Lattice dynamics of complex metallic alloys

Abstract

Throughout this thesis the lattice dynamics in CMA phases with different structural and dynamical peculiarities have been studied in experiment and simulation. While inelastic neutron and X-ray scattering enabled an experimental approach to dynamical quantities as dispersion curves, vibrational density of states or dynamical structure factors, the theoretical approach was based on ab-initio and molecular dynamics simulations. Experimental results could be analyzed and interpreted by means of computer simulations, thus yielding insight into dynamical processes on an atomistic level. Indeed, this combination of experiment and simulation proved to be a powerful tool for the investigation of different dynamical phenomena. In the Mg-Zn system the impact of structural complexity on vibrational properties was studied. Pure hcp Zn and the MgZn2 Laves phase were used as rather simple reference structures and compared to the structurally more complex Mg2Zn11 Pauling triacontahedral phase. While MgZn2 showed the behavior of an almost perfectly harmonic solid, Mg2Zn11 turned out to exhibit quite unusual dynamical features. In the case of MgZn2 experimental results from INS could be reproduced with high accuracy. For Mg2Zn11 experimental results and DFT calculations first evidenced non-negligible discrepancies. After reinvestigating the structure of Mg2Zn11 with both, experimental and computational methods, a partially occupied Zn site could be spotted as possible source of the occurring discrepancies. Surprisingly, the partially vacant Zn1 position, at the center of the mini-Bergman cluster proved to exert a strong influence on stability and dynamics of this system. After taking vacancy disorder into account, the experimental results could be decently reproduced and differences could be understood. With this knowledge the experimental GVDOS was finally interpreted in terms of distinct atomic motions, thus connecting macroscopic properties with processes on atomistic scale. The second Zn-based CMA phase that was explored, is the ScZn6 1/1-approximant. The structure of this phase is closely related to the Cd-based binary icosahedral quasicrystals in the Cd-Yb and Cd-Ca system, thus making it an interesting phase with respect to structure and dynamics of quasicrystals like Mg-Zn-Sc. Secondly, the ScZn6 1/1-approximant evidences an order-disorder phase transition at about 150 K. The dynamical aspects of this phase transition were investigated throughout this work, using quasielastic neutron scattering and molecular dynamics methods. Interestingly, the phase transition could be shown to be closely related to a freezing in of the tetrahedral shell in the center of the Tsai-type cluster building blocks. In fact, experiment and calculation clearly evidenced a dynamic disorder of the tetrahedral shell above the transition temperature. The tetrahedral shell is constantly reorienting between different, energetically equivalent configurations. From neutron scattering experiments the residence time between two tetrahedron jumps could be estimated to be of the order of a few ps, while it was overestimated by the conducted MD simulations. These results thus answer the controversially debated question about the nature of the disorder in ScZn6 in favor of a dynamic process. Finally the dynamic reorientations of the tetrahedron are highly interesting with respect to entropical stabilization, a possible candidate for quasicrystal stabilization. In the last part of the thesis the clathrate system Ba-Ge-Ni, was studied with respect to its cage-like structure and the resulting effects on its dynamical properties. Inelastic neutron scattering experiments nicely evidenced a flat dispersionless optic-like phonon branch, which by means of DFT could be shown to stem from localized motions of the encaged Ba atoms - so-called rattling modes. The cage structure of the Ba-Ge-Ni clathrates furthermore made a decomposition into different subsystems possible, such that their contributions to the vibrational spectrum could be analyzed. A comparison to a hypothetical Ge46 structure could be used to elaborate the influence of the encaged Ba-atoms and the host-lattice, respectively. Interestingly, the introduction of Ba-atoms creates a localized, dispersionless phonon branch at rather low energy, which interacts with the acoustic modes of the host structure, resulting in a reduction of the velocity of sound. Thus the low lattice thermal conductivity in this phase seems to be related to both, rattling modes of Ba guest atoms and reduced velocity of sound of the host framework.

Im Rahmen dieser Arbeit wurde die Gitterdynamik verschiedener CMA-Phasen mit Neutronen- bzw. Röntgenstreuung experimentell untersucht und dann anhand von Simulationen bezüglich verschiedener dynamischer Aspekte analysiert und interpretiert. Im Mg-Zn System konnte der konkrete Einfluss von struktureller Komplexität auf die Vibrationseigenschaften studiert und am Beispiel der bei den Phasen MgZn2 und Mg2Zn11 dargestellt werden. Ein besonderes Augenmerk wurde hierbei auf die Analyse der auftretenden niedrigenergetischen Moden in Mg2Zn11 gelegt. Für den ScZn6 1/1-Approximanten gelang es, den engen Zusammenhang der inneren Tetraederschalen mit dem Ordnungs-Unordnungs-Phasenübergang herauszuarbeiten und zu zeigen, dass die Unordnung oberhalb des Phasenüberganges von dynamischer Natur ist. Somit war es möglich, die viel diskutierte Frage über die Tetraederunordnung oberhalb des Phasenüberganges eindeutig zu beantworten. Im Clathrat-System Ba-Ge-Ni wurde das Phononenspektrum mit Bezug auf die niedrige thermische Leitfähigkeit des Gitters im Detail untersucht. Die bei tiefen Energien auftretenden rattling modes sowie deren Einfluss auf die die akustischen Phononmoden konnten hier als Ursachen für die niedrige thermische Leitfähigkeit ausgemacht werden.