Discussion on clusters, phasons and quasicrystal stabilisation

Henley, Christopher L.; Boissieu, M. de; Steurer, Walter

doi:10.1080/14786430500419395

Cited by 70 publications

(64 citation statements)

References 67 publications

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“…This approach allows modelling and refinement of the structure against experimental diffraction data in a way similar to that achieved for 3D periodic crystals 6 . Although much progress has been achieved recently, for instance in the i-AlPdMn phase 7 , the models proposed so far are still being debated 8 . Indeed, the amount of observed diffraction data is in general rather limited, which precludes a detailed refinement of the chemical order in ternary QCs.…”

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confidence: 99%

Atomic structure of the binary icosahedral Yb–Cd quasicrystal

et al. 2006

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Icosahedral quasicrystals (i-QCs) are long-range ordered solids that show non-crystallographic symmetries such as five-fold rotations. Their detailed atomic structures are still far from completely understood, because most stable i-QCs form as ternary alloys suffering from chemical disorder. Here, we present the first detailed structure solution of i-YbCd 5.7 , one of the very few stable binary i-QCs, by means of X-ray structure determination. Three building units with unique atomic decorations arrange quasiperiodically and fill the space. These also serve as building units in the periodic approximant crystals. The structure is not only chemically feasible, but also provides a seamless structural understanding of the i-YbCd 5.7 phase and its series of related i-QCs and approximant crystals, revealing hierarchic features that are of considerable physical interest.Icosahedral quasicrystals (i-QCs) are the only class to show quasiperiodicity in three dimensions 1,2 . Obviously, structural knowledge is essential for understanding the physical properties, stability and tailoring applications of these exotic materials 3 . However, in contrast to other types of QCs that show periodic order in at least one direction, i-QCs cannot make effective use of two-dimensional (2D) imaging techniques such as high-resolution electron microscopy or high-angle annular dark-field scanning transmission electron microscopy for their structural characterization 4 . The i-QCs' structure determination is best achieved in the context of hyperspace crystallography 5,6 , where the structure can be described as a periodic crystal in higher dimensions. For i-QCs, the periodic space is 6D and decomposes into two orthogonal 3D subspaces: the parallel (physical) space and the perpendicular (complementary) space. The 6D unit cell is decorated by 3D objects known as 'occupation domains' (OD), the 3D QCs being obtained as a section of this decorated 6D lattice. This approach allows modelling and refinement of the structure against experimental diffraction data in a way similar to that achieved for 3D periodic crystals 6 . Although much progress has been achieved recently, for instance in the i-AlPdMn phase 7 , the models proposed so far are still being debated 8 . Indeed, the amount of observed diffraction data is in general rather limited, which precludes a detailed refinement of the chemical order in ternary QCs. Therefore, the atomic order in i-QCs remains a challenging and outstanding question. The recent discovery of the first stable binary icosahedral YbCd 5.7 QCs 9,10 has been a breakthrough and led to discoveries of a whole series of related ternary i-QCs 11 . This i-QC offers a unique opportunity for the structural analysis of i-QCs. Indeed, the i-YbCd 5.7 phase can be obtained as high-quality single grains. Furthermore, it is binary and exhibits very good X-ray contrast between Cd (Z = 48) and Yb (Z = 70) atoms.Finally, there is a series of periodic 'approximant crystals' (ACs) to the QC, having almost the same chemical composition and for...

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Atomic structure of the binary icosahedral Yb–Cd quasicrystal

et al. 2006

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“…The same is true for their physical properties, which do not differ significantly from those of highorder approximant crystals (ACs). ACs are built from the same structural subunits, frequently called 'clusters' (see also Steurer, 2006a;Henley et al, 2006;, but in a periodic way. In contrast, we do not know enough about the thermodynamic stability of QCs, and the way they form and grow from the melt.…”

Section: Introductionmentioning

confidence: 99%

Quasicrystals: What do we know? What do we want to know? What can we know?

Steurer

2018

Acta Crystallogr A Found Adv

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More than 35 years and 11 000 publications after the discovery of quasicrystals by Dan Shechtman, quite a bit is known about their occurrence, formation, stability, structures and physical properties. It has also been discovered that quasiperiodic self-assembly is not restricted to intermetallics, but can take place in systems on the meso-and macroscales. However, there are some blank areas, even in the centre of the big picture. For instance, it has still not been fully clarified whether quasicrystals are just entropy-stabilized high-temperature phases or whether they can be thermodynamically stable at 0 K as well. More studies are needed for developing a generally accepted model of quasicrystal growth. The state of the art of quasicrystal research is briefly reviewed and the main as-yet unanswered questions are addressed, as well as the experimental limitations to finding answers to them. The focus of this discussion is on quasicrystal structure analysis as well as on quasicrystal stability and growth mechanisms.

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“…In quasicrystals, clusters are densely packed maintaining the long-range quasiperiodic translational order. An ongoing and persistent discussion (see, e.g., [1]) is whether or not these entities in quasicrystals are also responsible for physical properties.…”

Section: Introductionmentioning

confidence: 99%

Are clusters physical entities?

Rösch¹,

Trebin

2008

Zeitschrift Für Kristallographie

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Abstract. Complex metallic alloys frequently exhibit atomic clusters as structural units. In quasicrystals, they are densely arranged retaining an overall long-range quasiperiodic translational order. Thus, clusters are geometrical entities. The often posed question is, whether these are also responsible for physical properties. Fracture experiments and simulations strongly point towards a positive answer. However, a recent analysis of experimental fracture surfaces questions a signature of the clusters. From the self-affine behavior of a model system it is shown that the findings cannot vitiate the role of the clusters in the fracture process.

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Discussion on clusters, phasons and quasicrystal stabilisation

Cited by 70 publications

References 67 publications

Atomic structure of the binary icosahedral Yb–Cd quasicrystal

Atomic structure of the binary icosahedral Yb–Cd quasicrystal

Quasicrystals: What do we know? What do we want to know? What can we know?

Are clusters physical entities?

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