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Many technological aspects of everyday life are based on practical applications of the magnetic properties of the materials. Miniaturization is a key technological aspect; electronic circuits and storage devices are nowadays steadily decreasing in size and will eventually reach molecular dimensions. The understanding and predictions of the properties of matter at atomic levels represents one of the great achievements of the last years in science. In the present thesis, the aim is to present a complete study of the electronic structure of selected materials, by means of experimental and theoretical methods. The class of materials which are presented in this thesis, are belonging to the magnetic molecules and intermetallic compounds. The electronic structure of the single molecule system named ferric star molecule has been studied. From the resonant X-ray emission study the trend observed for the FeFe3 star gives a signature for the high-spin structure, or more precisely of strong magnetic systems like FeO or Heusler alloys. For the case of intermetallic alloys and compounds, the Mn 2p core-level presents a visible split structure, which is arising from the exchange interactions between the core-hole and the unpaired 3d electrons. The interpretation of this splitting can be regarded as an evidence of local magnetic moments belonging to the Mn site.

Thermoelectric materials are playing an increasingly significant role in the global effort to develop sustainable energy technologies. Consequently, the demand for materials with greater thermoelectric efficiency has stimulated the development of state-of-the-art interstitially doped skutterudite-based materials. However, since intermetallics are often embrittled by interstitial substitution, optimal skutterudite-based device design, manufacture, and operation require thorough assessment of the fracture toughness of interstitially doped skutterudites. This research determines whether the fracture toughness of skutterudites is sacrificed upon interstitial doping. Both pure and interstitially doped cobalt antimonide skutterudites were synthesized via a solid-state technique in a reducing atmosphere with antimony vapor. Their crystal structures were analyzed by X-ray diffraction, and then sintered by hot uniaxial pressing into dense pellets. The electronic properties of the sintered samples were characterized. Fracture toughness of the pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ samples was evaluated by the Vicker's indentation technique and by loading beam-shaped singe-edge vee-notched bend specimens (SEVNB) in 4-point flexure. The intrinsic crack-tip toughness of both materials was determined by measuring the crack-tip opening displacements (COD's) of radial cracks introduced from Vicker's indentations. The intrinsic crack-tip toughness of both pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ were found to be similar, 0.523 and 0.494 MPa√m, respectively. The fracture toughness of both pure and interstitially doped skutterudites, derived from SEVNB specimens in 4-point flexure were also found to be statistically identical, 0.509 and 0.574 MPa√m , respectively, and are in agreement with the intrinsic crack-tip toughness values. However, the magnitude of the toughness was found to be much lower than previously reported. Moreover, fracture toughness values derived from Vickers's indentations were found to be misleading when compared to the results obtained from fracture toughness tests carried out on the micronotched (SEVNB) specimens loaded in 4-point flexure. / Graduation date: 2012

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The main aim of this thesis was to design and develop new Aluminium based compositionally complex alloys (CCAs) using the high entropy alloy (HEA) concept, and to understand their evolution of microstructures during casting and also after the secondary process which is heat-treatment, and finally to evaluate their subsequent mechanical properties. Prior to the development of alloys, a computational technique ThermoCalc was used which helped in understanding the phase formation in various results. Use of thermodynamic physical parameters for predicting the stability of single-phase fields was done to assess their validity in predicting the compositional regions of the alloys developed. The first alloy developed is Al73.6Mg18Ni1.5Ti1.9Zr1Zn4 in at% (NiTiZrZn) CCA. The microstructure consists of the FCC as a primary phase with ~49% volume fraction along with β-AlMg and intermetallic (IM) phases including Al3Ni, Al3Ti, and Al3Zr. After casting, the microstructure showed some presence of eutectic structures. The Al3Ti, and Al3Zr IM phases seemed to precipitate early which led to less homogenization of Ti and Zr, causing deviation in the amount of these elements in the matrix. Further, the CCA was heat-treated at 375 oC for 24hrs and 48hrs and the evolution of microstructure along with its hardness and phase transformation characterisation was investigated. The second developed alloy was quaternary Al65.65Mg21.39Ag10.02Ni2.94 in at% (AgNi) CCA. In the as-cast state, the main phase (matrix) was FCC with ~64 % volume fraction along with BCC, β-AlMg and Al3Ni IM phases. There was a good level homogenization of all elements in the alloy. They were further heat- treated at 400 oC for 24 hrs and 48 hrs and were studied for any change in microstructure along with its hardness and thermal stability. This CCA had the highest hardness value from all developed CCAs. Lastly, in order to check how Ni affects the microstructure and properties of (AgNi) CCA, a ternary Al67.2Mg22.09Ag10.7 in at% (Ag) CCA was developed. The composition was kept such that it is exactly 97% by excluding the Ni. During the development of this alloy, the cast was cooled in two ways first being the normal cooled just like other CCAs and second being a fast cooling method. Both of these alloys consists of the FCC phase as a primary phase with 72% volume fraction along with BCC and β-AlMg. Both of them were also heat treated at 400 oC for 24 hrs and 48 hrs to evaluate any changes in microstructure and also to assess its hardness and thermal stability.

Neste trabalho investigou-se o comportamento sistemático do campo hiperfino magnético nos compostos intermetálicos Gd(Ni,Pd,Cu)In , Tb(Ni,Pd)In, Dy(Ni,Pd)In e Ho(Ni,Pd)In, com a técnica de Correlação Angular γ-γ Perturbada, nos sítios dos elementos terras raras com o núcleo de prova 140Ce e no sítio do In com o núcleo de prova 111Cd. Os resultados obtidos da dependência com a temperatura do Campo Hiperfino Magnético (Bhf) de cada composto foram primeiramente comparados com a teoria do campo molecular por meio da função de Brillouin, da qual foram extrapolados os valores de saturação de Bhf para cada composto estudado. Os valores extrapolados a zero Kelvin de Bhf (Bhf(T=0)) para os compostos estudados neste trabalho foram usados na comparação com valores de Bhf da literatura para outros compostos contendo o mesmo elemento R (R=Terra Rara). O resultado mostrou uma dependência linear com a temperatura de ordenamento magnético, o mesmo previsto pelo modelo RKKY o que indica que a principal contribuição para Bhf vem da polarização dos elétrons de condução (CEP) via interação de contato de Fermi. O comportamento de Bhf saturado para cada família de compostos de terras raras pesados RNiIn e RPdIn como função da projeção do spin 4f do elemento R mostrou também uma relação linear, com exceção dos resultados para os compostos RNiIn obtidos com 111Cd onde foi observado um leve desvio da linearidade. Foi feita uma investigação dos parâmetros hiperfinos elétricos em função da temperatura para os compostos RPdIn e GdNiIn com o núcleo de prova 111Cd, e os resultados mostram que para o composto GdPdIn pode ocorrer desordem Gd-In a altas temperaturas. / Systematic behavior of magnetic hyperfine field (Bhf) in the intermetallic compounds Gd(Ni,Pd,Cu)In , Tb(Ni,Pd)In, Dy(Ni,Pd)In and Ho(Ni,Pd)In was studied by Perturbed Gamma-Gamma Angular Correlation spectroscopy. The measurements of Bhf were carried out at the rare earth atom and In sites using the nuclear probes 140Ce and 111In respectively. The variation of hyperfine field with temperature, in most cases, follows the Brillouin function predicted from the molecular field theory. The hyperfine field values at rare earth atom sites obtained from 140Ce probe as well as at In sites obtained from 111In probe for each series of compounds were extrapolated to zero Kelvin Bhf(T=0) from these curves. These values were compared with the values of the literature for other compounds containing the same rare earth element and all of them show a linear relationship with the ordering temperature. This indicates that the main contribution to Bhf comes from the conduction electron polarization (CEP) through Fermi contact interaction and the principal mechanism of magnetic interaction in these compounds can be described by the RKKY type interaction. The values of Bhf(T=0) for each family of intermetallic compounds RNiIn and RPdIn when plotted as a function of 4f spin projection of rare earth element also shows a linear relationship. Exceptions are the results for the compounds RNiIn obtained with 111Cd probe where a small deviation from linearity is observed. The results of the measurements carried out with the 111Cd probe were also analyzed to obtain the hyperfine parameters of the quadrupole interaction as a function of temperature for RPdIn and GdNiIn compounds. The results show that for the compound GdPdIn there might be some Gd-In disorder at high temperature.

Neste trabalho investigou-se o comportamento sistemático do campo hiperfino magnético nos compostos intermetálicos Gd(Ni,Pd,Cu)In , Tb(Ni,Pd)In, Dy(Ni,Pd)In e Ho(Ni,Pd)In, com a técnica de Correlação Angular γ-γ Perturbada, nos sítios dos elementos terras raras com o núcleo de prova 140Ce e no sítio do In com o núcleo de prova 111Cd. Os resultados obtidos da dependência com a temperatura do Campo Hiperfino Magnético (Bhf) de cada composto foram primeiramente comparados com a teoria do campo molecular por meio da função de Brillouin, da qual foram extrapolados os valores de saturação de Bhf para cada composto estudado. Os valores extrapolados a zero Kelvin de Bhf (Bhf(T=0)) para os compostos estudados neste trabalho foram usados na comparação com valores de Bhf da literatura para outros compostos contendo o mesmo elemento R (R=Terra Rara). O resultado mostrou uma dependência linear com a temperatura de ordenamento magnético, o mesmo previsto pelo modelo RKKY o que indica que a principal contribuição para Bhf vem da polarização dos elétrons de condução (CEP) via interação de contato de Fermi. O comportamento de Bhf saturado para cada família de compostos de terras raras pesados RNiIn e RPdIn como função da projeção do spin 4f do elemento R mostrou também uma relação linear, com exceção dos resultados para os compostos RNiIn obtidos com 111Cd onde foi observado um leve desvio da linearidade. Foi feita uma investigação dos parâmetros hiperfinos elétricos em função da temperatura para os compostos RPdIn e GdNiIn com o núcleo de prova 111Cd, e os resultados mostram que para o composto GdPdIn pode ocorrer desordem Gd-In a altas temperaturas. / Systematic behavior of magnetic hyperfine field (Bhf) in the intermetallic compounds Gd(Ni,Pd,Cu)In , Tb(Ni,Pd)In, Dy(Ni,Pd)In and Ho(Ni,Pd)In was studied by Perturbed Gamma-Gamma Angular Correlation spectroscopy. The measurements of Bhf were carried out at the rare earth atom and In sites using the nuclear probes 140Ce and 111In respectively. The variation of hyperfine field with temperature, in most cases, follows the Brillouin function predicted from the molecular field theory. The hyperfine field values at rare earth atom sites obtained from 140Ce probe as well as at In sites obtained from 111In probe for each series of compounds were extrapolated to zero Kelvin Bhf(T=0) from these curves. These values were compared with the values of the literature for other compounds containing the same rare earth element and all of them show a linear relationship with the ordering temperature. This indicates that the main contribution to Bhf comes from the conduction electron polarization (CEP) through Fermi contact interaction and the principal mechanism of magnetic interaction in these compounds can be described by the RKKY type interaction. The values of Bhf(T=0) for each family of intermetallic compounds RNiIn and RPdIn when plotted as a function of 4f spin projection of rare earth element also shows a linear relationship. Exceptions are the results for the compounds RNiIn obtained with 111Cd probe where a small deviation from linearity is observed. The results of the measurements carried out with the 111Cd probe were also analyzed to obtain the hyperfine parameters of the quadrupole interaction as a function of temperature for RPdIn and GdNiIn compounds. The results show that for the compound GdPdIn there might be some Gd-In disorder at high temperature.

The main focus of this work is the preparation, chemical and structural characterization along with the investigation of physical properties of intermetallic clathrates. Starting from the history of clathrate research, classification of clathrate types, their structural properties and possible application areas are evaluated in chapter 2. The methodologies of sample preparation and materials characterization as well as quantum chemical calculations are discussed in chapter 3. The complete characterization of Ba8Ge433 ( is a Schottky-symbol standing for vacancies),12-14 which is a parent compound for the variety of ternary variants, is the subject of chapter 4. Ba8Ge433 is a high temperature phase,12 which was prepared for the first time as single phase bulk material in this work.15, 16 In this way, the intrinsic transport properties could be investigated without influence of grain boundary and impurity effects. The transport behavior is analyzed at low and high temperatures and referred to the former results. In addition, crystal structure and vacancy ordering in terms of the reaction conditions are discussed. Chemical bonding in Ba8Ge433 is investigated by topological analysis of the electron localizability indicator and the electron density. Chapter 5 deals with the preparation, phase analysis, crystal structure and physical properties of BaGe5, which constitutes a new clathrate type oP60.17, 18 So far, two clathrate types were known in the binary system Ba – Ge, namely the clathrate cP124 Ba6Ge25,19-21 and the clathrate-I Ba8Ge433. Originally, BaGe5 was detected by optical and scanning electron microscopy within the grains of Ba8Ge433.12 Once the preparation of phase-pure Ba8Ge433 was achieved, it became possible to make detailed investigations of its decomposition along with the formation of BaGe5. A detailed theoretical and experimental analysis on the relation between crystal structure and physical properties of BaGe5 is presented. In chapter 6, a thorough structural characterization and the physical properties of clathrates in the system Ba – Ni – Ge is presented based on the subtle relation between the crystal structure containing vacancies and the thermoelectric properties. During the investigations in this system, a large single crystal was grown by Nguyen et al. 22, 23 from the melt with the composition Ba8Ni3.5Ge42.10.4. A systematic reinvestigation of the phase relations in this system was performed and the influence of different Ni content to the crystal structure and physical properties is evaluated. The Si-based ternary clathrate with composition Ba8–δNixySi46–x–y is the subject of chapter 7. The phase relations and the homogeneity range are established. The crystal structure taking into account vacancies in the framework is discussed. Physical properties of bulk pieces are analyzed and the results are related to the sample composition. In addition, first-principles electronic structure calculations are carried out to assess variations in the electronic band structure, phase stability and chemical bonding.24 Chapter 8 reports on the intermetallic compound Ba3Si4,25, 26 which was encountered during the investigations on the Ba – Ni – Si phase diagram. The discussion covers issues related to preparation, crystal structure, phase diagram analysis, electrical and magnetic properties, NMR measurements, quantum mechanical calculations and oxidation to nanoporous silicon with gaseous HCl. Besides my contributions to the NoE CMA, I studied under the Priority Program 1178 of Deutsche Forschungsgemeinschaft “Experimental electron density as the key for understanding chemical interactions” with the project of “Charge distribution changes by external electric fields: investigations of bond selective redistributions of valence electron densities”. Chapter 9 deals with the preparation of chalcopyrites ZnSiP2 and CuAlS2 for experimental charge density analysis. Both phases show semiconducting properties and have non-centrosymmetric structures with high space group symmetry as needed to investigate the structural changes induced by external electric field. In this chapter, I describe the preparation and the crystal structure analyses of ZnSiP2 and CuAlS2 including issues related to the data collection as well as the results of NMR investigation.

Gamma-TiAl is an ordered intermetallic compound characterized by high strength to density ratio, good oxidation resistance, and good creep properties at elevated temperatures. However, it is intrinsically brittle at room temperature. This thesis investigates the potential for the use of grinding to process TiAl into useful shapes. Grinding is far from completely understood, and many aspects of the individual mechanical interactions of the abrasive grit with the material and their effect on surface integrity are unknown. The development of new synthetic diamond superabrasives in which shape and size can be controlled raises the question of the influence of those variables on the surface integrity. The goal of this work is to better understand the fundamentals of the abrasive grit/material interaction in grinding operations. Experimental, analytical, and numerical work was done to characterize and predict the resultant deformation and surface integrity on ground lamellar gamma-TiAl. Grinding tests were carried out, by analyzing the effects of grit size and shape, workpiece speed, wheel depth of cut, and wear on the subsurface plastic deformation depth (PDD). A practical method to assess the PDD is introduced based on the measurement of the lateral material flow by 3D non-contact surface profilometry. This method combines the quantitative capabilities of the microhardness measurement with the sensitivity of Nomarski microscopy. The scope and limitations of this technique are analyzed. Mechanical properties were obtained by quasi-static and split Hopkinson bar compression tests. Residual stress plots were obtained by x-ray, and surface roughness and cracking were evaluated. The abrasive grit/material interaction was accounted by modeling the force per abrasive grit for different grinding conditions, and studying its correlation to the PDD. Numerical models of this interaction were used to analyze boundary conditions, and abrasive size effects on the PDD. An explicit 2D triple planar slip crystal plasticity model of single point scratching was used to analyze the effects of lamellae orientation, material anisotropy, and grain boundaries on the deformation.