No description available.

No description available.

Thesis (Ph. D.)--Colorado State University, 2004. / Includes bibliographical references.

Intermetallic compounds are solid phases containing two or more metallic elements, whose crystal structure differs from that of its constituents [1]. The largest group among these compounds with more than 1400 binary and ternary representatives are the so-called Laves phases. The classification of an intermetallic compound as a Laves phase is solely based on the atomic configuration and the component ratio in the crystal structure. With the ideal composition AB2, the Laves phases crystallize in three closely related structure types which are named after their representatives, MgCu2 (C15, cubic), MgZn2 (C14, hexagonal) and MgNi2 (C36, hexagonal). Laves phases are built by almost all metals of the periodic system of the elements. A significant feature of many of these is the formation of broad homogeneity ranges by mutual substitution of atoms in combination with composition or temperature dependent phase transformations between the different Laves phase polytypes. Laves phases have received considerable attention in recent years as potential structural and functional materials. They combine high melting points with considerable creep resistance, high strength and fracture toughness and good corrosion and oxidation resistance. Some Laves phases like NbFe2 [2] or TaFe2 [3] show intriguing magnetic and electronic properties which provide a deeper inside into phenomenons like quantum criticality. Especially transition-metal based Laves phases like NbCr2 [4] and ZrCr2 [5] are promising candidates for the development of new high-temperature structural materials. The major drawback of the Laves phases, however, is their low-temperature brittleness. Many experimental and theoretical investigations have shown, that the low-temperature ductility can be improved by controlling the crystal structure with the help of phase transformations, by mechanical twinning or the addition of third elements. The addition of ternary alloying elements can alter the physical and electronic properties of the Laves phases and plays an important role in the composition dependent stability of the different polytypes. Some ternary Laves phases show an interesting phenomenon called site occupation reversal. It describes a composition dependent behavior of the alloying elements which prefer to occupy different crystallographic sites at different concentrations. The understanding of the point defect structure/mechanism and the site occupation of the alloying elements is thus of critical importance for the proper description of phase stability. The basis for the broad application of any metallic material is the knowledge of the corresponding phase diagram. The experimental determination of phase diagrams however, is tedious, time consuming and expensive work and the huge abundance of Laves phase makes this an impractical task. Thus, the time it takes to discover new advanced materials and to move them from the laboratory to the commercial market place is fairly long today. A cheap and fast enhancement for the development of new materials is the calculation of phase diagrams and physical properties using techniques like CALPHAD (CALculation of PHAse Diagrams) and DFT (Density Functional Theory). Very recently the Office of Science and Technology Policy of the United States White House announced to provide a budget of $100 million to launch the Materials Genome Initiative [6, 7]. The aim of this initiative is to provide the infrastructure and training needed to discover, develop, manufacture, and deploy advanced materials in a more expeditious and economical way [8]. One of the project’s three supporting legs is the calculation and prediction of crystal structures and physical properties using advanced Computational tools. "An early benchmark will be the ability to incorporate improved predictive modeling algorithms of materials behavior into existing product design tools. For example, the crystal structure and physical properties of the materials [. . . ]." [8]. Their computational tools of choice are the same as used in this work to predict crystal structures and site occupation factors. Contents of this work is the investigation of the substitutional disorder in binary and ternary Laves phases. This includes the experimental determination of the composition dependent stability of the Laves phase polytypes and the distribution of the substitution atoms in the crystal lattice of the respective phases, i.e., the site occupation factors (s.o.f.). For this purpose, detailed experimental studies on the two systems Cr–Co–Nb and Fe–Ta–V were performed and the Laves phase polytypes, their homogeneity ranges, the lattice parameters and the site occupation factors were determined. The experimental results are compared with the results obtained from quantum mechanical calculations. DFT is used to determine the composition dependent enthalpies of formation which serve as a measure for the stability of the different Laves phase polytypes. Additionally, the applicability of various approximations and their influence on the results has been checked. This study is thus also supposed to develop and improve the tools necessary for the calculation of phase stability and homogeneity ranges in ternary phases. Chapter two in the first part of this work describes the crystal structures of the Laves phases in detail with focus on the polytype stability, the site occupation and the c/a-ratio of hexagonal C14 Laves phases. Subsequently, the phase diagrams of the investigated systems and the occurring Laves phases are discussed. Chapter three briefly describes the experimental and theoretical methods used in this work. The last section of part one gives a detailed explanation of how the phase stability, the lattice parameters and the site occupation factors are calculated. The second part "Results and discussion" contains the discussion of the experimental and theoretical results for the intensively investigated systems Co–Cr–Nb (chapter five) and Fe–Ta–V (chapter six). Several other ternary C14 Laves phases and their site occupation behavior are studied in chapter seven. The thesis is concluded with a summary in chapter eight. Several additional information is contained in the appendix.

Intermetallic compounds are solid phases containing two or more metallic elements, whose crystal structure differs from that of its constituents [1]. The largest group among these compounds with more than 1400 binary and ternary representatives are the so-called Laves phases. The classification of an intermetallic compound as a Laves phase is solely based on the atomic configuration and the component ratio in the crystal structure. With the ideal composition AB2, the Laves phases crystallize in three closely related structure types which are named after their representatives, MgCu2 (C15, cubic), MgZn2 (C14, hexagonal) and MgNi2 (C36, hexagonal). Laves phases are built by almost all metals of the periodic system of the elements. A significant feature of many of these is the formation of broad homogeneity ranges by mutual substitution of atoms in combination with composition or temperature dependent phase transformations between the different Laves phase polytypes. Laves phases have received considerable attention in recent years as potential structural and functional materials. They combine high melting points with considerable creep resistance, high strength and fracture toughness and good corrosion and oxidation resistance. Some Laves phases like NbFe2 [2] or TaFe2 [3] show intriguing magnetic and electronic properties which provide a deeper inside into phenomenons like quantum criticality. Especially transition-metal based Laves phases like NbCr2 [4] and ZrCr2 [5] are promising candidates for the development of new high-temperature structural materials. The major drawback of the Laves phases, however, is their low-temperature brittleness. Many experimental and theoretical investigations have shown, that the low-temperature ductility can be improved by controlling the crystal structure with the help of phase transformations, by mechanical twinning or the addition of third elements. The addition of ternary alloying elements can alter the physical and electronic properties of the Laves phases and plays an important role in the composition dependent stability of the different polytypes. Some ternary Laves phases show an interesting phenomenon called site occupation reversal. It describes a composition dependent behavior of the alloying elements which prefer to occupy different crystallographic sites at different concentrations. The understanding of the point defect structure/mechanism and the site occupation of the alloying elements is thus of critical importance for the proper description of phase stability. The basis for the broad application of any metallic material is the knowledge of the corresponding phase diagram. The experimental determination of phase diagrams however, is tedious, time consuming and expensive work and the huge abundance of Laves phase makes this an impractical task. Thus, the time it takes to discover new advanced materials and to move them from the laboratory to the commercial market place is fairly long today. A cheap and fast enhancement for the development of new materials is the calculation of phase diagrams and physical properties using techniques like CALPHAD (CALculation of PHAse Diagrams) and DFT (Density Functional Theory). Very recently the Office of Science and Technology Policy of the United States White House announced to provide a budget of $100 million to launch the Materials Genome Initiative [6, 7]. The aim of this initiative is to provide the infrastructure and training needed to discover, develop, manufacture, and deploy advanced materials in a more expeditious and economical way [8]. One of the project’s three supporting legs is the calculation and prediction of crystal structures and physical properties using advanced Computational tools. "An early benchmark will be the ability to incorporate improved predictive modeling algorithms of materials behavior into existing product design tools. For example, the crystal structure and physical properties of the materials [. . . ]." [8]. Their computational tools of choice are the same as used in this work to predict crystal structures and site occupation factors. Contents of this work is the investigation of the substitutional disorder in binary and ternary Laves phases. This includes the experimental determination of the composition dependent stability of the Laves phase polytypes and the distribution of the substitution atoms in the crystal lattice of the respective phases, i.e., the site occupation factors (s.o.f.). For this purpose, detailed experimental studies on the two systems Cr–Co–Nb and Fe–Ta–V were performed and the Laves phase polytypes, their homogeneity ranges, the lattice parameters and the site occupation factors were determined. The experimental results are compared with the results obtained from quantum mechanical calculations. DFT is used to determine the composition dependent enthalpies of formation which serve as a measure for the stability of the different Laves phase polytypes. Additionally, the applicability of various approximations and their influence on the results has been checked. This study is thus also supposed to develop and improve the tools necessary for the calculation of phase stability and homogeneity ranges in ternary phases. Chapter two in the first part of this work describes the crystal structures of the Laves phases in detail with focus on the polytype stability, the site occupation and the c/a-ratio of hexagonal C14 Laves phases. Subsequently, the phase diagrams of the investigated systems and the occurring Laves phases are discussed. Chapter three briefly describes the experimental and theoretical methods used in this work. The last section of part one gives a detailed explanation of how the phase stability, the lattice parameters and the site occupation factors are calculated. The second part "Results and discussion" contains the discussion of the experimental and theoretical results for the intensively investigated systems Co–Cr–Nb (chapter five) and Fe–Ta–V (chapter six). Several other ternary C14 Laves phases and their site occupation behavior are studied in chapter seven. The thesis is concluded with a summary in chapter eight. Several additional information is contained in the appendix.

Ce mémoire de thèse concerne l’étude des composés ternaires Terre rare–magnésium–nickel utilisable pour le stockage de l’hydrogène. Ces composés ont été obtenus par fusion ou par mécanosynthèse. Les intermétalliques YNi4-xAlxMg, dérivant des phases de Laves de structure cubique ont été étudiés. Ils réagissent de manière réversible avec l’hydrogène à P et T ambiantes. Le comportement structural lors d’une hydruration a été étudié par DRX in situ. Le composé conserve sa symétrie cubique mais avec diminution de la cristallinité. Cette étude est complétée par l’étude de composés : (i) riche en terre rare (e.g. Gd4NiMg) qui absorbe l’hydrogène à température ambiante de manière irréversible. La structure de l’intermétallique et de l’hydrure sont déterminées. La décomposition de l’hydrure à température supérieure à 90°C est expliquée. (ii) riche en magnésium. Nous avons pu identifié un nouveau composé de formulation proche de Mg77Gd9Ni14.5 de structure CFC. / This work deals with the study of ternary compounds Rare Earth – magnesium - nickel used ofr hydrogen storage. All the compounds are prepared by fusion and by mechanical alloying method.. The compounds YNi4-xAlxMg, derived from the cubic Laves phases have been studied in the first part. It reacts reversibly towards hydrogen at atmospheric pressure and room temperature. The structural behaviour during the hydrogen sorption has been studied by in situ XRD. The compound remains cubic with a decrease of the crystallinity. This study is completed by the study of compounds : (i) rich in rare earth (e.g. Gd4NiMg) that absorb hydrogen at room temperature but irreversibly. Structures of both the intermetallic and the hydride have been determined. The decomposition of the hydride at temperature higher than 90°C is also explained. (ii) rich in magnesium. A new compounds with a formulation closed to Mg77Gd9Ni14.5 has been identified and it crystallized with a cubic faces centred structure.

O objeto desta pesquisa consiste em investigar as propriedades estruturais, magnéticas e hiperfinas dos compostos pseudobinários de fases de Laves: Hf(Fe(1-x)Cr(x))2 e (Nb(1-x)Zr(x))Fe2. Para o Hf(Fe(1-x)Cr(x))2, preparamos as amostras como ligas policristalinas e as fundimos por síntese nas concentrações: x = 0,0; 0,1; 0,2; 0,3; 0,4; 0,5; 0,6; 0,7; 0,8; 0,9 e 1,0. O mesmo foi feito para as amostras de (Nb(1-x)Zr(x))Fe2 nas concentrações x = 0,0; 0,1; 0,2; 0,3; 0,4 e 0,5. Todas as amostras fundiram-se num forno de fusão de arco, sob atmosfera de argônio ultrapuro (99.999%). Em seguida investigamos a estrutura cristalina das ligas pelo método do pó, com a técnica de difratometria de Raios X (XRD), obtendo-se os parâmetros de rede e confirmando-se a estrutura de fase hexagonal C14 para as amostras Hf(Fe(1-x)Cr(x))2 nas concentrações 0,0 <= x < 0,9, bem como em todas as outras amostras de (Nb(1-x)Zr(x))Fe2. Depois, determinamos as propriedades magnéticas das ligas Hf(Fe(1-x)Cr(x))2 pela técnica de magnetização a baixas temperaturas em baixos campos magnéticos aplicados de 0 a 7 T e em altos campos magnéticos aplicados de 0 a 16 T. As suscetibilidades AC e DC a baixos campos magnéticos com temperaturas de 4,2 K a 300 K, FC e ZFC, nos indicaram que as ligas de concentrações 0,4 <= x < 0,8 apresentam comportamento \'vidro de spin\', sendo que em x <= 0,3 são aglomerados magnéticos com interação de curto alcance e em x = 0,9 é um superparamagnético. Por tanto, os valores dos momentos magnéticos por átomo de Fe foram calculados para todas as amostras. As medidas de spectroscopia Mössbauer das mesmas amostras de Hf(Fe(1-x)Cr(x))2, na temperatura ambiente, apresentam dois sextetos para a mostras com x = 0,2 e dois dubletos quadrupolares para as demais composições, atribuídos aos sítios cristalográficos 2a e 6h do Fe. Por outra parte, os espectros Mössbauer das amostras de (Nb(1-x)Zr(x))Fe2 à temperatura de 4,2 K, sem campo magnético aplicado e com campo magnético aplicado de 6 e 12 T, sugerem que estes compostos se encontram em um balanço em que coexistem as fases ferromagnéticas e antiferromagnéticas. Finalmente, notamos que no composto (Nb0.6Zr0.4)Fe2 há, uma possível existência de comportamento paramagnético nos Fe do sítio cristalino 2a e, ao mesmo tempo, pouca certeza que o valor do momento magnético seja nulo neste sítio cristalino. / The object of this research consists of investigating the structural, magnetic and hiperfine properties of the pseudobinar Laves phases compounds Hf(Fe(1-x)Cr(x))2 and (Nb(1-x)Zr(x))Fe2. We prepared polycristaline samples alloys and for synthesis melting in the concentrations: x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 e 1.0, for the Hf(Fe(1-x)Cr(x))2 and in the concentrations: x = 0.0, 0.1, 0.2, 0.3, 0.4 e 0.5 for (Nb(1-x)Zr(x))Fe2. We melted them in an arc furnace under pure Argon (99.999%) gas atmosphere. We investigated the cristaline structure of the alloys by the powder XRD technique, obtaining lattice parameters and confirming the structure of hexagonal phase C14 for the samples Hf(Fe(1-x)Cr(x))2 in the concentrations 0.0 <= x <= 0.9 and also in all the other samples produced of (Nb(1-x)Zr(x))Fe2. We investigate the magnetic properties of Hf(Fe(1-x)Cr(x))2 alloys bye the technique of magnetization at low temperatures and low magnetic field applied until 7 T and high magnetic field applied until 16 T. The susceptibility AC and DC at low magnetic fields and temperatures of 4.2 K until 300 indicated that alloys of concentrations 0.4 <= x < 0.8 show spin glass behavior, in x <= 0.3 they are magnetic clusters with short range interactions, and in x = 0.9 is superparamagnetic. The values of the magnetic moments for atom of Fe were calculated for all samples. We measured Mössbauer spectra of the same samples of Hf(Fe(1-x)Cr(x))2 at room temperature, obtaning two sextets for the samples with x < 0.2 and two quadrupolar doublets for the other compositions, that would be attributed to the cristalographic sites 2a and 6h. Also the Mössbauer spectrum of the samples (Nb(1-x)Zr(x))Fe2 at temperature of 4.2 K without magnetic field applied and with magnetic field applied of 6 and 12 T, suggest that those compounds show coexisting ferromagnetic and antiferromagnetic phases. We could note for the compound (Nb0.6Zr0.4)Fe2 a possible existence of the paramagnetic behavior in the Fe of the cristalographic site 2a, but the magnetic moment in this site is not zero.

O objeto desta pesquisa consiste em investigar as propriedades estruturais, magnéticas e hiperfinas dos compostos pseudobinários de fases de Laves: Hf(Fe(1-x)Cr(x))2 e (Nb(1-x)Zr(x))Fe2. Para o Hf(Fe(1-x)Cr(x))2, preparamos as amostras como ligas policristalinas e as fundimos por síntese nas concentrações: x = 0,0; 0,1; 0,2; 0,3; 0,4; 0,5; 0,6; 0,7; 0,8; 0,9 e 1,0. O mesmo foi feito para as amostras de (Nb(1-x)Zr(x))Fe2 nas concentrações x = 0,0; 0,1; 0,2; 0,3; 0,4 e 0,5. Todas as amostras fundiram-se num forno de fusão de arco, sob atmosfera de argônio ultrapuro (99.999%). Em seguida investigamos a estrutura cristalina das ligas pelo método do pó, com a técnica de difratometria de Raios X (XRD), obtendo-se os parâmetros de rede e confirmando-se a estrutura de fase hexagonal C14 para as amostras Hf(Fe(1-x)Cr(x))2 nas concentrações 0,0 <= x < 0,9, bem como em todas as outras amostras de (Nb(1-x)Zr(x))Fe2. Depois, determinamos as propriedades magnéticas das ligas Hf(Fe(1-x)Cr(x))2 pela técnica de magnetização a baixas temperaturas em baixos campos magnéticos aplicados de 0 a 7 T e em altos campos magnéticos aplicados de 0 a 16 T. As suscetibilidades AC e DC a baixos campos magnéticos com temperaturas de 4,2 K a 300 K, FC e ZFC, nos indicaram que as ligas de concentrações 0,4 <= x < 0,8 apresentam comportamento \'vidro de spin\', sendo que em x <= 0,3 são aglomerados magnéticos com interação de curto alcance e em x = 0,9 é um superparamagnético. Por tanto, os valores dos momentos magnéticos por átomo de Fe foram calculados para todas as amostras. As medidas de spectroscopia Mössbauer das mesmas amostras de Hf(Fe(1-x)Cr(x))2, na temperatura ambiente, apresentam dois sextetos para a mostras com x = 0,2 e dois dubletos quadrupolares para as demais composições, atribuídos aos sítios cristalográficos 2a e 6h do Fe. Por outra parte, os espectros Mössbauer das amostras de (Nb(1-x)Zr(x))Fe2 à temperatura de 4,2 K, sem campo magnético aplicado e com campo magnético aplicado de 6 e 12 T, sugerem que estes compostos se encontram em um balanço em que coexistem as fases ferromagnéticas e antiferromagnéticas. Finalmente, notamos que no composto (Nb0.6Zr0.4)Fe2 há, uma possível existência de comportamento paramagnético nos Fe do sítio cristalino 2a e, ao mesmo tempo, pouca certeza que o valor do momento magnético seja nulo neste sítio cristalino. / The object of this research consists of investigating the structural, magnetic and hiperfine properties of the pseudobinar Laves phases compounds Hf(Fe(1-x)Cr(x))2 and (Nb(1-x)Zr(x))Fe2. We prepared polycristaline samples alloys and for synthesis melting in the concentrations: x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 e 1.0, for the Hf(Fe(1-x)Cr(x))2 and in the concentrations: x = 0.0, 0.1, 0.2, 0.3, 0.4 e 0.5 for (Nb(1-x)Zr(x))Fe2. We melted them in an arc furnace under pure Argon (99.999%) gas atmosphere. We investigated the cristaline structure of the alloys by the powder XRD technique, obtaining lattice parameters and confirming the structure of hexagonal phase C14 for the samples Hf(Fe(1-x)Cr(x))2 in the concentrations 0.0 <= x <= 0.9 and also in all the other samples produced of (Nb(1-x)Zr(x))Fe2. We investigate the magnetic properties of Hf(Fe(1-x)Cr(x))2 alloys bye the technique of magnetization at low temperatures and low magnetic field applied until 7 T and high magnetic field applied until 16 T. The susceptibility AC and DC at low magnetic fields and temperatures of 4.2 K until 300 indicated that alloys of concentrations 0.4 <= x < 0.8 show spin glass behavior, in x <= 0.3 they are magnetic clusters with short range interactions, and in x = 0.9 is superparamagnetic. The values of the magnetic moments for atom of Fe were calculated for all samples. We measured Mössbauer spectra of the same samples of Hf(Fe(1-x)Cr(x))2 at room temperature, obtaning two sextets for the samples with x < 0.2 and two quadrupolar doublets for the other compositions, that would be attributed to the cristalographic sites 2a and 6h. Also the Mössbauer spectrum of the samples (Nb(1-x)Zr(x))Fe2 at temperature of 4.2 K without magnetic field applied and with magnetic field applied of 6 and 12 T, suggest that those compounds show coexisting ferromagnetic and antiferromagnetic phases. We could note for the compound (Nb0.6Zr0.4)Fe2 a possible existence of the paramagnetic behavior in the Fe of the cristalographic site 2a, but the magnetic moment in this site is not zero.

Sc(AlxNi1-x)2 is a pseudobinary C14 Laves phase and a potential interstitial hydrogen storage material or anode in a Ni-MH battery. A previous study showed that Sc1Al1Ni1 can store hydrogen reversibly; both interstitially and trough decomposition into ScH2 and AlNi. It is also known that the exact composition is very important for the hydrogen storage properties of pseudobinary Laves phases. This thesis work is aimed at synthesising Sc(AlxNi1-x)2 and study the effect of the Ni/Al ratio on the hydrogen absorption/desorption process as well as the interstitial storage capacity. Compositions with high nickel content had the highest capacity (at least 0.67wt% for ScAl0.66Ni1.34) and ones with high aluminium content had the lowest total storage capacity (0wt% for ScAl1.28Ni0.62). The former composition was also shown to absorb and desorb hydrogen during multiple cycles. Desorption of interstitial hydrogen from ScAl0.66Ni1.34 requires 4.6kJ/mol in activation energy.

Laves-Phasen sind intermetallische Verbindungen der Zusammensetzung AB2, die in den Strukturtypen C14 (MgZn2), C15 (MgCu2), C36 (MgNi2) oder deren Abkömmlingen kristallisieren. Diese sind Polytypen mit einem gemeinsamen grundlegenden Strukturmuster. Insgesamt sind über 1400 binäre und ternäre Laves-Phasen bekannt. Sie stellen damit die größte Gruppe der bislang bekannten intermetallischen Verbindungen dar. Laves-Phasen wurden intensiv untersucht um grundlegende Aspekte der Phasenstabilität zu verstehen. Geometrische und elektronische Faktoren haben sich in ihrer Vorhersagekraft bezüglich des Auftretens und der Stabilität einer Laves-Phase aber nur in wenigen Fällen als hilfreich erwiesen. Das Auftreten von Homogenitätsbereichen und damit einhergehender struktureller Defekte ist in den meisten Fällen immer noch unklar und spiegelt grundsätzliche Probleme in der Chemie intermetallischer Verbindungen wider: Das unvollständige Bild der chemischen Bindung, die Tendenz zur Bildung ausgedehnter Homogenitätsbereiche sowie der Einfluss von Minoritätskomponenten auf Struktur und Phasenstabilität ist bei intermetallischen Verbindungen größer als bei vielen anderen Verbindungsklassen. Daher sind die Informationen über Struktur, Stabiblität und physikalische Eigenschaften intermetallischer Verbindungen im Allgemeinen unvollständig und mitunter unzuverlässig oder widersprüchlich. Um diese Probleme anzugehen wurden in dieser Arbeit Laves-Phasen in den Systemen Nb--TM (TM = Cr, Mn, Fe, Co) und Nb--Cr--TM (TM = Co, Ni) als Modellsysteme ausgewählt. Das Ziel der Untersuchung ist, das Wechselspiel zwischen chemischer Bindung, Struktur und Phasenstabilität für die Laves-Phasen auf der Grundlage genauer experimenteller Daten sowie quantenmechanischer Rechnungen zu beleuchten. Die Untersuchungen des binären Systems Nb--Co nehmen hier eine Schlüsselposition ein. Eine Neubestimmung des Phasendiagramms des Systems Nb--Co im Bereich der Laves-Phasen bestätigt die Existenz von Phasen mit C14-, C15- und C36-Struktur. Dabei wurden schmale Zweiphasenfelder C15 + C36 und C15 + C14 sowie ein schmaler, aber signifikanter Homogenitätsbereich der C36-Phase experimentell nachgewiesen. Die Kristallstrukturen von C36-Nb(1-x)Co(2+x) (x = 0,265), C15-Nb(1-x)Co(2+x) (x = 0,12), C15-NbCo2 und C14-Nb(1+x)Co(2-x) (x = 0,07) wurden mittels Einkristall-Röntgenstrukturanalyse verfeinert. Im Falle von C36-Nb(1-x)Co(2+x) (x = 0,265) und C15-Nb(1-x)Co(2+x) (x = 0,12) wird bestätigt, dass der Homogenitätsbereich durch Substitution von Nb durch Co erzeugt wird. Im Fall von C14-Nb(1+x)Co(2-x) werden Abweichungen von der Zusammensetzung NbCo2 durch Substitution von Co durch überschüssiges Nb erzeugt, wobei nur eine der beiden Co-Lagen gemischt besetzt wird. Quantenmechanische Rechnungen zeigen, dass dieses Besetzungsmuster energetisch bevorzugt ist. Weder mittels Röntgenbeugung noch mittels hochauflösender Elektronenmikroskopie und Elektronenbeugeng wurden Ordnungsvarianten oder Stapelvarianten der Laves-Phasen beobachtet. In der Kristallstruktur von C36-Nb(1-x)Co(2+x) (x = 0,265) ist mehr als ein Viertel des Nb durch überschüssiges Co ersetzt. Von zwei kristallographischen Nb-Lagen wird eine bevorzugt von Co besetzt, so dass sich der Co-Anteil der beiden Lagen etwa wie 2:1 verhält. Co-Antistrukturatome sind relativ zu der Nb-Position verschoben. Triebkraft dieser Verschiebungen ist die Bildung von Nb--Co-Kontakten innerhalb der A-Teilstruktur. Gemischte Besetzung der Nb-Lagen, die Verteilung der Co-Antistrukturatome und mit der Substitution einhergehende Verzerrungen führen zu einer komplizierten Realstruktur. Zur Beschreibung der elektronischen Struktur von C36-Nb(1-x)Co(2+x) (x = 0,265) werden daher verschiedene Modelle verwendet, die Tendenzen sowohl zur beobachteten Mischbesetzung als auch zur Verzerrung der Kristallstruktur aufzeigen. Die elektronische Struktur und chemische Bindung von C14-, C15- und C36-NbCo2 wurde vergleichend untersucht. Berechnungen der Gesamtenergie zeigen sehr geringe Energiedifferenzen zwischen den drei Strukturen, die mit einer sehr ähnlichen Bindungssituation der Polytypen im Einklang ist. In den Systemen Nb--Cr und Nb--Fe wurde der Verlauf der Gitterparameter innerhalb des gesamten Homogenitätsbereichs der Laves-Phase bei ausgewählten Temperaturen untersucht. Die Kristallstrukturen von C15-NbCr2 und C14-NbFe2 wurden erstmals verfeinert. Vorläufige Untersuchungen bestätigen die Existenz von zwei Hochtemperaturmodifikationen (C14 und C36) von NbCr2. Im System Nb--Mn wurde die Mn-reiche Seite des Homogenitätsbereichs bei 800 °C und 1100 °C an aus zweiphasigen (Mn(Nb) + C14) Präparaten isolierten Einkristallen untersucht. Bei 800 °C wird ein Kristall der Zusammensetzung NbMn2 erhalten, während bei 1100 °C ausgeprägte Löslichkeit von Mn in der C14-Phase beobachtet wird. Die Summenformel kann als Nb(1-x)Mn(2+x) (x = 0,13) geschrieben werden. Die Substitution von Nb durch Mn führt zu Verschiebungen der Antistrukturatome bezüglich der Nb-Lagen und damit zur Bildung kurzer Nb--Mn-Abstände. In den ternären Systemen Nb--Cr--Co und Nb--Cr--Ni wurden die Kristallstrukturen der C14-Phasen C14-Nb(Cr(1-x)Co(x))2 und C14-Nb(Cr(1-x)Ni(x))2 am Einkristall untersucht. Neben den auch für die binären C14-Phasen beobachteten Verzerrungen zeigen die Kristallstrukturen eine teilweise geordnete Verteilung von Cr und Co bzw. Cr und Ni auf die beiden kristallographischen Lagen der B-Teilstruktur. Die bevorzugte Besetzung wurde auf der Grundlage von Extended-Hückel-Rechnungen untersucht. Zwar können diese Rechnungen kein quantitatives Bild liefern, jedoch werden Tendenzen im System Nb--Cr--Co richtig wiedergegeben. Im System Nb--Cr--Ni liefern die Rechnungen jedoch dem Experiment widersprechende Ergebnisse. Die Vorhersagekraft der Methode ist also begrenzt. Vergleichende Untersuchungen der Reihe NbTM2, TM = Cr, Mn, Fe, Co mittels Röntgenabsorptionsspektroskopie und Bandstrukturrechnungen zeigen, dass die chemische Bindung der untersuchten Verbindungen im wesentlichen ähnlich ist, aber dass durchaus Entwicklungen innerhalb der Reihe festgestellt werden können. Diese Entwicklung wird besonders in der Verzerrung der C14-Phasen und hier speziell der B-Teilstruktur deutlich, die in den experimentell zugänglichen C14-Phasen in NbMn2 deutlicher ausgeprägt ist als in NbFe2. Analysen der chemischen Bindung mit Hilfe der COHP-Methoden zeigen eine ähnliche Tendenz zur Verzerrung, die vereinfacht auch als Funktion der Valenzelektronenkonzentration aufgefasst werden kann. Berechnungen der Gesamtenergie unterstützen diese Interpretation. Im Gesamtbild der elektronischen Struktur ist eine leichte Zunahme des ionischen Bindungsanteils von TM = Cr zu TM = Co zu erkennen. Die Natur der Laves-Phasen in Systemen der Übergangsmetalle ist ein sehr vielschichtiges Problem, das weiterhin intensive und interdisziplinäre Forschung erfordert. Insbesondere mit der Charakterisierung nichtstöchiometrischer Laves-Phasen wurden aber bereits wichtige Beiträge zum Verständnis der Bildung der Homogenitätsbereiche erarbeitet.