However, the components of SOFCs must still be improved before commercialization can be reached. Of particular interest is the solid electrolyte, which conducts oxygen ions from the cathode to the anode. Samarium-doped ceria (SDC) is the electrolyte of choice in most SOFCs today, due mostly to its high ionic conductivity at low temperatures. However, the underlying principles that contribute to high ionic conductivity in doped ceria remain unknown, and so it is difficult to improve upon the design of SOFCs. This thesis focuses on identifying the atomistic interactions in SDC which contribute to its favourable performance in the fuel cell. Unfortunately, information as basic as the structure of SDC has not yet been found due to the difficulty in experimentally characterizing and computationally modelling the system. For instance, to evaluate 10.3% SDC, which is close to the 11.1% concentration used in fuel cells, one must investigate 194 trillion configurations, due to the numerous ways of arranging the Sm ions and oxygen vacancies in the simulation cell. As an exhaustive search method is clearly unfeasible, we develop a genetic algorithm (GA) to search the vast potential energy surface for the low-energy configurations, which will be most prevalent in the real material. With the GA, we investigate the structure of SDC for the first time at the DFT+U level of theory. Importantly, we find key differences in our results from prior calculations of this system which used less accurate methods, which demonstrate the importance of accurately modelling the system. Overall, our simulation results of the structure of SDCagree with experimental measurements. We identify the structural significance of defects in the doped ceria lattice which contribute to oxygen ion conductivity. Thus, the structure of SDC found in this work provides a basis for developing better solid electrolytes, which is of significant scientific and technological interest. Following the structure search, we perform an investigation of the electronic properties of SDC, to understand more about the material. Notably, we compare our calculated density of states plot to XPS measurements of pure and reduced SDC. This allows us to parameterize the Hubbard (U) term for Sm, which had not yet been done. Importantly, the DFT+U treatment of the Sm ions also allowed us to observe in our simulations the magnetization of SDC, which was found by experiment. Finally, we also study the SDC surface, with an emphasis on its structural similarities to the bulk. Knowledge of the surface structure is important to be able to understand how fuel oxidation occurs in the fuel cell, as many reaction mechanisms occur on the surface of this porous material. The groundwork for such mechanistic studies is provided in this thesis.

Es werden die Verbindungen YMn2O5 und YFeMnO5 diskutiert. Die erstere zeigt unterhalb von TN = 45 K Ferromagnetismus und in der magnetischen Phase schwache Ferroelektrizität. Die elektrische Polarisation wird mit Symmetriebrechung durch die Spinstruktur erklärt, die zur Aufhebung der Inversionssymmetrie führt (sog. unechtes Ferroelektrikum). Isostrukturelle Ersetzung von Mn durch Fe führt zu YFeMnO5, einer Verbindung, die bei T<165 K ferrimagnetisch, jedoch nicht ferroelektrisch ist. Die Spin-Strukturen beider Verbindungen sind bereits eingehend untersucht und zeigen charakteristische Unterschiede. Für Verbindungen der Zusammensetzung YFe(x)Mn(2-x)O5 wurden Röntgenbeugungs-und Absorptionsfeinstruktur-Experimente zur Bestimmung der Kristallstrukur in Abhängigkeit vom Fe-Anteil x durchgeführt und ausgewertet. Die Ergebnisse zeigen, daß die Strukturparameter einen nahezu linearen Verlauf zwischen den aus der Literatur bekannten Grenzfällen YFeMnO5 und YMn2O5 nehmen. Fe ersetzt dabei das Mn auf der fünffach koordinierten Lage innerhalb einer Sauerstoff-Pyramide. Besonders markant ist die unterschiedliche Position von Mn bzw. Fe in dieser Umgebung. Mit Hilfe der Strukturdaten wurden kollineare DFT-Rechnungen im LSDA+U-Formalismus in skalar-relativistischer Näherung durchgeführt. Für YFeMnO5 konnte der experimentell bekannte Grundzustand im Rahmen der Näherung reproduziert werden, obgleich eine Bandlücke nur in Abhängigkeit von der U-Korrektur auftritt. Der berechnete Grundzustand von YMn2O5 gibt die komplizierte magnetische Struktur dieser Verbindung nicht wieder, weil die gewählte Elementarzelle des Gitters dafür zu klein ist. Statt dessen ist der berechnete Grundzustand hier sehr ähnlich zu dem von YFeMnO5. Eine ausführliche Untersuchung der projizierten Zustandsdichten der Metallatome ermöglicht die Diskussion der Kristallfeldaufspaltung im Zusammenhang mit deren Position innerhalb der Sauerstoffpolyeder. Durch Berechnung mehrerer Spinstrukturen in einer kristallographischen Elementarzelle mit erniedrigter Symmetrie konnten die Austauschparameter eines Heisenberg-Modells zwischen den lokalisierten Spins der Metallatome berechnet werden. Die Größenverhältisse dieser Parameter können mit den aus der Literatur bekannten Spinstrukturen in Einklang gebracht werden. Die Wechselwirkungen sind überwiegend antiferromagnetisch, in Übereinstimmung mit den GKA-Regeln für den Superaustausch. Bei YMn2O5 wird insbesondere eine der schwächeren Kopplungen in der magnetischen Struktur periodisch frustriert. Man geht davon aus, daß dies eine mögliche Ursache für das Auftreten von Ferroelektrizität in der magnetischen Phase ist. Bei YFeMnO5 ist der berechnete Wert dieser Kopplung wesentlich größer und die magnetische Struktur beinhaltet keine Frustration. Dies ist eine mögliche Erklärung für die Abwesenheit der magnetisch induzierten Ferroelektrizität in YFeMnO5. Im zweiten Teil stehen das in Perowskitstruktur kristallisierende SrTiO3 und die durch Hinzufügen von SrO daraus hervorgehenden Kristallstrukturen der sog. Ruddlesden-Popper-Phasen (RP) im Mittelpunkt. Die Daten von Nahkanten-Elektronenenergieverlustspektren (ELNES) an der Sauerstoff K-Kante in SrTiO3, SrO und einer RP-Phase wurden ausgewertet und mit DFT-berechneten projizierten Zustandsdichten (PDOS) der 2p-Orbitale der O-Atome in diesen Verbindungen verglichen. Bei ELNES-Nahkantenspektren ist ein solcher Vergleich mit Experimenten im Bereich hoher Elektronenenergien möglich, weil die Auswahlregel auch für die inelastischen Elektronenstöße zutrifft. Die Spektren zeigen für jede Verbindung charakteristische Maxima, deren Ursache die unterschiedliche nähere Umgebung der Sauerstoffatome ist. Weiterhin wurden Experimente an SrTiO3-Einkristallen unter Einfluß elektrischer Gleichströme und -felder durchgeführt. Bei Experimenten an Einkristall-Wafern waren Hinweise auf lokale Veränderungen der Kristallstruktur unter diesen Bedingungen gefunden worden. Ergänzend dazu wurden mikroskopische einkristalline Proben untersucht. Bei geringen Stromstärken zeigte sich dabei das bereits bekannte Degradationsverhalten des elektrischen Widerstands. Bei hohen Stromstärken kommt es zum elektrischen Durchbruch und dauerhafter Erniedrigung des Widerstands. Röntgenbeugungsmessungen ergaben keine Hinweise auf Veränderungen an der Kristallstruktur oder in Form von Zwillings-oder Bruchstückbildung. Im dritten Teil werden Röntgenbeugungsmessungen an CeCu2Si2-Einkristallen diskutiert. Bei der Auswertung älterer Messungen fielen nach der Strukturbestimmung charakteristische Maxima der Restelektronendichte auf, deren Ursprung nicht erklärt werden konnte und die bei mehreren Kristallen beobachtet wurden. Mit erneuten Messungen und Simulationen konnte nun gezeigt werden, daß diese Maxima von einer fehlerhaften Auswertungsmethode verursacht wurden.

Organic and inorganic nanostructured materials, nano- and mesoscale objects and devices, and their integration into existing microelectronic technologies have been at the center of recent fundamental and applied research in nanotechnology. One of the critical needs is to develop an enhanced predictive capability of structure-property correlations and enable robust high performance systems by design. My thesis work was concerned with the theoretical and experimental studies of electronic and mechanical properties of chemically functionalized nanowires. I will describe a theoretical approach for investigating structure-property correlations in atomic-sized metallic wires based on the Density Functional Theory (DFT) for structure calculations and the Non-equilibrium Green's Function (NEGF) technique for electronic transport properties simulations. This synergistic approach is shown to yield the atomic structure of the smallest niobium nanowires. Furthermore, the method was applied to simulate electronic properties of chemically functionalized graphene nanoribbons. Further, I will demonstrate an experimental technique for simultaneous measurements of force and conductance in atomic-size objects based on quartz tuning fork piezoelectric sensors. A peculiar scaling effect, relevant for a broad range of test and measurement applications, namely the squeeze film effect, was observed during the development of the sensors. Using theoretical analysis based on finite element simulations of the hydrodynamic behavior of the sensors in a broad range of ambient conditions, I explain the observed phenomenon.

One of the most important challenges in low-temperature fuel cell technology is improving the catalytic efficiency at the electrode-catalyst where the oxygen reduction reaction (ORR) occurs. Platinum is the best pure catalyst for this reaction but its high cost and scarcity hinder the commercial implementation of fuel cells in automobiles. Pt-based alloys are promising alternatives to substitute platinum while maintaining the efficiency and life-time of the pure catalyst. However, the acid medium and the oxidation of the surface reduce the activity and durability of the alloy catalyst through changes in its local composition and structure. Molecular simulation techniques are applied to characterize the thermodynamics and dynamic evolution of the surface of platinum-based alloy catalysts under reaction conditions.1-10 A simulation scheme of the surface oxidation is proposed which combines classical molecular dynamics (MD) and density functional theory (DFT). This approach is able to reproduce the main features of the oxidation phenomena observed experimentally, it is concluded that the dissolution mechanism of metal atoms involves: 1) Surface segregation of alloy atoms, 2) oxygen absorption into the subsurface of the catalyst, and 3) metal detachment through the interaction with ions in the solvent. Therefore, to improve the durability of platinum-based alloy catalysts, the steps of the dissolution mechanism must be prevented. A versatile 3-D kinetic Monte Carlo (KMC) code is developed to study the degradation and dealloying in nanocatalysts. The results on the degradation of Pt nanoparticles under different potential regimes demonstrate that the dissolution depends on the potential path to which the nanocatalyst is exposed. Metal atoms detach from the boundaries of (111) facets expecting a reduction in the activity of the nanoparticle. Also, the formation of Pt hollow nanoparticles by the Kirkendall effect is addressed, the role of vacancies is crucial in the removal of the non-noble core that yields to hollow nanoparticles. To investigate the reasons for the experimentally found enhanced ORR activity in porous/hollow nanoparticles, the effect of subsurface vacancies on the main ORR activity descriptors is studied with DFT. It is found that an optimum amount of vacancies may enhance the ORR activity of Pt-monolayer catalysts over certain alloy cores by changing the binding energies of O and OH.

Ο κύριος στόχος της παρούσας εργασίας είναι η αποσαφήνιση του τρόπου εναπόθεσης και της τοπικής δομής των ειδών κοβαλτίου που σχηματίζονται στην διεπιφάνεια «τιτάνια / ηλεκτρολυτικό διάλυμα» κατά το στάδιο του εμποτισμού. Προκειμένου να επιτευχθεί ο στόχος αυτός προβήκαμε σε κατάλληλη θεωρητική και υπολογιστική επεξεργασία δεδομένων, που προέρχονται από την εφαρμογή ηλεκτροχημικών τεχνικών, την εκτέλεση πειραμάτων προσρόφησης και την εφαρμογή φασματοσκοπικών τεχνικών, καθώς επίσης και σε ab-initio υπολογισμούς για την εξακρίβωση του είδους των επιφανειακών οξυγόνων, των φορτίων τους και των συγκεντρώσεών τους. Συγκεκριμένα, εκτελέστηκαν πειράματα στα οποία μελετήθηκε η μεταβολή του pH κατά την εναπόθεση των ειδών του κοβαλτίου στην επιφάνεια της τιτάνιας, πειράματα τιτλοδοτήσεων Co-H+ σε σταθερό pH, καθώς και πειράματα “adsorption edges” προκειμένου να ληφθεί μια γενική εικόνα της έκτασης της προσρόφησης των ιόντων κοβαλτίου σε μια ευρεία περιοχή pH. Επιπλέον, μελετήθηκε η μεταβολή του σημείου μηδενικού φορτίου και του ισοηλεκτρικού σημείου της τιτάνιας, εκτελώντας πειράματα ποτενσιομετρικών τιτλοδοτήσεων μάζας και μικροηλεκτροφόρησης, αντιστοίχως, παρουσία των ιόντων αυτών. Όλα τα παραπάνω, σε συνδυασμό με την φασματοσκοπική τεχνική της διάχυτης ανάκλασης υπέδειξαν το σχηματισμό μονοπυρηνικών / ολιγοπυρηνικών συμπλόκων εσωτερικής σφαίρας κατά την εναπόθεση των ιόντων [Co(H2O)6]2+ στη διεπιφάνεια “τιτάνιας / ηλεκτρολυτικού διαλύματος”. Η αποσαφήνιση όμως της ακριβούς δομής των συμπλόκων αυτών καθώς και η εξακρίβωση της σχετικής τους συγκέντρωσης, σε διάφορες επιφανειακές συγκεντρώσεις Co(II) κατέστη δυνατή με την συνδυασμένη χρήση ημιεμπειρικών κβαντομηχανικών υπολογισμών, στερεοχημικών θεωρήσεων καθώς και προσομοίωσης των δεδομένων εναπόθεσης. Στο συμπαγές τμήμα της διεπιφάνειας “τιτάνιας / ηλεκτρολυτικού διαλύματος” και για χαμηλές καθώς και μεσαίες επιφανειακές συγκεντρώσεις Co(II) σχηματίζονται μονοπυρηνικά σύμπλοκα. Είναι πολύ πιθανός ο σχηματισμός μιας μονο-υδρολυμένης Ti2O-TiO διαμόρφωσης καθώς και μιας δι-υποκατεστημένης TiO-TiO, η οποία έχει υποστεί δυο υδρολύσεις. Στην πρώτη διαμόρφωση ένα υδατικός υποκαταστάτης αντικαθίσταται από ένα γεφυρωμένο επιφανειακό οξυγόνο ενώ ένας άλλος από ένα ακραίο επιφανειακό οξυγόνο. Στην δεύτερη διαμόρφωση δυο υδατικοί υποκαταστάτες αντικαθίσταται από δυο ακραία επιφανειακά οξυγόνα. Επίσης σε υψηλές επιφανειακές συγκεντρώσεις Co(II) παρατηρείται και ο σχηματισμός διπυρηνικών και τριπυρηνικών συμπλόκων εσωτερικής σφαίρας. / The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H2O)6]2+ on the “titania/electrolyte solution” interface, the structure of the inner-sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse-reflectance spectroscopy. These suggested the formation of mononuclear/ oligonuclear inner-sphere complexes on deposition of the [Co(H2O)6]2+ ions at the “titania/electrolyte solution” interface. The joint application of semiempirical quantum-mechanical calculations, stereochemical considerations and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co(II) surface concentrations to be determined. It was found that the interface speciation depends on the Co(II) surface concentration. Mononuclear complexes are formed at the compact layer of the “titania/electrolyte solution” interface for low and medium Co(II) surface concentrations. Formation of mono-hydrolyzed Ti2O–TiO and the dihydrolyzed TiO–TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co (H2O)6]2+ ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H2O)6]2+ ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner-sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co(II) surface concentrations

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work first we perform a study about the stability, and the electronic properties of SiC growth in the [111] direction when defects are present. We use the supercell method and the dangling bonds on the surface of the nanowire are saturated using hydrogen atoms. We also study antisites and substitutional oxygen impurity in this nanowire. For this study, we perform total energy and band structure calculations in order to find the most stable positions for the defects and the influence of defects on the electronic properties. The first principles calculations are based in the density functional theory (DFT). The Generalized Gradient Approximation (GGA) is used for the exchange-correlation term and the ion-electron interactions are replaced by norm-conserving fully separable Troullier-Martins pseudopotentials. For the calculations we use the SIESTA-code and the standard Kohn-Shan (KS) equations are solved in a fully selfconsistent way. The Khon-Sham orbitals are expanded using a linear combination of numerical pseudo-atomic orbitals (PAOs). All calculations use a split-valence double-zeta quality basis set enhanced with a polarization function. Our results show that the most stable antisite is a carbon atom occupying a silicon site (CSi). The substitutional oxygen impurity is most stable in a carbon site (OC). Both defects present a greater stability in the surface of the nanowire when compared with the core of the nanowire. The analysis of electronic structure of bands shows that these defects give rise to electronic levels localized in the band gap of the nanowire. Keywords: density functional theory; SiC nanowires, antisites, impurity. / Neste trabalho inicialmente realizamos um estudo da estabilidade e das propriedades eletrônicas de nanofios de SiC crescido na direção [111]. Foi utilizado o método de supercélula e as ligações pendentes da superfície do nanofio de SiC foram saturadas com átomos de H. Em seguida analisamos estes nanofios na presença de antissítios e impureza substitucional de oxigênio. Para estes defeitos procurou-se as posições energeticamente mais estáveis e as influências dos defeitos nas propriedades eletrônicas. Os cálculos teóricos foram de primeiros princípios fundamentados na Teoria do Funcional da densidade (DFT). Utilizamos para descrever o funcional de trocacorrelação a Aproximação do Gradiente Generalizado (GGA) e para a interação elétron-íon pseudopotenciais de norma conservada de Troullier-Martins. As densidades de carga são obtidas resolvendo as equações de Kohn-Sham, com as funções de onda de Khon-Sham expandidas em uma combinação linear de orbitais atômicos. Nossos resultados mostram que o antissítio mais estável é um átomo de carbono ocupando o sítio de um átomo de silício (CSi). A impureza substitucional de oxigênio apresenta uma maior estabilidade quando ocupando o sítio do átomo de carbono (OC). Ambos os defeitos são energeticamente mais estáveis na superfície do nanofio de SiC. A análise da estrutura eletrônica apresenta que níveis de defeitos podem estar presentes no gap do nanofio, porém nos sítios mais estáveis não observa-se níveis de defeitos no gap.

Covalent functionalization significantly enhances the utility of carbon nanomaterials for many applications. In this study, we investigated the functionalization of double nitrogen doped graphene by the addition of different alkyl and phenyl functional groups at N atoms in syn and anti-configurations. Density functional theory calculations at the B3LYP/def-SV(P) level were employed to understand the syn versus anti preference on functionalization. The bond lengths, bond angles, relative energies, deformation energies and HOMO-LUMO energy gaps, of the syn and anti-configurations of the functionalized 2N-doped graphenes, have been compared. Functionalization with two groups leads to considerable deformation of 2N-doped graphene, which is confirmed by the change in C–N bond lengths by attachment of the functional groups. The attachment of larger functional groups deforms 2N-doped graphene to a greater extent than smaller functional groups. The HOMO-LUMO energy gap values are the least for the alkyl functionalized products, indicating that these structures are kinetically less stable than the phenyl functionalized products.

abstract: Fuel cells, particularly solid oxide fuel cells (SOFC), are important for the future of greener and more efficient energy sources. Although SOFCs have been in existence for over fifty years, they have not been deployed extensively because they need to be operated at a high temperature (&sim;1000 &deg;C), are expensive, and have slow response to changes in energy demands. One important need for commercialization of SOFCs is a lowering of their operating temperature, which requires an electrolyte that can operate at lower temperatures. Doped ceria is one such candidate. For this dissertation work I have studied different types of doped ceria to understand the mechanism of oxygen vacancy diffusion through the bulk. Doped ceria is important because they have high ionic conductivities thus making them attractive candidates for the electrolytes of solid oxide fuel cells. In particular, I have studied how the ionic conductivities are improved in these doped materials by studying the oxygen-vacancy formations and migrations. In this dissertation I describe the application of density functional theory (DFT) and Kinetic Lattice Monte Carlo (KLMC) simulations to calculate the vacancy diffusion and ionic conductivities in doped ceria. The dopants used are praseodymium (Pr), gadolinium (Gd), and neodymium (Nd), all belonging to the lanthanide series. The activation energies for vacancy migration between different nearest neighbor (relative to the dopant) positions were calculated using the commercial DFT code VASP (Vienna Ab-initio Simulation Package). These activation energies were then used as inputs to the KLMC code that I co-developed. The KLMC code was run for different temperatures (673 K to 1073 K) and for different dopant concentrations (0 to 40%). These simulations have resulted in the prediction of dopant concentrations for maximum ionic conductivity at a given temperature. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

Abstract Density functional theory (DFT) is a well-established tool for calculating the properties of materials. The volume of DFT-related publications doubles every 5–6 years, which has resulted in the appearance of continuously growing open material databases, containing information on millions of compounds. Furthermore, the results of DFT computations are frequently coupled with experimental ones to strengthen the computational findings. In this thesis, several applications of DFT related to physics and chemistry are discussed. The conductivity between MoS₂ and transition metal nanoparticles is evaluated by calculating the electronic structure of two different models for the nanoparticles. Chemical bonding of Ni to the MoS₂ host is proven by the system’s band alignment. To meet the demand for cleaner fuel, the applicability of the (103) edge surface of molybdenum disulfide in relation to the early stages of the hydrodesulfurization (HDS) reaction is considered. The occurrence of the (103) edge surface of molybdenum disulfide in the XRD patterns is explained. A method for calculating oxidation states based on partial charges using open materials databases is suggested. We estimate the applicability of the method in the case of mixed valence compounds and surfaces, showing that DFT calculations can be used for the estimation of oxidation states. / Tiivistelmä Tiheysfunktionaaliteoria (density functional theory, DFT) on yleisesti käytetty työkalu laskennallisessa materiaalitutkimuksessa. DFT:llä tuotettujen julkaisujen määrä kaksinkertaistuu 5–6 vuoden välein, minkä johdosta käytettävissä on jatkuvasti kasvava määrä avoimia materiaalitietokantoja, joihin on talletettu miljoonien yhdisteiden ominaisuuksia. DFT-laskujen tuloksia täydennetään myös usein kokeellisilla tuloksilla. Tässä työssä tarkastellaan tiheysfunktionaaliteorian sovelluksia fysiikassa ja kemiassa. MoS₂:n ja metallisten nanopartikkelien välistä johtavuutta on tutkittu mallintamalla erilaisia nanopartikkeleita. Nikkelin ja MoS₂:n välinen kemiallinen sidos selittyy systeemin energiavöiden kohdistumisella. MoS₂:n (103)-pinnan soveltuvuutta rikinpoistoreaktion varhaisissa vaiheissa on tutkittu tarkoituksena löytää uusia menetelmiä puhtaan polttoaineen tuottamiseksi. Myös (103)-pinnan esiintyminen röntgendiffraktiokuvissa selitetään. Työssä on myös esitetty menetelmä hapetustilojen laskemiseksi tietokannoista löytyvien laskettujen varausjakaumien avulla. Menetelmän soveltuvuutta on tarkasteltu erilaisille yhdisteille ja pinnoille. Tämä tarkastelu osoittaa, että DFT-tuloksia voidaan käyttää hapetustilojen laskemiseen.

SILVA, Bruno Poti e. Propriedades estruturais, eletrônicas, ópticas e vibracionais do cristal de ureia sob formalismo DFT. 2014. 97 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2014. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2014-09-12T19:37:01Z No. of bitstreams: 1 2014_dis_bpsilva.pdf: 12912641 bytes, checksum: fc4c9bac87d198c8c4aa0d75b058e861 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-09-12T19:51:21Z (GMT) No. of bitstreams: 1 2014_dis_bpsilva.pdf: 12912641 bytes, checksum: fc4c9bac87d198c8c4aa0d75b058e861 (MD5) / Made available in DSpace on 2014-09-12T19:51:21Z (GMT). No. of bitstreams: 1 2014_dis_bpsilva.pdf: 12912641 bytes, checksum: fc4c9bac87d198c8c4aa0d75b058e861 (MD5) Previous issue date: 2014 / Urea, [CO(NH2)2], was the first organic compound artificially synthesized in 1828 by Friedrich Whoeler. Since then, have been widely studied in various fields as nonlinear optics, protein denaturation and so on. The presence of electronegative atoms in the urea molecule, Oxygen and Nitrogen, in the carboxyl and amine groups, makes it possesses great capacity of formation hydrogen bonds, enabling the formation of complexes networks of urea molecules. The ability to formation hydrogen bonds causes that urea interacts strongly with water. Several studies have tried to explain the effect of urea in the water structure. Due interesting properties of urea crystals in the nonlinear optics, scientists have calculated, using various methods of first principle calculations, the properties of the urea crystals. The first theoretical calculation done on the urea crystal properties was made by Dovesi {it et al.} In the present work, the studies convergence were made using GGA and LDA functionals of exchange-correlation and dispersion corrections to urea crystal, which has symmetry tetragonal and spacial group P-421m. The study of energies cutoff for convergence of the crystal was done. With this evaluation of the study of the convergence, showed that the functional that described the geometrical parameters of the urea crystal was the GGA+TS. The study of electronic, optical and vibrational properties was also conducted. The energy gap obtained to the urea crystal was 5.12 eV, which is in good agreement with experimental result, obtained by optical absorption, 5.85 eV, an error of -0.63 eV (-11{\%}). The effective mass calculations along differents pathways in the crystal, and using differents exchange-correlation functionals, proved to be consistent when results are compared between the functionals, obtained an maximum error of 10({\%}). The infrared spectra obtained both crystal and the molecule of urea showed excellent agreement with data reported in the literature. In this work also calculated, using DFT, the Raman spectra of urea crystal in different pressure values, 0.0, 0.2, 0.4, 0.6 e 0.8 GPa. The results showed that the bands presents in lower wavenumbers are more sensitive to application of pressure to the crystal, having the ocurrence of a blue shift in these, which can be characterized an atomic rearrangement in the crystal of urea. / Ureia, [CO(NH2)2], foi o primeiro composto orgânico sintetizado artificialmente em 1828 por Friedrich Whoeler. Desde então tem sido largamente estudada em diversos campos como óptica não linear, desnaturação de proteínas, dentre outros. A presença de átomos eletronegativos na molécula de ureia, Oxigênio e Nitrogênio, nos grupos carboxila e amina, faz com que esta possua grande capacidade de formação de ligações de hidrogênio, possibilitando a formação de redes complexas de moléculas de ureia. Esta capacidade de formação de ligação de hidrogênio faz com que a ureia interaja fortemente com a água. Diversos estudos tem tentado explicar o efeito da ureia na estrutura da água. Devido interessantes propriedades dos cristais de ureia na óptica não linear, cientistas tem calculado, utilizando vários métodos de primeiros princípios, as propriedades dos cristais de ureia. O primeiro cálculo teórico feito sobre as propriedades eletrônicas foi feito por Dovesi et al. No presente trabalho foram feitos estudos de convergência utilizando funcionais de correlação-troca GGA e LDA, bem como correções de dispersão para o cristal de ureia, que possui simetria tetragonal e grupo espacial P-421m. Foi também feito um estudo da energia de corte utilizada na convergência do cristal. Com isso a avaliação do estudo de convergência mostrou que o funcional que melhor descrevia os parâmetros geométricos do cristal de ureia foi o GGA+TS. Foi realizado também um cálculo da estrutura eletrônica, propriedades ópticas e vibracionais, com este funcional. O gap de energia obtido para o cristal de ureia foi de 5.12 eV, que está em boa concordância com o resultado experimental, obtido por absorção óptica, de 5.85 eV, um erro de -0,63 eV (-11%). O cálculo de massas efetivas ao longo de diferentes direções no cristal, e utilizando diferentes funcionais de correlação-troca, mostrou-se coerente quando são comparados os resultados entre os funcionais, obtendo-se um erro máximo de 10{\%} do melhor funcional com os outros. Os espectros de Infravermelho obtidos tanto para o cristal quanto para molécula de ureia apresentaram excelente concordância com os dados relatados na literatura. Neste trabalho, também foram calculados, via DFT, os espectros Raman do cristal de ureia em diferentes pressões, 0.0, 0.2, 0.4, 0.6 e 0.8 GPa. Os resultados mostraram que as bandas presentes em menores números de onda são mais sensíveis a aplicação da pressão no cristal, havendo a ocorrência de um blue shift nestas, que pode caracterizar um rearranjo atômico no cristal de ureia.