The Search for New Quasicrystals

TitleThe Search for New Quasicrystals
Publication TypeThesis
Year of Publication2000
AuthorsLu, Peter J.
UniversityPrinceton University
Thesis TypeUndergraduate
Abstract

Having examples of quasicrystals with a large variety of compositions and stoichiometries is important in exploring physical properties and finding practical applications. The discovery of newquasicrystals has historically depended on serendipity. The goal of this thesis is determine if a more systematic approach can be developed. Based on a theoretical understanding of quasicrystal diffraction properties, a computer search scheme has been developed to identify icosahedral quasicrystals based on records in the international Powder Diffraction File, a collection of about 70,000 x-ray powder diffraction patterns. The scheme is first tested by determining if it can successfully identify and index the known icosahedral patterns in agreement with published analyses. Then, the scheme is applied to the remainder of the Powder Diffraction File (materials described as crystalline) to identify the most promising quasicrystal candidates. The leading candidates are examined with electron microscopy. The electron diffraction patterns of the most thoroughly investigated of the mineral candidates to date, krennerite, strongly indicates that it is an incommensurately modulated crystal, only the second such incommensurate mineral known. The fact that the mineral arose from a search for icosahedral quasicrystals means that its structure is not only incommensurate, but the incommensurate ratios of diffraction wavevectors are simply related to the golden ratio. A generic incommensurate crystal can have arbitrary irrational ratios. The finding suggests that the incommensurability may be related to an icosahedral phase, perhaps one obtained by varying the stoichiometry slightly. The system merits synthesis and further testing in the laboratory.

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Peter J. Lu  |  Harvard University  |  Cambridge, MA 02138 USA |  plu_at_fas.harvard.edu