Slide 1
Atoms can have order without a repeating unit
Quasicrystals have long-range atomic order without the repeating unit found in an ordinary periodic crystal.
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A tiled floor suggests an easy picture of order: shift the pattern by one tile and it matches itself again. Quasicrystals show another possibility. Their atomic arrangements possess long-range order without the translational repetition expected of an ordinary periodic crystal.
Slide 2
Try shifting the whole pattern
A periodic arrangement matches itself after a suitable shift. An ordered aperiodic pattern has no repeating unit of that kind.
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In a periodic pattern, suitable translations bring the entire arrangement back into alignment. An aperiodic ordered pattern lacks that repeating unit. A two-dimensional tiling can illustrate the distinction, but it is an analogy for atomic order, not a literal image of a quasicrystal.
Slide 3
Scattered beams let scientists test the arrangement
Diffraction reveals internal structure through the way a material scatters a beam. Its pattern can show order even when the symmetry does not fit the usual repeating picture.
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Diffraction probes how a material's internal arrangement scatters a beam. The observed sharp pattern indicated order, while its symmetry conflicted with the accepted periodic picture. The difficulty was not simply an unusual-looking specimen; it was evidence that challenged the definition used to classify it.
Slide 4
Shechtman’s 1982 observation challenged the old category
Dan Shechtman’s observation in 1982 prompted a reassessment of crystalline order. The evidence called for a wider definition of a crystal.
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Dan Shechtman's 1982 observation began a difficult reassessment of crystalline order. As the evidence was developed and accepted, crystallography widened its understanding of what a crystal could be. The change concerned a scientific category, not a suspension of nature's laws.
Slide 5
Keep reliable observations even when a definition must change
Quasicrystals separated order from repetition. They showed how reliable observations can outgrow a familiar definition.
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Repetition had been treated as essential to a crystal's order. Quasicrystals separated those ideas. A successful definition must accommodate what reliable observations reveal, even when the new case makes a familiar visual intuition insufficient.
Sources
- The Discovery of Quasicrystals
Royal Swedish Academy of Sciences / Nobel Prize