Logo: to the web site of Uppsala University

The icosahedral quasicrystal (i-QC), an aperiodic crystal, exhibits non-crystallographic symmetry in three-dimensional space. Its unique structural properties continue to draw significant attention in Condensed Matter Physics. Tsai-type i-QCs for example, have been the focus of extensive research since 2000. However, due to challenges in determining the structure of i-QCs, their exact crystal chemistry and the relationship between their structure and physical properties remains unclear. So-called quasicrystal approximants (ACs) share similar atomic arrangements and chemical compositions with their aperiodic siblings. The exploration of the correlation between structural and physical properties in periodic ACs thus permits to gain deeper insights into local structures that resemble those of QCs, and thus on the crystal chemistry of QCs.

This thesis deepens our understanding of the Tsai-type ACs and QCs, focusing on optimizing synthesis methods and using single-crystal X-ray diffraction (SCXRD) to elucidate the crystal chemistry of ACs. Structural evaluations of various ACs employ elemental tuning and substitution in binary and ternary Tsai-type systems such as Gd-Cd and Gd-Au-Al; the results are connected to the observed magnetic properties.

Chapter 1 provides an overview of QCs and ACs, covering their history, structure, and physical properties to give readers a solid background on the topic. Chapter 2 outlines the synthesis methods and sample characterization techniques used in this research. Chapter 3 presents the key findings, including a detailed structural evaluation of ACs, AC-related superstructures, and more complex AC structures. This analysis clarifies the structural mechanisms associated with phase stability and connect them to their physical (magnetic) properties. Finally, a summary of the thesis main contribution to the research community, and concluding remarks on the chemical insights into the structural and physical properties of ACs and QCs are given in Chapter 4.