<p>This thesis is based on experimental studies in chemical physics. Titanium dioxide (TiO₂) and phthalocyanine’s (Pc’s), interesting in many future perspectives, have been deposited as thin films and studied as follows. Information has been obtained on e.g. molecular orientation, crystal structure, depth profile of the chemical composition, electrochemical properties and electronic structure. This has been achieved by means of a combination of techniques: X-ray photoelectron spectroscopy (XPS), near edge x-ray absorption fine structure (NEXAFS), density functional theory calculations (DFT), UV-visible absorption spectroscopy (UVVIS) and cyclic voltammetry (CV).</p><p>Metal-free phthalcyanine (H₂Pc) has been shown to form films with different crystal structure and molecular orientation depending on deposition method, evaporation/sublimation or powder deposition, on commercial conducting glass (fluorine doped tin oxide, FTO), which is used e.g. in solar cells and organic light emitting devices (OLEDs). The unoccupied molecular orbitals are divided in x, y and z space coordinates of the molecule and also divided in inequivalent nitrogen components. </p><p>The electronic structure is also studied for a sublimated titanyl phthalocyanine (TiOPc) film and related to the metal-free phthalocyanine. The ligand field around the titanium atom in TiOPc is compared with that of TiO₂ to delineate the unoccupied levels recorded by means of x-ray absorption spectroscopy.</p><p>Nanostructured TiO₂ films were manufactured by screen printing/doctor blading on FTO. Such films were additionally covered with lutetium diphthalocyanine (LuPc₂) by means of surface assembly from solution. LuPc₂^-, LuPc₂⁺ and LuPc₂H were identified and the stability of the electrochromic reactions in this system was monitored.</p><p>Chemical vapor deposition (CVD) has been used to grow nanometer sized anatase TiO₂ crystals on pre-oxidized Si (111) without formation of interfacial carbon and with an interface layer of the size of 15- 25Å. The interface layer was found to be amorphous TiSi_xO_y with graded stoichiometry. </p>

<p>Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis.</p><p>The compounds TlCo₂S₂, TlCo₂Se₂ and their solid solution TlCo₂Se_2-xS_x, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo₂S₂, the net result is ferromagnetism, while TlCo₂Se₂ exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo₂Se₂ (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0).</p><p>The systems TlCo_2-xMe_xSe₂ show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu₂Se₂, but now with the moments perpendicular to the metal sheets.</p><p>TlCrTe₂ forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr³⁺.</p>

<p>The elastic constants of Mg_(1-x)Al_xB₂ have been calculated in the regime 0<x<0.25. The calculations show that the ratio, B/G, between the bulk- and the shear-modulus stays well below the empirical ductility limit, 1.75, for all concentrations, indicating that the introduction of Al will not change the brittle behaviour of the material considerably. Furthermore, the tetragonal elastic constant C’ has been calculated for the transition metal alloys Fe-Co, Mo-Tc and W-Re, showing that if a suitable tuning of the alloying is made, these materials have a vanishingly low C'. Thermal expansion calculations of the 4d transition metals have also been performed, showing good agreement with experiment with the exception of Nb and Mo. The calculated phonon dispersions of the 4d metals all give reasonable agreement with experiment. First principles calculations of the thermal expansion of hcp Ti have been performed, showing that this element has a negative thermal expansion along the c-axis which is linked to the closeness of the Fermi level to an electronic topological transition. Calculations of the EOS of fcc Au give support to the suggestion that the ruby pressure scale might underestimate pressures with ~10 GPa at pressures ~150 GPa. The high temperature bcc phase of the group IV metals has been calculated with the novel self-consistent ab-initio dynamical (SCAILD) method. The results show good agreement with experiment, and the free energy resolution of < 1 meV suggests that this method might be suitable for calculating free energy differences between different crystallographic phases as a function of temperature.</p>

SiGe nanostructures on crystalline Si substrates with (001) orientation are among the most studied system in condensed matter physics and nanoscience. This interest has been mainly driven by the important potential applications in opto and nano-electronic devices thanks to the improvement of the optical and electronic properties compared to bulk systems. These features come essentially from the possibility of engineering the strain field within the nanostructures using the lattice mismatch of ~ 4 % between Ge and Si and from the spatial confinement, capable of modifying the electronic band structure leading to an increase of the charge carrier mobility. It is obvious that these applications largely depend on the control of surface processes during the growth of the nanostructures, and their performance are strongly dependent on strain relaxation and dislocation injection. Besides the technological interest, the SiGe/Si(001) system has received much attention since it is also a model for understanding the fundamental processes occurring during 3D island formation and self-organization phenomena. In fact, the lattice mismatch between Ge and Si introduces a stress field which has dramatic effects on the growth process and is responsible for a number of structural and electronic phenomena. In particular, the stored elastic energy can be partially relieved by spontaneous formation of 3D objects of nanometric size on top of a pseudomorphic SiGe wetting layer. This growth mode, called Stranski-Krastanov (SK), is a way of easily forming self-assembled nanostructures, which can be used to obtain quantum confinement of charge carriers in nanoelectronics device applications. In recent years, considerable efforts have been devoted to the growth of hetero-epitaxial SiGe nanostructures with well controlled size, shape and positioning, and with defined stoichiometry and strain state. However, some aspects still need to be addressed for a complete understanding of this system, including: (i) the competition between kinetic and thermodynamic factors for island formation, (ii) the mechanisms governing the relative growth of individual nanostructures, (iii) the interplay between SiGe intermixing and strain relaxation mechanisms. In the present work, we carry out an experimental investigation of the relationship between morphology, elemental composition, strain state and electronic structure of self-assembled and lithographically defined SiGe nanostructures by means of several spectro-microscopy techniques. The Si and Ge diffusion dynamics and the self-organization phenomena during the growth of SiGe islands have been studied by Scanning Auger Microscopy (SAM) and Atomic Force Microscopy (AFM). Micro-Raman, SAM and Scanning Transmission Electron Microscopy coupled with Electron Energy Loss Spectroscopy (STEM-EELS) techniques have been used for the investigation of the interplay between strain relaxation mechanisms and SiGe intermixing in self-assembled islands. The effects of strain and composition on the electronic band structure in lithographically defined SiGe nanostructures, in layout very close to those used in prototype devices, have been characterized with nanoscale spatial resolution joining information from Tip Enhanced Raman Spectroscopy (TERS), nanofocused X-Ray Diffraction (XRD) and Energy-Filtered PhotoElectron Emission Microscopy (PEEM). The thesis is conceptually divided in two main parts: the first, to which belong Chapters 1, 2 and 3, deals with the experimental investigation of the Ge surface diffusion and of the self-organization phenomena of SiGe islands grown in a bottom-up approach; the second, including Chapters 4 and 5, is based on the experimental characterization of the strain state and of the strain-induced effects on the electronic band structure of lithographically defined SiGe nanostructures obtained in a top-down approach Chapter 1 presents an overview on the basic processes occurring during hetero-epitaxial growth of thin solid films. In the Chapter 2 the surface diffusion of Ge on a clean and C covered Si(001) surface promoted by annealing at high temperatures in UHV of pure Ge stripes is experimentally investigated by means of in-situ Scanning Auger Microscopy. The influence of a controlled carbon coverage on the Ge surface diffusion is quantitatively studied, showing that the diffusion coefficient presents a strong dependence on carbon coverage (see Fig. 1(a)). Chapter 3 deals with the experimental investigation of the growth process of self-assembled SiGe islands on Si(001) (see Fig. 1(b)). From the size and density evolution exhibited by the nucleated islands, we propose a scenario where the island growth is essentially driven by kinetic factors within a diffusion limited regime. Finally, we investigated the interplay among SiGe intermixing and plastic relaxation, showing that the surface thermal diffusion growth method leads to the formation of coherent islands (dislocation-free), as shown in Fig. 1(c), larger than those attainable by MBE and CVD. Chapter 4 presents the mapping with nanoscale resolution of strain, composition, local work function and valence band structure of lithographically defined SiGe embedded nano-stripes using TERS and Energy-Filtered PEEM (see Fig. 1(d) showing the Ge concentration mapping of the nano-stripes as obtained by PEEM analysis). In Chapter 5 are presented the first results of a direct characterization of the strain state of lithographically defined SiGe nano-ridges using the recently developed nanofocused XRD technique. The work presented in this thesis is the outcome of an experimental PhD research project developed at the Politecnico di Milano (Milano, Italy) in co-tutorship with the École Polytechnique (Paris, France) and the French Atomic Energy Commission (CEA-Saclay, France). SAM and AFM have been performed at Department of Physics of the Politecnico di Milano. Micro-Raman Spectroscopy has been carried out at the Materials Science Department of the Università Milano-Bicocca. PEEM measurements have been realized at CEA and during two standard experimental runs at the TEMPO beamline of SOLEIL Synchrotron (France). TERS and preliminary TEM analysis have been performed at the École Polytechnique, while more extensive TEM and STEM-EELS measurements have been developed at IMM-CNR in Catania. The nano-XRD experiment has been carried out during a standard experimental run at ID13 beamline of the European Synchrotron Radiation Facility (ESRF). The close collaboration with the laboratory L-NESS in Como made available the set of the lithographically-defined investigated samples. The experimental results have been exploited in close collaboration with a theory group at the Materials Science Department of the Università Milano-Bicocca for a deeper insight into the atomic level mechanisms during island growth process.

The Thesis is focused on the properties of self-organized nanostructures. Atomic and electronic properties of different systems have been investigated using methods of electron diffraction, scanning tunneling microscopy and photoelectron spectroscopy. Implementation of the STM technique (including design, construction, and tuning of the UHV experimental set-up) has been done in the framework of present work. This time-consuming work is reported to greater detail in the experimental part of this Thesis. <br><br> The scientific part starts from the study of quantum-size effects in the electronic structure of a two-dimensional Ag film on the supporting substrate Ni(111). Distinct quantum well states in the sp-band of Ag were observed in photoelectron spectra. Analysis of thickness- and angle-dependent photoemission supplies novel information on the properties of the interface. For the first time the Ni(111) relative band gap was indirectly probed in the ground-state through the electronic structure of quantum well states in the adlayer. This is particularly important for Ni where valence electrons are strongly correlated. Comparison of the experiment with calculations performed in the formalism of the extended phase accumulation model gives the substrate gap which is fully consistent with the one obtained by ab-initio LDA calculations. It is, however, in controversy to the band structure of Ni measured directly by photoemission. These results lend credit to the simplest view of photoemission from Ni, assigning early observed contradictions between theory and experiments to electron correlation effects in the final state of photoemission. <br><br> Further, nanosystems of lower dimensionality have been studied. Stepped surfaces W(331) and W(551) were used as one-dimensional model systems and as templates for self-organization of Au nanoclusters. Photon energy dependent photoemission revealed a surface resonance which was never observed before on W(110) which is the base plane of the terrace microsurfaces. The dispersion E(k) of this state measured on stepped W(331) and W(551) with angle-resolved photoelectron spectroscopy is modified by a strong umklapp effect. It appears as two parabolas shifted symmetrically relative to the microsurface normal by half of the Brillouin zone of the step superlattice. The reported results are very important for understanding of the electronic properties of low-dimensional nanostructures. <br><br> It was also established that W(331) and W(551) can serve as templates for self-organization of metallic nanostructures. A combined study of electronic and atomic properties of sub-monolayer amounts of gold deposited on these templates have shown that if the substrate is slightly pre-oxidized and the temperature is elevated, then Au can alloy with the first monolayer of W. As a result, a nanostructure of uniform clusters of a surface alloy is produced all over the steps. Such clusters feature a novel sp-band in the vicinity of the Fermi level, which appears split into constant energy levels due to effects of lateral quantization. <br><br> The last and main part of this work is devoted to large-scale reconstructions on surfaces and nanostructures self-assembled on top. The two-dimensional surface carbide W(110)/C-R(15x3) has been extensively investigated. Photoemission studies of quantum size effects in the electronic structure of this reconstruction, combined with an investigation of its surface geometry, lead to an advanced structural model of the carbide overlayer. <br><br> It was discovered that W(110)/C-R(15x3) can control self-organization of adlayers into nanostructures with extremely different electronic and structural properties. Thus, it was established that at elevated temperature the R(15x3) superstructure controls the self-assembly of sub-monolayer amounts of Au into nm-wide nanostripes. Based on the results of core level photoemission, the R(15x3)-induced surface alloying which takes place between Au and W can be claimed as driving force of self-organization. The observed stripes exhibit a characteristic one-dimensional electronic structure with laterally quantized d-bands. Obviously, these are very important for applications, since dimensions of electronic devices have already stepped into the nm-range, where quantum-size phenomena must undoubtedly be considered. <br><br> Moreover, formation of perfectly uniform molecular clusters of C₆₀ was demonstrated and described in terms of the van der Waals formalism. It is the first experimental observation of two-dimensional fullerene nanoclusters with "magic numbers". Calculations of the cluster potentials using the static approach have revealed characteristic minima in the interaction energy. They are achieved for 4 and 7 molecules per cluster. The obtained "magic numbers" and the corresponding cluster structures are fully consistent with the results of the STM measurements. / Die aktuelle Doktorarbeit ist auf die Eigenschaften von selbst-organisierten Nanostrukturen fokussiert. Die strukturellen und elektronischen Eigenschaften von verschiedenen Systemen wurden mit den Methoden Elektronenbeugung, Rastertunnelmikroskopie und Photoelektronenspektroskopie untersucht. Insbesondere wurde die fuer die Rastertunnelmikroskopie in situ praeparierter Proben eingesetzte Apparatur im Rahmen dieser Arbeit konstruiert und aufgebaut. Einzelheiten hierzu sind im experimentellen Kapitel zu finden. <br><br> Der wissenschftliche Teil beginnt mit Untersuchungen von Quantentrogeffekten in der elektronischen Struktur einer Ag-Schicht auf Ni(111)-Substrat. Charakteristische Quantentrogzustaende im Ag-sp-Band wurden in Photoelektronenspektren beobachtet. Die Analyse von schichtdicken- und winkelabhaengiger Photoemission hat neue und wesentliche Informationen ueber die Eigenschaften des Ag/Ni-Systems geliefert. Insbesondere konnte zum ersten Mal eine relative Bandluecke im Ni-Substrat durch das Verhalten der Quantentrogzustaende indirekt vermessen werden. Das ist fuer Ni besonders wichtig, weil es sich bei Ni um ein stark korreliertes Elektronensystem handelt. Die Ergebnisse wurden mit Rechnungen auf der Basis des erweiterten Phasenmodelles verglichen. Der Vergleich ergibt eine Bandluecke, die sehr gut mit ab-initio-Rechnungen auf Basis der lokalen Elektronendichte-Naehrung uebereinstimmen. Dennoch widersprechen die Daten der Ni-Bandstruktur, die direkt mit Photoemission gemessen wird. Diese Kontroverse zeigt deutlich, dass der Unterschied zwischen Theorie und Experiment Korrelationeffekten im Endzustand der Photoemission zugeordnet werden kann. <br><br> Des weiteren wurden Nanosysteme von noch niedrigerer Dimensionalitaet untersucht. Gestufte Oberflaechen W(331) und W(551) wurden als eindimensionale Modellsysteme fuer die Selbstorganisation von Au-Nanoclustern benutzt. Photonenenergieabhaengige Photoemission hat eine neue Oberflaechen-resonanz aufgedeckt, die auf der Basisebene der Terrassen dieser Systeme auftritt. Die Dispersion E(k) von diesem Zustand, die mit winkelaufgeloester Photoemission vermessen wurde, zeigt deutlich die Einwirkung von Umklapp-Effekten. Diese zeigen sich als zwei Parabeln, die relativ zu der Terrassennormale symmetrisch um die Haelfte der Oberflaechen-Brillouinzone verschoben sind. Die erzielten Ergebnisse sind sehr wichtig fuer das Verstaendnis der elektronischen Eigenschaften von eindimensionalen Nanostrukturen. <br><br> Ausserdem wurde gezeigt, dass W(331) und W(551) als Vorlage fuer selbstorganisierte metallische Nanostrukturen dienen koennen. Eine kombinierte Untersuchung von strukturellen und elektronischen Eigenschaften von unter-monolagen Mengen von Au auf diesen Substraten wurde durchgefuehrt. Es hat sich gezeigt, dass Au mit dem Substrat an der Oberflaeche legieren kann, wenn die Oberflaeche ein wenig oxidiert und die Temperatur erhoert ist. Als Folge formiert sich auf den Stufen eine Nanostruktur von gleichen (aber nicht regelmaessig verteilten) Nanoclustern aus dieser Au-W Legierung. Diese Oberflaechenlegierung bildet ein neuartiges sp-Band in der Naehe der Fermi-Kante. Zudem spaltet dieser neue elektronische Zustand in konstante Energieniveaus auf. Das beobachtete Phaenomen wird als laterale Quantisierung interpretiert. <br><br> Das letzte Kapitel dieser Doktorarbeit bildet auch den Hauptteil. Es handelt von Selbstorganisierungsphaenomenen auf einer Oberflaechenrekonstruktion und den Eigenschaften von so hergestellten Nanostrukturen. Das zweidimensionale Oberflaechen-Karbid W(110)/C-R(15x3) wurde intensiv untersucht. Beobachtete Quantentrogeffekte in der Photoemission in Kombination mit den Ergebnissen der Rastertunnelmikroskopuntersuchungen fuehren zu einem verbesserten Strukturmodell fuer das Oberflaechenkarbid. <br><br> Es wurde auch gezeigt, dass W(110)/C-R(15x3) die Selbstorganisierung von Nanostrukturen mit sehr verschiedenen elektronischen und strukturellen Eigenschaften steuern kann. Es wurde gefunden, dass bei erhoehter Temperatur die R(15x3)-Ueberstruktur die Bildung von Nanostreifen aus unter-monolagiger Au Bedeckung, von denen jede 1 nm breit ist, kontrolliert. Die hergestellten Nanostreifen besitzen eine charakteristische eindimensionale elektronische Struktur mit lateral quantisierten d-Baendern. Basierend auf der Photoemission von Rumpfniveaus wird eine Kohlenstoff-induzierte Oberflaechenlegierung zwischen Au und W als Grund fuer die beobachtete Organisierung vorgeschlagen. Solche Phaenomene sind sehr wichtig fuer Anwendungen, seit die Mikroelektronik in den nm-Massstab eingetreten ist, in welchem mit Quantentrogeffekten zu rechnen ist. <br><br> Zusaetzlich wurde die Bildung von perfekt uniformen molekularen Nanoclustern von C₆₀ auf W(110)/C-R(15x3) demonstriert. Dieses Phaenomen kann im van-der-Waals Formalismus beschrieben werden. Die berichteten Ergebnisse sind eine erstmalige experimentelle Beobachtung von zweidimensionalen Fulleren-Nanoclustern mit "magischen Zahlen". Berechnungen der Clusterpotentiale in der statischen Naeherung im Girifalco-Modell zeigen Minima der Wechselwirkungsenergie fuer Cluster aus 4 und 7 C₆₀-Molekuelen. Diese "magischen Zahlen" sowie die entsprechenden Clusterkonfigurationen sind vollkommen konsistent mit den Ergebnissen des STM-Experiments.

Es ist seit einigen Jahren bekannt, dass Elektronen unter bestimmten Bedingungen in dünne Filme eingeschlossen werden können, selbst wenn diese Filme aus Metall bestehen und auf Metall-Substrat aufgebracht werden. In Photoelektronenspektren zeigen diese Filme charakteristische diskrete Energieniveaus, und es hat sich herausgestellt, dass sie zu großen, technisch nutzbaren Effekten führen können, wie der oszillatorischen magnetischen Kopplung in modernen Festplatten-Leseköpfen. <br><br> In dieser Arbeit wird untersucht, inwieweit die der Quantisierung in zweidimensionalen Filmen zu Grunde liegenden Konzepte auf niedrigere Dimensionalität übertragbar sind. Das bedeutet, dass schrittweise von zweidimensionalen Filmen auf eindimensionale Nanostrukturen übergegangen wird. Diese Nanostrukturen sind zum einen die Terrassen auf atomar gestuften Oberflächen, aber auch Atomketten, die auf diese Terrassen aufgebracht werden, bis hin zu einer vollständigen Bedeckung mit atomar dünnen Nanostreifen. Daneben werden Selbstorganisationseffekte ausgenutzt, um zu perfekt eindimensionalen Atomanordnungen auf Oberflächen zu gelangen. <br><br> Die winkelaufgelöste Photoemission ist als Untersuchungsmethode deshalb so geeignet, weil sie das Verhalten der Elektronen in diesen Nanostrukturen in Abhängigkeit von der Raumrichtung zeigt, und unterscheidet sich darin beispielsweise von der Rastertunnelmikroskopie. Damit ist es möglich, deutliche und manchmal überraschend große Effekte der eindimensionalen Quantisierung bei verschiedenen exemplarischen Systemen zum Teil erstmals nachzuweisen. Die für zweidimensionale Filme wesentliche Rolle von Bandlücken im Substrat wird für Nanostrukturen bestätigt. Hinzu kommt jedoch eine bei zweidimensionalen Filmen nicht vorhandene Ambivalenz zwischen räumlicher Einschränkung der Elektronen in den Nanostrukturen und dem Effekt eines Übergitters aus Nanostrukturen sowie zwischen Effekten des Elektronenverhaltens in der Probe und solchen des Messprozesses. Letztere sind sehr groß und können die Photoemissionsspektren dominieren. <br><br> Abschließend wird der Effekt der verminderten Dimensionalität speziell für die d-Elektronen von Mangan untersucht, die zusätzlich starken Wechselwirkungseffekten unterliegen. Auch hierbei treten überraschende Ergebnisse zu Tage. / It has been known for several years that under certain conditions electrons can be confined within thin layers even if these layers consist of metal and are supported by a metal substrate. In photoelectron spectra, these layers show characteristic discrete energy levels and it has turned out that these lead to large effects like the oscillatory magnetic coupling technically exploited in modern hard disk reading heads. <br><br> The current work asks in how far the concepts underlying quantization in two-dimensional films can be transferred to lower dimensionality. This problem is approached by a stepwise transition from two-dimensional layers to one-dimensional nanostructures. On the one hand, these nanostructures are represented by terraces on atomically stepped surfaces, on the other hand by atom chains which are deposited onto these terraces up to complete coverage by atomically thin nanostripes. Furthermore, self organization effects are used in order to arrive at perfectly one-dimensional atomic arrangements at surfaces. <br><br> Angle-resolved photoemission is particularly suited as method of investigation because is reveals the behavior of the electrons in these nanostructures in dependence of the spacial direction which distinguishes it from, e. g., scanning tunneling microscopy. With this method intense and at times surprisingly large effects of one-dimensional quantization are observed for various exemplary systems, partly for the first time. The essential role of bandgaps in the substrate known from two-dimensional systems is confirmed for nanostructures. In addition, we reveal an ambiguity without precedent in two-dimensional layers between spacial confinement of electrons on the one side and superlattice effects on the other side as well as between effects caused by the sample and by the measurement process. The latter effects are huge and can dominate the photoelectron spectra. <br><br> Finally, the effects of reduced dimensionality are studied in particular for the d electrons of manganese which are additionally affected by strong correlation effects. Surprising results are also obtained here. <br><br>----------------------------<br> Die Links zur jeweiligen Source der im Appendix beigefügten Veröffentlichungen befinden sich auf Seite 83 des Volltextes.

The elastic constants of Mg(1-x)AlxB2 have been calculated in the regime 0&lt;x&lt;0.25. The calculations show that the ratio, B/G, between the bulk- and the shear-modulus stays well below the empirical ductility limit, 1.75, for all concentrations, indicating that the introduction of Al will not change the brittle behaviour of the material considerably. Furthermore, the tetragonal elastic constant C’ has been calculated for the transition metal alloys Fe-Co, Mo-Tc and W-Re, showing that if a suitable tuning of the alloying is made, these materials have a vanishingly low C'. Thermal expansion calculations of the 4d transition metals have also been performed, showing good agreement with experiment with the exception of Nb and Mo. The calculated phonon dispersions of the 4d metals all give reasonable agreement with experiment. First principles calculations of the thermal expansion of hcp Ti have been performed, showing that this element has a negative thermal expansion along the c-axis which is linked to the closeness of the Fermi level to an electronic topological transition. Calculations of the EOS of fcc Au give support to the suggestion that the ruby pressure scale might underestimate pressures with ~10 GPa at pressures ~150 GPa. The high temperature bcc phase of the group IV metals has been calculated with the novel self-consistent ab-initio dynamical (SCAILD) method. The results show good agreement with experiment, and the free energy resolution of &lt; 1 meV suggests that this method might be suitable for calculating free energy differences between different crystallographic phases as a function of temperature.

Lithium ion insertion into electrode materials is commonly used in rechargeable battery technology. The insertion implies changes in both the crystal structure and the electronic structure of the electrode material. Side-reactions may occur on the surface of the electrode, which is exposed to the electrolyte and form a solid electrolyte interface (SEI). The understanding of these processes is of great importance for improving battery performance. The chemical and physical properties of water and alcohols are complicated by the presence of strong hydrogen bonding. Various experimental techniques have been used to study geometrical structures and different models have been proposed to view the details of how these liquids are geometrically organized by hydrogen bonding. However, very little is known about the electronic structure of these liquids, mainly due to the lack of suitable experimental tools. This thesis addresses the electronic structure of liquids and lithium battery materials using resonant inelastic X-ray scattering (RIXS) at high brightness synchrotron radiation sources. The electronic structure of battery electrodes has been probed, before and after lithiation, studying the doping of electrons into the host material. The chemical composition of the SEI on cycled graphite electrodes was determined. The local electronic structure of water, methanol and mixtures of the two have been examined using a special liquid cell. Results from the study of liquid water showed a strong influence on the 3a1 molecular orbital and orbital mixing between molecules upon hydrogen bonding. The study of methanol showed the existence of ring and chain formations in the liquid phase and the dominating structures are formed of 6 and 8 molecules. Upon mixing of the two liquids, a segregation at the molecular level was found and the formation of new structures, which could explain the unexpected low increase of the entropy.

In this thesis I have studied some problems of condensed matter at high pressures and temperatures by means of numerical simulations based on Density Functional Theory (DFT). The stability of MgCO3 and CaCO3 carbonates at the Earth's mantle conditions may play an important role in the global carbon cycle through the subduction of the oceanic crust. By performing ab initio electronic structure calculations, we observed a new high-pressure phase transition within the Pmcn structure of CaCO3. This transformation is characterized by the change of the sp-hybridization state of carbon atom and indicates a change to a new crystal-chemical regime. By performing ab initio Molecular Dynamics simulations we show the new phase to be stable at 250 GPa and 1000K. Thus, the formation of sp3 hybridized bonds in carbonates can explain the stability of MaCO3 and CaCO3 at pressures corresponding to the Earth's lower mantle conditions. We have also calculated phase transition sequence in CaCO3, SrCO3 and BaCO3, and have found that, despite the fact that these carbonates are isostructural and undergo the same type of aragonite to post-aragonite transition, their phase transformation sequences are different at high pressures. The continuous improvement of the high-pressure technique led to the discovery of new composite structures at high pressures and complex phases of many elements in the periodic table have been determined as composite host-guest incommensurate structures. We propose a procedure to accurately describe the structural parameters of an incommensurate phase using ab initio methods by approximating it with a set of analogous commensurate supercells and exploiting the fact that the total energy of the system is a function of structural parameters. By applying this method to the Sc-II phase, we have determined the incommensurate ratio, lattice parameters and Wyckoff positions of Sc-II in excellent agreement with the available experimental data. Moreover, we predict the occurrence of an incommensurate high-pressure phase in Ca from first-principle calculations within this approach. The implementation of DFT in modern electronic structure calculation methods proved to be very successful in predicting the physical properties of a solid at low temperature. One can rigorously describe the thermodynamics of a crystal via the collective excitation of the ionic lattice, and the ab initio calculations give an accurate phonon spectra in the quasi-harmonic approximation. Recently an elegant method to calculate phonon spectra at finite temperature in a self-consistent way by using first principles methods has been developed. Within the framework of self-consistent ab initio lattice dynamics approach (SCAILD) it is possible to reproduce the observed stable phonon spectra of high-temperature bcc phase of Ti, Zr and Hf with a good accuracy. We show that this method gives also a good description of the thermodynamics of hcp and bcc phases of Ti, Zr and Hf at high temperatures, and we provide a procedure for the correct estimation of the hcp to bcc phase transition temperature.

The analysis of nature of bonding in non-classical structures is always an intriguing area of research. Typical examples of such systems are polyhedral boranes that exhibit fascinating cluster bonding where the traditional 2-center-2-electron (2c-2e) bond model fails. This thesis involves the investigation of such polyhedral borane structures and their reactivity by employing both qualitative and quantitative tools of electronic structure theory. There is an intense current interest in the macropolyhedral boranes for their applications pharmaceuticals and materials chemistry. The mno rule had been formulated to account for the electronic requirements for the macropolyhedral structures. Though useful in explaining and designing structures, electron counting rules provide a yes or no answer; not all the molelcules having stipulated number of electrons are equally stable. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns such as closo-closo, closo-nido and nido-nido. One of the major problems in polyhedral boron cage chemistry has been the lack of general synthetic routes for the construction of large cage systems . With this view, we explored the mechanism of the reaction of macropolyhedron B20H16 with MeCN and similar ligands, which provide an understanding of the skeletal rearrangement that occur in macropolyhedral boranes. This can help in the design and synthesis of new macropolyhedral boranes. The early examples of metallaboranes were found to adopt structures which are analogous to that of boranes and carboranes. Hypercloso metallaboranes have closo structure with less number of electrons than required by Wades rule. We have carried out a detailed DFT analysis to explore the structure and electronic relationship of 9-12 vertex closo and hypercloso structures of both borane and metallaboranes. Calculations show that in vertex hypercloso metallaborane needs only n skeleton electron pairs rather than n+1 as suggested by Wade’s rules. Stabilization of supraicosahedral boranes with more than 12 vertices by substituting BH groups by transition metal fragments is also explored with DFT calculations. Calculations show that as the number and the size of the metal atom increases the stability of supraicosahedral and condensed supraicosahedral borane structures also increases. These studies will open up new possibilities for the development of polyhedral clusters of extraordinary size.