The research presented in this thesis concerns experimental studies of thin manganese silicide and germanide layers, grown by solid phase epitaxy on the Si(111)7×7 and the Ge(111)c(2×8) surfaces, respectively. The atomic and electronic structures, as well as growth modes of the epitaxial Mn-Si and Mn-Ge layers, were investigated by low-energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), core-level spectroscopy (CLS), and scanning tunneling microscopy and spectroscopy (STM and STS). The magnetic properties of the Mn-Ge films were investigated by X-ray magnetic circular dichroism (XMCD). The Mn-Si layers, annealed at 400 °C, showed a √3×√3 LEED pattern, consistent with the formation of the stoichiometric monosilicide MnSi. Up to 4 monolayers (ML) of Mn coverage, island formation was observed. For higher Mn coverages, uniform film growth was found. Our results concerning morphology and the atomic and electronic structure of the Mn/Si(111)-√3×√3 surface, are in good agreement with a recent theoretical model for a layered MnSi structure and the √3×√3 surface structure. Similar to the Mn-Si case, the grown Mn-Ge films, annealed at 330 °C and 450 °C, showed a √3×√3 LEED pattern. This indicated the formation of the ordered Mn5Ge3 germanide. A strong tendency to island formation was observed for the Mn5Ge3 films, and a Mn coverage of about 32 ML was needed to obtain a continuous film. Our STM and CLS results are in good agreement with the established model for the bulk Mn5Ge3 germanide, with a surface termination of Mn atoms arranged in a honeycomb pattern. Mn-Ge films grown at a lower annealing temperature, 260 °C, showed a continuous film at lower coverages, with a film structure that is different compared to the structure of the Mn5Ge3 film. XMCD studies showed that the low-temperature films are ferromagnetic for 16 ML Mn coverage and above, with a Curie temperature of ~250 K.

Les propriétés électroniques et thermoélectriques de matériaux basés sur Mg2Si ont été étudiées par calculs DFT et semi-classiques (théorie de Boltzmann). Les effets d’abaissement de dimensionalité et de contraintes ont été étudiés. Les calculs ont été effectués sur les films monocristallins orientés 001, 110 et 111 et sur les films polycristallins. Seul le film monocristallin orienté 110 a montré des propriétés thermoélectriques intéressantes. Trois types de contraintes ont été investiguées: uniaxiale, biaxiale et isotrope. L’augmentation de la contrainte sur Mg2Si produit un décalage du maximum du facteur de puissance (PF) vers les basses températures. Comparé à Mg2Si non contraint, le coefficient Seebeck (S) augmente uniquement sous contrainte isotrope. On montre l’équivalence des propriétés thermoélectriques entre Mg2Si contraint dans la direction [110] et celles du film orienté 110. Les contraintes de tension isotropes ont été modélisées en insérant des atomes Sb dans Mg2Si massif conduisant aux structures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Seul Mg2Si:4Sb produit une contrainte isotrope. Les effets de substitutions de Sn pour Si dans Mg2Si massif sont similaires à ceux observés pour Mg2Si sujet à des contraintes en tension uniaxiales et biaxiales. Pour les films Mg2Si1−xSnx orientés 110 le S du matériau dopé p est supérieur à celui des massifs Mg2Si et Mg2Si1−xSnx. Concernant les nanostructures, le super-réseau Mg2Si/Mg2Sn est le plus intéressant lorsque faiblement dopé p et à basse température. Les assemblages de fils sont les meilleurs en tant que matériaux faiblement dopés n et à basse température: le PF est quasiment doublé par rapport à celui de Mg2Si massif. / The electronic and thermoelectric properties of Mg2Si-based materials have been investigated by means of DFT calculations and semi-classical Boltzmann theory. The low-dimensional and strain effects on these properties have been studied. The properties have been investigated on 001-, 110- and 111-oriented Mg2Si monocrystalline films, and on polycrystalline Mg2Si film. Only the 110-oriented monocrystalline film has been found to have interesting thermoelectric properties. Three types of strains have been investigated: uniaxial, biaxial and isotropic. Increasing the intensity of the strain on Mg2Si induces a shift of the power factor (PF) maximum towards low temperature. Compared with unstrained Mg2Si, the Seebeck coefficient (S) increases only under isotropic strain. We evidence an equivalence in the thermoelectric properties between Mg2Si material constrained in the [110] direction and the 110-oriented Mg2Si film. Isotropic tensile strains have been modeled by inserting Sb atoms in bulk Mg2Si leading to the stuctures Mg2Si:Sb, Mg2Si:3Sb and Mg2Si:4Sb. Only Mg2Si:4Sb is found to induces such type of constraints. The effects of the Sn for Si substitutions in bulk Mg2Si are very similar to those observed for Mg2Si subjected to uniaxial and biaxial tensile strains. For (110)-oriented Mg2Si1−xSnx films S of the n−doped material outperforms that of the bulk Mg2Si and bulk Mg2Si1−xSnx. Regarding nanostructures, the Mg2Si/Mg2Sn superlattice is most interesting as a p-doped material at low carrier concentration/low temperature. The stick assemblage is best as a n-doping material at low carrier concentration/low temperature where its PF is almost twice as high as that of bulk Mg2Si.

Silizide spielen ein wesentliche Rolle in den technologisch fortschrittlichsten CMOS Bauteilen. Sie finden Verwendung als Kontaktmaterial auf den Aktivgebieten und dem Silizium Gatter von Transistoren. Diese Arbeit beschäftigt sich mit den Systemen: Co-Si, Co-Ni-Si und Ni-Si. Sowohl in situ Hochtemperatur-SR-XRD Experimente als auch CBED wurden zur Phasenidentifikation herangezogen. AES erlaubte es, Elementverteilungen in Schichtstapeln zu bestimmen. Für Studien über Agglomerationserscheinungen wurde REM eingesetzt. TEM und analytisches TEM trugen nicht nur zu Einblicken in Schichtstrukturen und Kornformen bei, sondern lieferten auch Daten zu Elementverteilungen in Silizidschichten. Diese Dissertation gliedert sich in zwei Hauptteile. Der erste Teil beschäftigt sich mit den Phasenbildungsabfolgen und den Phasenbildungs- und Umwandlungstemperaturen in nanoskaligen dünnen Schichten. Als Trägermaterial wurden einkristalline und polykristalline Siliziumsubstrate verwendet. Der Einfluß verschiedener Dotierungen im Vergleich zu undotierten Substraten sowie die Beeinflussung der Silizidierung durch eine Deckschicht wurden untersucht. Im zweiten Teil waren Größeneffekte verschiedener Schichtdicken und Agglomerationserscheinungen Gegenstand von Untersuchungen. Unterschiede bei der Silizidierung in Zusammenhang mit unterschiedlichen Schichtdicken wurden bestimmt. Darüberhinaus wurde eine ternäre CoTiSi Phase gefunden und identifiziert. Außerdem konnte die stark eingeschränkte Mischbarkeit der Monosilizide CoSi und NiSi gezeigt werden. Der thermische Ausdehnungskoeffizient von NiSi im Temperaturbereich 400?700°C und sein nicht-lineares Verhalten wurden bestimmt. / Silicides are an essential part of state-of-the-art CMOS devices. They are used as contact material on the active regions as well as on the Si gate of a transistor. In this work, investigations were performed in the systems Co-Si, Co-Ni-Si, and Ni-Si. In situ high temperature SR-XRD and CBED techniques were used for phase identification. AES enabled the determination of elemental concentrations in layer stacks. SEM was applied to agglomeration studies. TEM imaging and analytical TEM provided insights into layer structures, grain morphology as well as information about the distribution of chemical elements within silicide layers. This thesis is divided into two main parts. The first part deals with the phase formation sequences and the phase formation and conversion temperatures in nanoscale thin films on either single crystal or polycrystalline Si substrates. The effect of different types of dopants vs. no doping and the impact of a capping layer on the phase formation and conversion temperatures were studied. In the second part, size effects and agglomeration of thin silicide films were investigated. The effect of different layer thicknesses on the silicidation process was studied. Additionally, the degree of agglomeration of silicide films was calculated. Furthermore, the ternary CoTiSi phase was found and identified as well as the severely limited miscibility of the monosilicides CoSi and NiSi could be shown. The CTE of NiSi between 400?700 ±C and its non-linear behavior was determined.

Die Dissertation beschreibt Untersuchungen der intermetallischen Verbindungen TbxY1-xNi2B2C bzw. TbxEr1-xNi2B2C, Tb2PdSi3 sowie Dy2PdSi3, die sich auszeichnen durch physikalische Erscheinungen, die durch die Wechselwirkung der magnetischen Momente der Seltenerd (SE)-Ionen mit den Leitungselektronen hervorgerufen werden. Sie umfasst Beiträge zur Konstitution der Legierungssysteme, methodische Untersuchungen zur Einkristallzüchtung, zur Charakterisierung der Homogenität und Perfektion der Kristalle und zu physikalischen Eigenschaften (Supraleitung, Magnetismus). Die für das Erstarrungs- und Schmelzverhalten der intermetallischen Phasen relevanten quasi-binären Schnitte der Tb-(Y, Er)-Ni-B-C bzw. Tb(Dy)-Pd-Si-Phasendiagramme, die bisher noch nicht bekannt waren, konnten bestimmt werden. Damit wurden die Prozessparameter der Einkristallzüchtung durch tiegelfreies Zonenschmelzen optimiert. Die unterschiedlichen Erstarrungstypen, peritektische Erstarrung (SENi2B2C) bzw. kongruentes Erstarren (SE2PdSi3), wurden durch unterschiedliche Ziehgeschwindigkeiten bei der Einkristallzüchtung berücksichtigt. An den massiven Einkristallen wurden Homogenitätsuntersuchungen hinsichtlich Elementkonzentration und physikalischer Eigenschaften (Tc, Tn, RRR) als Funktion der Längskoordinate durchgeführt, die geringe Eigenschaftsunterschiede der einkristallinen Proben nachweisen. Aus der Zusammensetzungsänderung über die Kristalllänge konnte in Verbindung mit in-situ Messungen der Zonentemperatur ein einfaches Prozessmodell des Zonenschmelzens der untersuchten Verbindungen entwickelt werden. Die Korrelation von magnetischen und supraleitenden Eigenschaften der Borokarbidmischreihen werden primär vom Verhältnis der Seltenen Erden bestimmt aber auch durch die Konzentration der weiteren Elemente. An Einkristallen konnte eine Anisotropie des oberen kritischen Feldes der Supraleitung von TbxY1-xNi2B2C gezeigt werden, die durch die magnetischen Tb-Ionen bestimmt wird. Die Untersuchungen der Tb2PdSi3- und Dy2PdSi3-Einkristalle ergaben eine Anisotropie der magnetoelektrischen Eigenschaften. / In this thesis investigations of the intermetallic compounds TbxY1-xNi2B2C, TbxEr1-xNi2B2C, Tb2PdSi3 and Dy2PdSi3 are presented. These compounds exhibit interesting physical phenomena caused by the interaction of the rare earth (RE) magnetic moments on the conduction electrons. Moreover, contributions on the constitution of the alloy systems, basic investigations of crystal growth process, homogeneity, microstructure and physical properties (superconductivity, magnetism) of the crystals are given. The quasi-binary sections of the Tb-(Y, Er)-Ni-B-C and Tb(Dy)-Pd-Si-phase diagrams which are relevant for the crystallisation of the different intermetallic phases have been determined for the first time. They were utilised for optimisation of the process parameters of single crystal growth by floating-zone melting. Because of the different solidification modes of RENi2B2C (peritectic solidification) and RE2PdSi3 (congruent solidification) different growth velocities have been employed in crystal growth. The composition and the physical properties (Tc, Tn, RRR) have been investigated as function of the crystal axis co-ordinate. As these properties show only a slight shift over the crystal length samples are representative for the whole crystal. From the slight composition shift over the crystal length a process model of the floating zone growth has been developed utilising the in-situ measurements of the zone-temperature. The correlation of magnetic and superconducting properties of the borocarbide solid solution compounds are mainly governed by the RE-fraction but they are also influenced by the concentration of the other elements. For TbxY1-xNi2B2C single crystals an anisotropic upper critical field of superconductivity has been detected which is induced by the magnetic Tb-ions. The investigations of Tb2PdSi3 and Dy2PdSi3 revealed an anisotropy of the magnetoresistive properties.

The present thesis reports on the transmission electron microscopy structure characterization of semiconducting thin films of higher manganese silicides (HMS or MnSi1.7) grown on (001)Si by different UHV deposition methods (the template method, reactive deposition and surfactant mediated reactive deposition). In this work electron diffraction technique was applied for the fist time to reveal the HMS phase growing in thin MnSi1.7 films. The obtained results suggest the presence of the shortest in c-axis length HMS phase, namely Mn4Si7, within our experiments. It has been shown that growth of epitaxial Mn4Si7 grains can be achieved by the template technique. In particular, the influence of the template thickness on the silicide layer quality has been investigated. It has been found that deposition of a thin Mn layer of 0.8 nm nominal thickness at room temperature prior to the Mn/Si codeposition at 550°C causes the formation of a silicide template that leads to the preferred epitaxial Mn4Si7 growth with (110)[4-41]Mn4Si7 || (001)[110]Si. Silicide crystallites of two additional orientation relations, (3-38)[-443]Mn4Si7 || (001)[110]Si and (001)[110] Mn4Si7 || (001)[110]Si, were present at the same template thickness to a lesser extent. Due to the crystal symmetry of Mn4Si7 and Si the epitaxial Mn4Si7 growth on (001)Si leads to the formation of a number of Mn4Si7 domains for each observed orientation. Additional experiments were carried out using the reactive deposition process. It has been shown that the deposition of Mn onto (001)Si at substrate temperatures higher then 600°C leads to the formation of large silicide islands growing with the major part of their elongated grains parallel to <110>Si. XRD investigations show the observed silicide grains to exhibit the following texture: (110)Mn4Si7 || (001)Si. The found island morphology of Mn4Si7 films can be modified by the deposition of about one monolayer of Sb (surfactant) onto (001)Si prior to the Mn-deposition. This process results in an increase of the silicide island density by about two orders of magnitude and decrease of the silicide grain dimensions to nanometer range. Furthermore, in the presence of Sb the silicide layers grow with the preferential orientation: (100)[010]Mn4Si7 || (001)[100]Si. The observed changes in the morphology and orientation of the Mn4Si7 layers can be explained by the reduced diffusion of Mn and Si atoms in the presence of the Sb overlayer. / In der vorliegenden Arbeit wird die Struktur von dünnen MnSi1.7-Schichten, die mit verschiedenen UHV-Herstellungsmethoden (template-Verfahren, reaktive Abscheidung und surfactant gesteuerte Abscheidung) auf (001)Si hergestellt wurden, mittels Elektronenmikroskopie charakterisiert. Die Ergebnisse der Elektronenbeugung an dünnen Mangansilicid-Schichten können vollständig interpretiert werden, wenn von den bekannten höheren Mangansiliciden (HMS) das Mn4Si7 als einzige vorliegende Phase angenommen wird. Der Hauptteil der Arbeit beschäftigt sich mit den mittels template-Verfahren abgeschiedenen Mn4Si7-Schichten. In diesen Experimenten wurde der Einfluss der template-Dicke auf die Morphologie und Orientierung der hergestellten Schichten untersucht. Es wird gezeigt, dass bei der Abscheidung von einer dünnen Mn-Schicht mit einer nominalen Dicke von 0,8 nm bei Raumtemperatur und weiterer Mn/Si-Koabscheidung bei einer Substrattemperatur von 550°C nahezu geschlossene Silicidschichten mit der bevorzugten Orientierungsbeziehung (110)[4-41]Mn4Si7 || (001)[110]Si entstehen. Weiterhin wachsen bei dieser template-Dicke Mn4Si7-Kristallite mit den Orientierungsbeziehungen: (3-38)[-443]Mn4Si7 || (001)[110]Si und (001)[110] Mn4Si7 || (001)[110]Si. Bei jeder gefundenen Orientierungsbeziehung treten beim Wachstum von Mn4Si7 auf (001)Si mehrere Domäne auf. Zusätzliche Experimente wurden unter Verwendung der reaktiven Schichtabscheidung durchgeführt. Sie verdeutlichen, dass bei reaktiver Abscheidung von Mn auf (001)Si ab einer Substrattemperatur von 600°C ein Wachstum von Mn4Si7-Inseln entlang den [110]-Richtungen des Siliciums erfolgt. XRD-Untersuchungen zeigen, dass diese Inseln die folgende Textur haben: (110)Mn4Si7 || (001)Si. Durch eine Modifizierung der Si-Oberfläche mit einer bis zu einer Monolage dicken Sb-Schicht (surfactant) kann das Mn4Si7-Inselwachstum beeinflusst werden. Die dabei gefundene Erhöhung der Mn4Si7-Inseldichte wird hier auf die reduzierte Mn- und Si-Diffusion zurükgeführt. Weiterhin wurde gefunden, dass dieser Abscheidungsprozess Mn4Si7-Kristallite der bevorzugten Orientierung (100)[010]Mn4Si7 || (001)[110]Si liefert.

The main focus of the present work was the Ge-rich part of the binary Ba – Ge system, in which by inspecting the behavior of the clathrate-I Ba8Ge43 under pressure, several new phases were found. The new phases in this system have the following compositions: BaGe3 (with two modifications), BaGe5, BaGe5.5 and BaGe6, therefore they are quite close in composition range: 75% - ~85% at. Ge. Concerning the conditions required for the synthesis of each phase, several combinations of temperature and pressure were employed in order to find a stability range. It was possible to establish such a formation range for all phases. In some cases two phases were found for a given conditions and in many other cases three or more phases were found to coexist. Thus, the stability range of pressure and temperature for single phase formation turned out to be very narrow. By inspecting of some structural features, for instance the interatomic distances, it is found that the average of the Ge – Ge distances change in line with the composition, i.e. the shorter contacts belong to BaGe6 while the longer distances are present in BaGe3 (both modification). An opposite trend is observed for the calculated density of each phase (neglecting the tI32 form of BaGe3): the lower density is found for BaGe3 and the denser compound is found to be BaGe6. Of course this is not coincidence, since due to the Ge content, BaGe6 has the largest molar mass. Similarly, by examining the density as a function of the interatomic distance. In such case, the denser compound is characterized by shorter Ge – Ge contacts, while the less dense phase holds the longest Ge – Ge contacts. This is in agreement with the building motifs within each crystal structure: columns in BaGe3 (open framework) passing through layers in BaGe5, ending in a three-dimensional network (closed framework) in BaGe6.

Metal silicon systems have a wide range of applications, ranging from the use in electronic industry, as superconductors, protective coatings and as high temperature structural materials. Mo- and Nb-based silicides have emerged as suitable high temperature materials and extensive studies are being conducted make it suitable for various applications. Because of very good strength to density ratio, Nb-based silicides have attracted maximum attention. This is basically a mixture of Nb solid solution and Nb5Si3 intermetallic compound. A very small amount of NbCr2 Laves phase could also be present because of Cr addition. Incorporation of other alloying elements, which are mainly partitioned to these phases, helps to achieve a property balance like, high temperature strength, high fracture toughness, high creep and oxidation resistance. The knowledge on diffusion parameters is useful to understand many physical and mechanical properties. In this thesis, diffusion couple technique is used in different temperature ranges to study the growth kinetics and diffusion of the phases in an interdiffusion zone in binary silicides, Nb/Si, Mo/Si and V/Si, binary solid solutions, Nb/Mo, Nb/Ti, Nb/Zr and ternary silicides, Nb-Mo/Si, Nb-Ti/Si, Nb-Zr/Si. The parabolic growth constant, the integrated diffusion coefficients and the tracer diffusion coefficients are calculated from the experimental results obtained in this study and also from the results already available in the literature on the binary silicides. The activation energy for growth kinetics and the diffusion coefficients are also calculated to gain knowledge on the diffusion mechanism. The atomic mechanism of the diffusing species in all the phases of Nb and Mo silicide are discussed with the help of crystal structure and possible defects present. Also, a detailed analysis is done on the growth mechanism of the phases in Nb/Si and Mo/Si systems. In the Nb/Si system, Si is found to have higher diffusion rate in both the NbSi2 and Nb5Si3 phases. The number of nearest neighbour Si bonds is higher than nearest neighbour Nb bonds and hence one may predict high concentration of Nb antisites to be present in the NbSi2 phase. The growth mechanism analysis following the physico chemical approach explains the absence of the Kirkendall plane in the Nb5Si3 phase and duplex morphology in the NbSi2 phase in the Nb/Si couple. In the Mo/Si system, Si diffusion is faster than Mo in all the three phases. In the MoSi2 phase, Mo is practically immobile due to the absence of vacancies on the Mo sublattice. Similar defect structure is expected in the Mo5Si3 and Mo3Si phases also with additional Si antisite defects to assist Si diffusion. The growth mechanism analysis explains the absence of the Kirkendall plane in the Mo5Si3 and Mo3Si phases and continuous columnar grains in the MoSi2 phase in the Mo/Si couple. In the V/Si system, the activation energy for integrated diffusion coefficient of the VSi2 phase is found to be reasonably lower than the other phases which could happen because of very high concentration of defects, and/or because of contribution from the grain boundary diffusion as it shows the presence of columnar grains. Problems associated with the analysis done in literature are also discussed. A diffusion study is performed in different temperature ranges for the three binary metallic solid solution systems to determine the interdiffusion coefficients over the entire composition range using the relation developed by Wagner. The change in activation energy for interdiffusion with composition is also determined. It is found that activation energy for interdiffusion in Nb/Mo system is much higher than that for Nb/Ti and Nb/Zr system. Further the impurity diffusion coefficients of the species are determined and compared with the available data in literature. It is found that the activation energy for the impurity diffusion of Nb in Ti, Zr and Mo is higher than that of Ti, Zr and Mo in Nb. Interdiffusion study is done in the ternary silicides with the aim to examine the role of alloying additions, such as, Ti, Mo and Zr on the growth kinetics and diffusion behaviour of the phases in the Nb/Si system. The average interdiffusion (or integrated) coefficients are calculated when possible. The reaction and dissociation of the species at the interfaces are considered to understand the growth mechanism of the phases. An attempt is made to understand the change in diffusion mechanism because of the presence of third element. It is found that none of the alloying elements participate in the diffusion process although they do alter the growth kinetics and diffusion rate in both the phases, NbSi2 and Nb5Si3. It is also found that Nb becomes immobile in the NbSi2 phase in the presence of the alloying elements. Mo reduces the growth of both the phases while Ti addition does not cause any change in the growth but affects the diffusivity. Zr addition also reduces growth of the Nb5Si3 phase. It however complicates the interdiffusion zone in the Nb(Zr)/Si couple, which limits to qualitative study only. The Growth and consumption rate of the end members become very significant in many practical applications. Hence, relations for the growth and consumption rate in systems with finite end member thickness is developed considering single and double phase layer in the interdiffusion zone. Two different methodologies are used, the diffusion based and the physico-chemical approach to develop the same relations. We have shown that the diffusion based approach is rather straightforward; however, the physico-chemical approach is much more versatile than the other method. It is found that the position of the marker plane becomes vague in the second stage of the interdiffusion process in such a system, where two phases grow simultaneously.

<p>Materials with new or improved properties are crucial for technological development. To provide the foundation for future successful products, it is important to prepare and characterise new chemical compounds that could show unusual properties. The properties of magnetic materials are closely related to their crystal, magnetic and electronic structures. This thesis focuses on the novel synthesis and structural characterisation of a number of new ternary or <i>pseudo</i>-ternary silicides and germanides of manganese with iridium, cobalt or palladium. To provide a more complete picture of the complex magnetic properties, crystal and magnetic structure refinements by the Rietveld method of X-ray and neutron powder diffraction data are complemented by single-crystal X-ray diffraction, electron diffraction, magnetisation measurements and Reverse Monte Carlo simulations of magnetic short-range order. The experimental results are corroborated by first-principles electronic structure and total energy calculations. </p><p>A commensurate non-collinear antiferromagnetic structure is found for most compounds of the solid solution Mn₃Ir_1-yCo_ySi_1-xGe_x. The non-collinearity is a result of geometric frustration in a crystal structure with magnetic Mn atoms located on a three-dimensional network of triangles. The close structural similarity to the β-modification of elemental manganese, which does not order magnetically, inspired a closer theoretical comparison of the Mn₃Ir_1-yCo_ySi_1-xGe_x propertieswith β-Mn.</p><p>Magnetic frustration is also observed for Mn₄Ir_7-xMn_xGe₆, and is an important factor underlying the dramatic change from commensurate antiferromagnetic order to spin glass properties induced by a small variation in Mn concentration. Magnetic short-range order with dominant antiferromagnetic correlation is observed for Mn₈Pd₁₅Si₇, and results from a random distribution of Mn atoms in-between the geometrically frustrated magnetic moments on the Mn octahedra. </p><p>An incommensurate cycloidal magnetic structure, observed for IrMnSi, is stabilised by an electronic structure effect, which also accounts for the non-collinearity of the Mn₃IrSi type magnetic structure.</p>

In dieser Dissertation wird die Darstellung tetrelreicher Seltenerdmetall–Verbindungen in den Systemen SE:Tt (SE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) und die Charakterisierung ihrer Eigenschaften beschrieben. Diese Verbindungen, welche mittels der Hochdruck–Hochtemperatur–Methode dargestellt wurden, zeigen neuartige Verknüpfungsmuster in der Tetrel–Partialstruktur. Neben der Charakterisierung der Verbindungen hinsichtlich der thermischen Stabilität und der physikalischen Eigenschaften bei Normaldruck wurde bei den Germanium–reichen Seltenerdmetall–Verbindungen eine Untersuchung der Veränderungen der Kristallstruktur bei Variation des Drucks oder der Temperatur vorgenommen. Die dargestellten Seltenerdmetall–Trisilicide SESi3 (SE = Gd, Ho, Lu) kristallisieren tetragonal isotyp zu YbSi3. LuSi3 zeigt Supraleitung mit Tc = 7.0 K. In den Systemen Gd–Si und SE–Ge (SE = La, Nd, Sm, Gd, Tb) wurden die Verbindungen GdSi5 und SEGe5 (SE = La, Nd, Sm, Gd, Tb) synthetisiert. Sie kristallisieren orthorhombisch isotyp zu LaGe5. Durch in–situ Röntgenbeugungsexperimente bei erhöhten Temperaturen kann die Existenz metastabiler Germanium–ärmerer Verbindungen SE2Ge9 (SE = Nd, Sm) nachgewiesen werden. Es handelt sich um Defektvarianten der Verbindungen SEGe5 (SE = Nd, Sm). Die strukturelle Verwandtschaft zum Aristotyp SEGe5 wird anhand einer Gruppe–Untergruppe–Beziehung aufgezeigt. Eine alternative Synthesemethode zur Darstellung tetrelreicher Verbindungen mit Gerüststrukturen bei extremen Reaktionsbedingungen stellt das Spark–Plasma–Sinterverfahren (SPS) dar. Die Darstellung der Clathratphase Ba8Ga16±xGe30∓x (x = 0, 1) erfolgte mittels SPS aus den Precursoren BaGa2±x (x = 0, 0.125) und Germanium. Die Untersuchungen der thermoelektrischen Eigenschaften zeigen, dass durch Variation der nominellen Zusammensetzung sowohl n–leitende als auch p–leitende Eigenschaften erhalten werden können. / In this thesis the synthesis of tetrel–rich rare–earth metal compounds in the systems RE:Tt (RE = La, Nd, Sm, Gd, Tb, Ho, Lu; Tt = Si, Ge) and the characterization of their properties is described. These compounds, synthesized by means of high–pressure high–temperature method, show new structural motifs in the tetrel partial structure. The compounds were characterized with respect to their thermal stability and their physical properties at ambient pressure. In addition, the changes in the crystal structure of the germanium-rich rare–earth metal compounds by variation of pressure or temperature were investigated. The synthesized rare–earth trisilicides SESi3 (SE = Gd, Ho, Lu) crystallize tetragonal, isotypic to YbSi3. LuSi3 is a superconductor with Tc = 7.0 K. In the systems Gd–Si and SE–Ge (SE = La, Nd, Sm, Gd, Tb) the compounds GdSi5 and SEGe5 (SE = La, Nd, Sm, Gd, Tb) were synthesized. They crystallize orthorhombic isotypic to LaGe5. Using in-situ high–temperature X-ray experiments the metastable germanium-poorer compounds SE2Ge9 (SE = Nd, Sm) could be observed. These compounds are defect variants of the pentagermanides SEGe5 (SE = Nd, Sm). The structural relationship to the aristotype SEGe5 is given via a group-subgroup relation. An alternative synthesis route for the preparation of tetrel–rich compounds with framework structures at extreme reaction conditions is the spark plasma sintering method (SPS). The clathrate phase Ba8Ga16±xGe30∓x (x = 0, 1) was synthesized from the precursors BaGa2±x (x = 0, 0.125) and germanium by means of SPS. The investigation of the thermoelectric properties shows, that through variation of the nominal composition both n-type and p-type conduction properties can be obtained.

Materials with new or improved properties are crucial for technological development. To provide the foundation for future successful products, it is important to prepare and characterise new chemical compounds that could show unusual properties. The properties of magnetic materials are closely related to their crystal, magnetic and electronic structures. This thesis focuses on the novel synthesis and structural characterisation of a number of new ternary or pseudo-ternary silicides and germanides of manganese with iridium, cobalt or palladium. To provide a more complete picture of the complex magnetic properties, crystal and magnetic structure refinements by the Rietveld method of X-ray and neutron powder diffraction data are complemented by single-crystal X-ray diffraction, electron diffraction, magnetisation measurements and Reverse Monte Carlo simulations of magnetic short-range order. The experimental results are corroborated by first-principles electronic structure and total energy calculations. A commensurate non-collinear antiferromagnetic structure is found for most compounds of the solid solution Mn3Ir1-yCoySi1-xGex. The non-collinearity is a result of geometric frustration in a crystal structure with magnetic Mn atoms located on a three-dimensional network of triangles. The close structural similarity to the β-modification of elemental manganese, which does not order magnetically, inspired a closer theoretical comparison of the Mn3Ir1-yCoySi1-xGex propertieswith β-Mn. Magnetic frustration is also observed for Mn4Ir7-xMnxGe6, and is an important factor underlying the dramatic change from commensurate antiferromagnetic order to spin glass properties induced by a small variation in Mn concentration. Magnetic short-range order with dominant antiferromagnetic correlation is observed for Mn8Pd15Si7, and results from a random distribution of Mn atoms in-between the geometrically frustrated magnetic moments on the Mn octahedra. An incommensurate cycloidal magnetic structure, observed for IrMnSi, is stabilised by an electronic structure effect, which also accounts for the non-collinearity of the Mn3IrSi type magnetic structure.