quasicrystal

quasicrystal. A quasicrystal is a solid structure that exhibits an ordered but non-periodic atomic arrangement, defying the classical rules of crystallography. This unique order produces diffraction patterns with rotational symmetry forbidden in conventional crystals, such as five-fold symmetry. The discovery challenged fundamental paradigms in materials science and solid-state physics, leading to a redefinition of the very concept of a crystal.

Definition and discovery

The defining characteristic is long-range orientational order without translational symmetry, a concept initially met with great skepticism. The pivotal discovery is credited to Dan Shechtman in 1982, while he was conducting electron diffraction experiments on a rapidly cooled aluminium-manganese alloy at the National Institute of Standards and Technology. His observation of a sharp ten-fold diffraction pattern contradicted established beliefs, famously leading to opposition from figures like Linus Pauling. This work was later validated by researchers such as John Cahn and Denis Gratias, culminating in the International Union of Crystallography revising its definition of a crystal in 1992. The recognition of this breakthrough was solidified when Shechtman was awarded the Nobel Prize in Chemistry in 2011.

Structure and classification

Quasicrystals lack a repeating unit cell but possess a mathematically deterministic structure often described by projection from a higher-dimensional hypercubic lattice, a method pioneered by theorists like Alan Mackay and Dov Levine. They are broadly classified by their symmetry; common types include icosahedral phases, exhibiting full icosahedral symmetry, and decagonal phases with periodic stacking in one direction. Key structural models include the Penrose tiling, conceived by Roger Penrose, which provides a two-dimensional analog. The study of these patterns involves advanced number theory and is linked to the golden ratio, a proportion frequently appearing in their geometry.

Physical properties

These materials exhibit a distinctive combination of properties that often differ markedly from both crystalline and amorphous materials. They typically display high hardness, low electrical conductivity, and very low thermal conductivity, akin to insulators like glass. Their surface energy is often exceptionally low, leading to non-stick and low-friction characteristics comparable to Teflon. The electronic structure of quasicrystals, studied extensively at institutions like the Ames Laboratory, reveals unusual electronic transport behavior, including a pseudogap at the Fermi level. Their magnetic properties are also complex, with many phases showing spin glass behavior.

Formation and synthesis

Most stable quasicrystals are ternary or multicomponent intermetallic alloys, with common systems involving aluminium paired with elements like copper, iron, or palladium. They were first synthesized in the laboratory via rapid solidification techniques such as melt spinning. The discovery of natural quasicrystals in a meteorite sample from the Khatyrka meteorite, found in the Koryak Mountains and studied by teams including Luca Bindi and Paul Steinhardt, proved they can form under geological conditions. Other synthesis methods now include solid-state reactions, electrodeposition, and thin-film growth, allowing for the creation of various forms from bulk samples to nanoparticles.

Applications and significance

Practical applications leverage their unique surface and mechanical properties. They have been used in commercial products such as non-stick coatings for cookware and durable coatings on surgical instruments and razor blades. Their low friction and high wear resistance make them candidates for reinforcing aluminium alloys in components like pistons. The scientific significance is profound, fundamentally expanding the understanding of long-range order in condensed matter physics and influencing fields from metallurgy to mathematics. Research continues at facilities like the Advanced Photon Source to explore potential uses in thermoelectric devices and as hydrogen storage materials due to their complex catalytic surfaces.

Category:Materials science Category:Solid-state chemistry Category:Condensed matter physics