The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

The objective of this investigation is to study the distinct physical and electronic properties of high-pressure solids, through state-of-the-art first-principles numerical computations. This thesis is composed of four distinct research topics.<p>The superconducting properties of several high-pressure solids were investigated based on the Migdal-Eliashberg theory within the framework of the BCS model. The possibility of pressure-induced superconductivity was investigated for selected materials, including dense Li, Xe, and Group IV hydrides. The pressure-induced phase transition FCC ¡÷ cI16 in Li and the superconducting properties in the FCC and cI16 phases were investigated. Noble gas Xe is predicted being a superconductor under pressure with a comparatively low Tc. Two Group IV hydrides, SiH4 and SnH4, were predicted to be good superconductors under high pressure. <p> The Bader¡¦s AIM analysis, IR and Raman spectroscopes were used as diagnostic tools to differentiate among candidate structural models for solid H2, O2, and SiH4. For solid H2, IR and Raman spectra are used to examine two recently proposed competing structures of the high-pressure phase III; the Cmcm and C2/c structures. For solid O2, the experiment observed structure, IR and Raman spectra of the recently solved C2/m structure of the high-pressure Õ phase were well produced. Using Bader¡¦s AIM method and from the analysis of the electron charge density, the preference on the formation of (O2)4 clusters in the C2/m structure and the nature of the interactions between O2 molecules is explained. For SiH4, IR and Raman spectra were calculated for our predicted P42/nmc structure and the agreement with available experiment results is very good. <p>On theoretical aspect, typical approaches for predicting/determining unknown high-pressure crystal structures usually involve dynamical processes. An alternate approach based on a recently proposed genetic algorithm was explored in this thesis. The focus is to predict stable and meta-stable structures at high pressure without any preference on initial structures. The high-pressure structures of Ca were investigated and two new stable structures that might explain the diffraction pattern of the Ca-IV and Ca-V phases were predicted. The high-pressure phase II and phase III of AlH3 were also investigated, and structures were successfully predicted for each phase. Another example presented is the prediction of a metastable single-bonded phase of nitrogen.<p>A first-principles approach was developed for the calculation of XAS within the framework of the DFT. The PAW method was used to reconstruct the core orbitals. These orbitals are essential for the calculation of the transition matrix elements. This approach provides a straightforward framework for the investigation of single particle core hole and electron screening effects, which have been demonstrated to be significant for all investigated materials. To test the implementation, the C, Si, and O K-edge XAS were calculated for diamond, fullerene C60, £-quartz and water molecule. In all cases, the calculated XAS agree very well with experiments. For water molecule, the quality of the calculated XAS sensitively depends on the delicate theoretical treatment of core hole potential and electron screening. The overall agreement between the calculated XAS and experiment is reasonable.

Moderne Hochdruckforschung entwickelt sich rasant zu einer der vielfältigstens und überraschensten Disziplinen der Festkörperphysik. Unter Benutzung von Diamantstempelzellen können Drücke erreicht werden, die den Bedingungen im Inneren unserer Erde ähneln. Eine Anwendung von Kernmagnetischen Resonanzexperimenten (NMR) in Diamantstempelzellen galt jedoch fur lange Zeit als unmöglich. In der vorliegenden Arbeit wird ein neuartiger Ansatz weiterentwickelt, der Radiofrequenz-(RF)-Mikrospulen benutzt, die direkt zwischen den Diamantstempeln platziert werden, und somit zu einer signififikanten Sensitivitatssteigerung führen. Es ist gelungen, Hochdruckzellen zu entwickeln, die fur die speziellen Anforderungen der NMR zugeschnitten sind. Des Weiteren konnte eine nicht metallische, nicht magnetische Dichtung entwickelt werden, die zudem zu einer signififikanten Stabilisierung des Probenvolumens führt. Eine breit angelegte Analyse der Leistungsfähigkeit dieser neuartigen NMR-Hochdruckprobenköpfe zeigt deren Leistungsfähigkeit mit sehr hohen Empfifindlichkeiten sowie einer exzellenten RF Anregung und Zeitauflösung. Drei Anwendungsbeispiele, die das Potenzial dieses Ansatzes in dieser Arbeit unterstreichen, werden vorgestellt. Bei Drücken von bis zu 4 GPa werden die elektronischen und dynamischen Eigenschaften von elementarem Gallium untersucht. Unter höheren Drücken ist es gelungen, einen druckinduzierten Isolator-Metall-Übergang in dem ternaren Chalkogenid AgInTe2 zu beobachten. Schlussendlich ist es gelungen, die strukturellen und elektronischen Eigenschaften von Rubin bei Drücken von bis zu 30.5 GPa zu untersuchen, was einer Verdreifachung des bisher zugänglichen experimentellen Druckbereiches entspricht und die NMR fur moderne Hochdruckanwendungen möglich macht.

The physical and chemical properties of materials are intimately related to their underlying crystal structure: the detailed arrangement of atoms and chemical bonds within. This thesis uses computational methods to predict crystal structure, with a particular focus on structures and stable phases that emerge at high pressure. We explore three distinct systems. We first apply the ab initio random structure searching (AIRSS) technique and density functional theory (DFT) calculations to investigate the high-pressure behaviour of beryllium, magnesium and calcium difluorides. We find that beryllium fluoride is extensively polymorphic at low pressures, and predict two new phases for this compound - the silica moganite and CaCl$_2$ structures - to be stable over the wide pressure range 12-57 GPa. For magnesium fluoride, our results show that the orthorhombic `O-I' TiO$_2$ structure ($Pbca$, $Z=8$) is stable for this compound between 40 and 44 GPa. Our searches find no new phases at the static-lattice level for calcium difluoride between 0 and 70 GPa; however, a phase with $P\overline{6}2m$ symmetry is energetically close to stability over this pressure range, and our calculations predict that this phase is stabilised at high temperature. The $P\overline{6}2m$ structure exhibits an unstable phonon mode at large volumes which may signal a transition to a superionic state at high temperatures. The Group-II difluorides are isoelectronic to a number of other AB$_2$-type compounds such as SiO$_2$ and TiO$_2$, and we discuss our results in light of these similarities. Compressed hydrogen sulfide (H$_2$S) has recently attracted experimental and theoretical interest due to the observation of high-temperature superconductivity in this compound ($T_c$ = 203 K) at high pressure (155 GPa). We use the AIRSS technique and DFT calculations to determine the stable phases and chemical stoichiometries formed in the hydrogen-sulfur system as a function of pressure. We find that this system supports numerous stable compounds: H$_3$S, H$_7$S$_3$, H$_2$S, H$_3$S$_2$, H$_4$S$_3$, H$_2$S$_3$ and HS$_2$, at various pressures. Working as part of a collaboration, our predicted H$_3$S and H$_4$S$_3$ structures are shown to be consistent with XRD data for this system, with H$_4$S$_3$ identified as a major decomposition product of H$_2$S in the lead-up to the superconducting state. Calcium and oxygen are two elements of generally high terrestrial and cosmic abundance, and we explore structures of calcium peroxide (CaO$_2$) in the pressure range 0-200 GPa. Stable structures for CaO$_2$ with $C2/c$, $I4/mcm$ and $P2_1/c$ symmetries emerge at pressures below 40 GPa, which we find are thermodynamically stable against decomposition into CaO and O$_2$. The stability of CaO$_2$ with respect to decomposition increases with pressure, with peak stability occurring at the CaO B1-B2 phase transition at 65 GPa. Phonon calculations using the quasiharmonic approximation show that CaO$_2$ is a stable oxide of calcium at mantle temperatures and pressures, highlighting a possible role for CaO$_2$ in planetary geochemistry, as a mineral redox buffer. We sketch the phase diagram for CaO$_2$, and find at least five new stable phases in the pressure/temperature ranges 0 $\leq P\leq$ 60 GPa, 0 $\leq T\leq$ 600 K, including two new candidates for the zero-pressure ground state structure.

Moderne Hochdruckforschung entwickelt sich rasant zu einer der vielfältigstens und überraschensten Disziplinen der Festkörperphysik. Unter Benutzung von Diamantstempelzellen können Drücke erreicht werden, die den Bedingungen im Inneren unserer Erde ähneln. Eine Anwendung von Kernmagnetischen Resonanzexperimenten (NMR) in Diamantstempelzellen galt jedoch fur lange Zeit als unmöglich. In der vorliegenden Arbeit wird ein neuartiger Ansatz weiterentwickelt, der Radiofrequenz-(RF)-Mikrospulen benutzt, die direkt zwischen den Diamantstempeln platziert werden, und somit zu einer signififikanten Sensitivitatssteigerung führen. Es ist gelungen, Hochdruckzellen zu entwickeln, die fur die speziellen Anforderungen der NMR zugeschnitten sind. Des Weiteren konnte eine nicht metallische, nicht magnetische Dichtung entwickelt werden, die zudem zu einer signififikanten Stabilisierung des Probenvolumens führt. Eine breit angelegte Analyse der Leistungsfähigkeit dieser neuartigen NMR-Hochdruckprobenköpfe zeigt deren Leistungsfähigkeit mit sehr hohen Empfifindlichkeiten sowie einer exzellenten RF Anregung und Zeitauflösung. Drei Anwendungsbeispiele, die das Potenzial dieses Ansatzes in dieser Arbeit unterstreichen, werden vorgestellt. Bei Drücken von bis zu 4 GPa werden die elektronischen und dynamischen Eigenschaften von elementarem Gallium untersucht. Unter höheren Drücken ist es gelungen, einen druckinduzierten Isolator-Metall-Übergang in dem ternaren Chalkogenid AgInTe2 zu beobachten. Schlussendlich ist es gelungen, die strukturellen und elektronischen Eigenschaften von Rubin bei Drücken von bis zu 30.5 GPa zu untersuchen, was einer Verdreifachung des bisher zugänglichen experimentellen Druckbereiches entspricht und die NMR fur moderne Hochdruckanwendungen möglich macht.

In the 1970s, the tandem shaped charge was developed to overcome countermeasures like active armor and thicker armor. It uses a leading charge to clear a path for the main trailing charge, theoretically improving penetration. However, disturbances from shock waves can cause under-performance. To address this, the leading charge's shock waves must be minimized to protect the main charge. Designing a part with shock wave attenuating properties can help achieve this.There is limited information on grenade structures that reduce shock wave stresses from impact and detonation. Most studies on shock wave attenuation focus on protecting buildings and humans, with few addressing structures designed to withstand and then halt the transmission of shock waves. Here we report on simulations in IMPETUS Afea where an explicit FEM solver approach have been made to compare four different concepts to an original structure. Physical theories about attenuation of shock waves in solids has been applied. The first concept utilize a mass with different area increases in the propagation path of the shock wave. The second concept use the characteristics of interfaces between two different materials, with different shock wave impedance which will reflect and transmit the incoming wave. The third concept incorporate a brittle ceramic which should shatter when a shock wave propagates to it, if the brittle structure fracture, no more shock waves can be transmitted. The fourth concept utilize the properties of porous metals. All concepts shows shock wave attenuating effects, but only the third concept which totally disconnects the leading shaped charge to the main charge might be a full solution. The fourth concept compared to the third has other advantages, it attenuate the shock wave to more than 90%, not halt the transmission. Configuration of the porosity in the structural part can be made to have characteristics that can solve the problem. Concept three and four are the main recommendations for a feasibility study. / På 1970-talet utvecklades tandem-RSV laddningar för att övervinna motmedel som aktivt pansar och tjockare pansar. De använder en förladdning för att skapa en väg för den efterföljande huvudladdningen, vilket teoretiskt sett förbättrar prestandan. Störningar från tryckvågor från olika källor orsaka prestandaförluster. För att hantera detta måste islagets och förladdningens tryckvågor minimeras för att skydda huvudladdningen. Att designa en mellandel med egenskaper som dämpar tryckvågor kan hjälpa till att uppnå detta. Information om granatstrukturer som minskar tryckvågsbelastningar från islag och detonation är begränsad. De flesta studier om dämpning av tryckvågor fokuserar på att skydda byggnader och människor, med få som behandlar strukturer designade för att motstå och sedan stoppa överföringen av tryckvågor. Här rapporterar vi om simuleringar genomförda i IMPETUS Afea med en explicit FEM-lösare för att jämföra fyra olika koncept mot en originalstruktur. Fysiska teorier om dämpning av tryckvågor i fasta material har tillämpats. Det första konceptet utnyttjar en massa med olika areaökningar i tryckvågens propagationsväg. Det andra konceptet använder egenskaperna hos gränsytor mellan två olika material med olika tryckvågsimpedanser som kommer att reflektera och transmittera den inkommande tryckvågen. I det tredje konceptet appliceras en spröd keram som bör splittras när en tryckvåg fortplantas till den. Om den spröda strukturen spricker kan då inga fler tryckvågor överföras genom strukturen. Det fjärde konceptet utnyttjar egenskaperna hos porösa metaller. Alla koncept visar tryckvågsdämpande effekter. Men endast det tredje konceptet som helt kopplar bort förladdningen från huvudladdningen kan vara en fullständig lösning. Det fjärde konceptet jämfört med det tredje har andra fördelar; det dämpar tryckvågen till mer än 90%, men stoppar inte överföringen helt. Konfigurationen av porositeten i den strukturella delen kan utformas för att ha egenskaper som kan lösa problemet. Koncept tre och fyra är de huvudsakliga rekommendationerna för en genomförbarhetsstudie.

Moderne Hochdruckforschung entwickelt sich rasant zu einer der vielfältigsten und überraschensten Disziplinen der Festkörperphysik. Unter Benutzung von Diamantstempelzellen können Drücke erreicht werden, die den Bedingungen im Inneren unserer Erde ähneln. Eine Anwendung von Kernmagnetischen Resonanzexperimenten (NMR) in Diamantstempelzellen galt jedoch für lange Zeit als unmöglich. In der vorliegenden Arbeit wird ein neuartiger Ansatz weiterentwickelt, der Radiofrequenz-(RF)-Mikrospulen benutzt, die direkt zwischen den Diamantstempeln platziert werden, und somit zu einer signifikanten Sensitivitätssteigerung führen. Es ist gelungen, Hochdruckzellen zu entwickeln, die für die speziellen Anforderungen der NMR zugeschnitten sind. Des Weiteren konnte eine nicht metallische, nicht magnetische Dichtung entwickelt werden, die zudem zu einer signfifikanten Stabilisierung des Probenvolumens führt. Eine breit angelegte Analyse der Leistungsfähigkeit dieser neuartigen NMR-Hochdruckprobenköpfe zeigt deren Leistungsfähigkeit mit sehr hohen Empfifindlichkeiten sowie einer exzellenten RF Anregung und Zeitauflösung. Drei Anwendungsbeispiele, die das Potenzial dieses Ansatzes in dieser Arbeit unterstreichen, werden vorgestellt. Bei Drücken von bis zu 4 GPa werden die elektronischen und dynamischen Eigenschaften von elementarem Gallium untersucht. Unter höheren Drücken ist es gelungen, einen druckinduzierten Isolator-Metall-Übergang in dem ternären Chalkogenid AgInTe2 zu beobachten. Schlussendlich ist es gelungen, die strukturellen und elektronischen Eigenschaften von Rubin bei Drücken von bis zu 30.5 GPa zu untersuchen, was einer Verdreifachung des bisher zugänglichen experimentellen Druckbereiches entspricht und die NMR für moderne Hochdruckanwendungen möglich macht.

ABAGARO, Bruno Tavares de Oliveira. Propriedades vibracionais de cristais de DL-leucina e L-prolina monohidratada submetidos a altas pressões. 2012. 116 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2012. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-20T18:43:46Z No. of bitstreams: 1 2012_tese_btoabagaro.pdf: 5633012 bytes, checksum: 021da558210c7a87a4904feee0d9a2b0 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-22T20:06:05Z (GMT) No. of bitstreams: 1 2012_tese_btoabagaro.pdf: 5633012 bytes, checksum: 021da558210c7a87a4904feee0d9a2b0 (MD5) / Made available in DSpace on 2015-05-22T20:06:05Z (GMT). No. of bitstreams: 1 2012_tese_btoabagaro.pdf: 5633012 bytes, checksum: 021da558210c7a87a4904feee0d9a2b0 (MD5) Previous issue date: 2012 / We studied the stability of two types amino acid crystal: a racemic crystal, DL-leucine (C6H13NO2); and a chiral one; L-proline monohydrate (C5H9NO2.H2O). Both were subjected to high pressures. The structural properties at ambient conditions (with the hydrogen-bonds data) of each crystal type were presented. For the DL-leucine crystal, polarized Raman scattering measurements were made in three different scattering geometries. The classification of the normal modes of vibration was made in terms of representations of irreducible factors groups Ci (for the crystal of DL-leucine) and C2 (crystal of L-proline monohydrate). Based on studies already carried out previously, tentative assignments were proposed for the Raman bands observed for the two crystals. The high pressure Raman experiments were performed in a diamond anvil cell using mineral oil as pressure media. For the DL-leucine crystal, the maximum pressure was studied up to about 5 GPa. Among the various observed effects, we found the discontinuity of the wavenumber versus pressure curves between 2.4 and 3.2 GPa, both for the region of the external modes as for region of internal modes, particularly remarkable. These effects were interpreted as resulting from a gradual change of the molecular conformation of leucine molecules in the unit cell, culminating with change of hydrogen bonds in the interval where the phase transition is observed. This behaviour suggests that the DL-leucine crystal is more stable than the chiral L-leucine in the sense that while the former presents only one phase transition, the latter presents at least two different transitions in the same pressure range considered. For the L-proline monohydrated crystal, which was subject to pressures up to about 8.0 GPa, several induced effects by this thermodynamic parameter were observed in the Raman spectra, such as the appearance and disappearance of bands, discontinuities in the wavenumber versus pressure curves in the regions of external and internal modes and intensity relative changes. These changes point to the fact that between (i) 1.1 e 1.7 GPa and (ii) 4.5 e 5.0 GPa, this hydrated crystal undergoes phase transitions accompanied by conformational molecular changes in the unit cell. / Neste trabalho, estudou-se a estabilidade de dois tipos de cristais de aminoácidos: um cristal racêmico; DL-leucina (C6H13NO2) e um quiral; L-prolina monohidratada (C5H9NO2.H2O), ambos submetidos a altas pressões. As propriedades estruturais em condições ambientes (com dados sobre as ligações de hidrogênio) de cada tipo de cristal foram apresentadas. Para o cristal de DL-leucina foram feitas medidas de espectrosocopia Raman polarizada em três diferentes geometrias de espalhamento. A distribuição dos modos normais de vibração foi feita em termos das representações irredutíveis dos grupos fatores Ci (para o cristal de DL-leucina) e C2 (cristal de L-prolina monohidratada). Foram propostas classificações tentativas para as bandas Raman observadas para os dois cristais, tendo como base estudos já realizados anteriormente. Os experimentos de espalhamento Raman sob altas pressões foram realizados em uma câmara de bigornas de diamantes utilizando-se óleo mineral como meio compressor. No caso do cristal de DL-leucina, a pressão máxima estudada foi de, aproximadamente, 5 GPa. Entre os diversos efeitos observados, destaca-se a descontinuidade das curvas de número de onda versus pressão entre 2,4 e 3,2 GPa, tanto para a região dos modos externos quanto para a região dos modos internos. Os efeitos foram interpretados como sendo decorrentes de uma mudança gradual de conformação das moléculas de leucina na célula unitária do cristal, culminando em uma mudança nas ligações de hidrogênio no intervalo onde a transição de fase foi observada. Esse comportamento sugere que o cristal de DL-leucina é mais estável sob altas pressões que o seu correspondente quiral, considerando que o cristal de L-leucina, o qual apresentou pelo menos duas transições de fase no mesmo intervalo de pressão considerado. Em relação ao cristal de L-prolina monohidratada, submetido a pressões de até cerca de 8,0 GPa, diversos efeitos induzidos por esse parâmetro termodinâmico nos espectros Raman foram observados, como o surgimento e desaparecimento de bandas,descontinuidades nas curvas de número de onda versus pressão nas regiões dos modos externos e internos, bem como mudanças de intensidade relativa. Essas modificações sugerem que entre (i) 1,1 e 1,7 GPa e (ii) 4,5 e 5,0 GPa, esse cristal hidratado sofre transições de fase acompanhadas por mudanças conformacionais das moléculas na célula unitária.

O estudo do interior da Terra apresenta diversos desafios, principalmente devido à impossibilidade de observações diretas de suas propriedades. Ondas sísmicas liberadas por terremotos são a melhor fonte de informação sobre a estrutura do planeta, mas sua correta interpretação depende do conhecimento das propriedades de seus elementos constituintes. Entretanto, estes estudos devem ser feitos nas condições extremas de temperatura e pressão do interior terrestre, condições difíceis de serem alcançadas em laboratório. Neste contexto, o estudo teórico de materiais tem sido muito importante na elaboração de modelos sobre a estrutura interna da Terra e na correta interpretação de dados sísmicos. Pesquisas recentes têm mostrado que a quantidade de carbono no manto inferior da Terra é maior do que se pensava anteriormente, e é importante compreender seus efeitos no interior profundo da Terra. Apesar da importância de entender os efeitos do carbono no interior da Terra, existem poucos estudos deste elemento nestas condições extremas de pressão e temperatura. Neste trabalho, utilizamos métodos e técnicas da física do estado sólido para estudar as propriedades de compostos de carbono nas condições de pressão e temperatura do manto inferior terrestre. Estudamos, primeiramente, as propriedades estruturais, eletrônicas e elásticas do MgSiO3 nas estruturas perovskita e pós-perovskita, considerado o principal mineral do manto inferior. Os resultados obtidos para as velocidades acústicas neste mineral mostraram variações maiores em relação às direções cristalinas, quando comparadas com mudanças devido à transição de fase estrutural. Isso indica que uma orientação preferencial dos eixos (anisotropia) pode ajudar a explicar algumas regiões com aumento descontínuo nas velocidades sísmicas. Posteriormente, foram obtidas as propriedades do MgCO3 e do CaCO3 em suas estruturas mais estáveis, em função da pressão. Nossos resultados foram comparados com os do MgSiO3 , mostrando que carbonatos de cálcio e de magnésio são estáveis nas condições do manto terrestre e que sua formação é energeticamente favorável. Resultados dos cálculos dos coeficientes elásticos e das velocidades acústicas nestes minerais mostram que as velocidades são menores que aquelas no MgSiO 3 . Dessa forma, em regiões ricas em carbono deve ocorrer a formação destes carbonatos e, por conseguinte, as velocidades sísmicas seriam menores nessas regiões. Isso pode explicar a existência das zonas de baixa velocidade na fronteira do manto inferior com o núcleo. Foram estudadas, também, as consequências da introdução de efeitos térmicos. Entretanto, obteve-se que os resultados não apresentam alterações significativas, de modo que mesmo nas altas temperaturas do interior da Terra nossas conclusões permanecem válidas, onde propomos que as regiões de baixa velocidade no manto inferior possam ser provocadas pela presença de carbono na forma de carbonatos e que a formação destes seria um modelo adicional para explicar onde e como o carbono pode ser armazenado no manto profundo. / Investigations on the Earths interior face several challenges, especially due to the infeasibility of direct observations of its properties. Earthquake seismic waves are the best information source about our planets structure, but its correct interpretation depends on the knowledge of its forming elements. However, these studies must consider the extreme pressures and temperatures of the Earths interior, hard to achieve experimentally. In this way, theoretical methods have emerged as an essential tool in elaborating models for the Earth internal structure and in the correct interpretation of seismic data. Recent studies have shown that the Earth must have much more carbon than previous thought, and it is important to understand its effects on the Earths deep interior. Despite its importance, there are few studies on carbon in these extreme conditions of pressure and temperature and on its effects in the Earths interior. In this investigation, we use theoretical solid state physics methods to investigate the properties of carbon compounds in the pressure and temperature conditions of Earths deep interior. First of all, we studied the electronic and elastic properties of MgSiO3 in the perovskite and post perovskite structures. This silicate is considered the main mineral in the Earths lower mantle. Our results show that seismic velocities have a larger variation with respect to the propagation direction than that with the phase transition. This indicates that a lattice preferred orientation can explain some seismic discontinuities. Thereafter, the properties of the MgCO3 and CaCO3 minerals were obtained in their more stable structures with respect to pressure. The results were compared with those of the MgSiO3, showing that calcium and magnesium carbonates are stable in the Earths mantle and that their formation is energetically favorable. The elastic coefficients and the acoustic velocities in these carbonates show seismic velocities considerably lower than those in the MgSiO3 . In this way, in regions with high carbon concentration the formation of carbonates could favorably occur and therefore the seismic velocities would be lower in those regions. This may explain the existence of low velocity zones near the bottom of Earths lower mantle. We also studied the consequences of the introduction of thermal effects. However, our results do not show any significant variation with temperature. Hence, even in the high temperatures of Earths interior, our conclusions are still valid where we propose that low velocity regions can be caused by the presence of carbon in the form of carbonates. Its formation could provide an additional model to explain where and how carbon can be stored in the deep mantle.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / Neste trabalho, estudou-se a estabilidade de dois tipos de cristais de aminoÃcidos: um cristal racÃmico; DL-leucina (C6H13NO2) e um quiral; L-prolina monohidratada (C5H9NO2.H2O), ambos submetidos a altas pressÃes. As propriedades estruturais em condiÃÃes ambientes (com dados sobre as ligaÃÃes de hidrogÃnio) de cada tipo de cristal foram apresentadas. Para o cristal de DL-leucina foram feitas medidas de espectrosocopia Raman polarizada em trÃs diferentes geometrias de espalhamento. A distribuiÃÃo dos modos normais de vibraÃÃo foi feita em termos das representaÃÃes irredutÃveis dos grupos fatores Ci (para o cristal de DL-leucina) e C2 (cristal de L-prolina monohidratada). Foram propostas classificaÃÃes tentativas para as bandas Raman observadas para os dois cristais, tendo como base estudos jà realizados anteriormente. Os experimentos de espalhamento Raman sob altas pressÃes foram realizados em uma cÃmara de bigornas de diamantes utilizando-se Ãleo mineral como meio compressor. No caso do cristal de DL-leucina, a pressÃo mÃxima estudada foi de, aproximadamente, 5 GPa. Entre os diversos efeitos observados, destaca-se a descontinuidade das curvas de nÃmero de onda versus pressÃo entre 2,4 e 3,2 GPa, tanto para a regiÃo dos modos externos quanto para a regiÃo dos modos internos. Os efeitos foram interpretados como sendo decorrentes de uma mudanÃa gradual de conformaÃÃo das molÃculas de leucina na cÃlula unitÃria do cristal, culminando em uma mudanÃa nas ligaÃÃes de hidrogÃnio no intervalo onde a transiÃÃo de fase foi observada. Esse comportamento sugere que o cristal de DL-leucina à mais estÃvel sob altas pressÃes que o seu correspondente quiral, considerando que o cristal de L-leucina, o qual apresentou pelo menos duas transiÃÃes de fase no mesmo intervalo de pressÃo considerado. Em relaÃÃo ao cristal de L-prolina monohidratada, submetido a pressÃes de atà cerca de 8,0 GPa, diversos efeitos induzidos por esse parÃmetro termodinÃmico nos espectros Raman foram observados, como o surgimento e desaparecimento de bandas,descontinuidades nas curvas de nÃmero de onda versus pressÃo nas regiÃes dos modos externos e internos, bem como mudanÃas de intensidade relativa. Essas modificaÃÃes sugerem que entre (i) 1,1 e 1,7 GPa e (ii) 4,5 e 5,0 GPa, esse cristal hidratado sofre transiÃÃes de fase acompanhadas por mudanÃas conformacionais das molÃculas na cÃlula unitÃria. / We studied the stability of two types amino acid crystal: a racemic crystal, DL-leucine (C6H13NO2); and a chiral one; L-proline monohydrate (C5H9NO2.H2O). Both were subjected to high pressures. The structural properties at ambient conditions (with the hydrogen-bonds data) of each crystal type were presented. For the DL-leucine crystal, polarized Raman scattering measurements were made in three different scattering geometries. The classification of the normal modes of vibration was made in terms of representations of irreducible factors groups Ci (for the crystal of DL-leucine) and C2 (crystal of L-proline monohydrate). Based on studies already carried out previously, tentative assignments were proposed for the Raman bands observed for the two crystals. The high pressure Raman experiments were performed in a diamond anvil cell using mineral oil as pressure media. For the DL-leucine crystal, the maximum pressure was studied up to about 5 GPa. Among the various observed effects, we found the discontinuity of the wavenumber versus pressure curves between 2.4 and 3.2 GPa, both for the region of the external modes as for region of internal modes, particularly remarkable. These effects were interpreted as resulting from a gradual change of the molecular conformation of leucine molecules in the unit cell, culminating with change of hydrogen bonds in the interval where the phase transition is observed. This behaviour suggests that the DL-leucine crystal is more stable than the chiral L-leucine in the sense that while the former presents only one phase transition, the latter presents at least two different transitions in the same pressure range considered. For the L-proline monohydrated crystal, which was subject to pressures up to about 8.0 GPa, several induced effects by this thermodynamic parameter were observed in the Raman spectra, such as the appearance and disappearance of bands, discontinuities in the wavenumber versus pressure curves in the regions of external and internal modes and intensity relative changes. These changes point to the fact that between (i) 1.1 e 1.7 GPa and (ii) 4.5 e 5.0 GPa, this hydrated crystal undergoes phase transitions accompanied by conformational molecular changes in the unit cell.