Quasicrystals imbue a symbolic power. They’re not like other crystals (their name means ‘almost crystals’) yet they share important properties. In solids, the constituent atoms are confined to a fixed arrangement. In crystals, the atoms are arranged in a pattern that periodically repeats itself. In quasicrystals, the atoms are arranged in a pattern that repeats itself at irregular, yet predictable, intervals.
Put this way, quasicrystals simply represent an incremental deviation from the natural order – but when you think about creating one in a lab or in nature, their quiet power comes to the fore. Creating quasicrystals is not easy because of why crystals form in the first place.
Extenuating circumstances
Take the crystals of common salt (NaCl) for example, whose atoms are arranged in a repeating cubic pattern. Why don’t they assume tetrahedral or rhomboidal patterns? Because the sodium and the chloride ions have different sizes and exert different electric forces, and a cubic pattern allows them to pack themselves in without upsetting each other, while also optimising for their density, thermal stability, etc.
Anything that is naturally abundant implies a natural preference for that form over other possibilities. So whenever sodium chloride crystals take shape, they adopt the cubic pattern, unless extenuating circumstances force them to pick something else. Quasicrystals, now, embody these extenuating circumstances.
Photograph of a single-grain icosahedral Ho-Mg-Zn quasicrystal made in the lab. The edges are 2.2 mm long. | Photo Credit: AMES lab, public domain
Quasicrystals are crystals that have defied a peaceful logic of crystal formation in favour of less-than-optimum, more contested patterns. In order to create them that way, the forces that shape them need to constantly nudge them away from the form they would rather take, if left alone, and towards a form that they can take. It’s not unlike keeping a spring compressed between your fingers: the spring can be compressed but it would rather be relaxed, so it pushes against you, exerts a force demanding freedom from your oppression.
By adopting a suboptimal crystal structure (an anthropocentric view, to be sure), quasicrystals offer a similar narrative: they may not be oppressed, kept in a state of stress, but the structure of their atomic lattice still contains the imprints of some stressful event.
In the lab, scientists can create these events in miniature, orchestrating forces to strike at just the right time, in carefully controlled conditions, adjusting the evolving crystal structure. But how might this happen in nature?
Meteors and nuclear explosions
The American-Israeli scientist Dan Shechtman discovered quasicrystals in the lab in 1982. In the late 1990s, scientists began looking for quasicrystals in nature. After an arduous decade-long quest, Luca Bindi, Paul Steinhardt, and others reported finding the first natural quasicrystal in 2009 – as microscopic grains in a fragment of the Khatyrka meteorite lying in the Koryak mountains of Russia.
Further analysis revealed at least three varieties: two of an icosahedrite and a decagonite, later joined by a “quasicrystal approximant” called proxidecagonite. The crystal structure of icosahedrite exhibited fivefold symmetry in two dimensions: the pattern repeated itself after being rotated by 72º. (Icosahedrite exhibited 20-fold symmetry in three dimensions, thus its name). Decagonite exhibited 10-fold symmetry (36º).
An X-ray diffraction pattern showing the arrangement of atoms in an icosahedrite crystal in the Khatyrka meteorite. | Photo Credit: Materialscientist/Wikimedia Commons, CC BY-SA 3.0
The Khatyrka meteorite is believed to have been involved in several collisions in space, over millions of years, at least some of which would have exerted 5 gigapascals (or 10,000 Earth-atmospheres) of pressure and heated it to 1,200º C. These conditions inspired a series of experiments in which physicists used ‘shock synthesis’ to create new varieties of quasicrystals in the lab. Their results inspired others to look for natural quasicrystals in places where similar shocks could have been in play.
In 2021, Bindi, Steinhardt, and others raised their hand with a quasicrystal in the remains of the first atomic weapon ever detonated: the Trinity test of the Manhattan Project on July 16, 1945. The thing, they wrote in their paper, “was found in a sample of red trinitite, a combination of glass fused from natural sand and anthropogenic copper from transmission lines used during the test.”
In both these incidents there were fiery contests between godlike forces. Like the meteorite’s cosmic tribulations, we know the Trinity test created temperatures of 1,500º C and a pressure of almost 8 gigapascals. Such infernal crucibles, it would seem, are the birthplaces of natural quasicrystals.
A black and white image of the first nuclear test at the Trinity site, seconds after the explosion took place. | Photo Credit: National Nuclear Security Administration/Nevada Site Office
A rare find
In a study published in December 2022, Bindi, Steinhardt, and others (again) extended this reputation as they reported finding a third forge of natural quasicrystals. In the Sand Hills dunes in northern Nebraska, they uncovered a metallic fragment in a long, tube-shaped mass of sand heated and fused by a heavy electric current. They also noticed a power line nearby had fallen to the ground. That’s where the metal could have come from, but they couldn’t tell where the current had originated: in the power line or as a lightning strike on a stormy night.
Whatever the source, it had melted the quartz at the site and formed a silicate glass – a process that needs to happen at least 1,700º C. The metallic portion was a mass of aluminium, chromium, manganese, nickel, and silicon. When Bindi et al. placed it under a powerful electron microscope, it revealed itself with atoms arranged in a 12-fold symmetry (30º). It was a dodecagonal quasicrystal, rare even for quasicrystals.
Electron back-scatter image showing the 12-fold symmetry in the crystal. | Photo Credit: https://doi.org/10.1073/pnas.2215484119
An electron-microscope image of the quasicrystal grain annotated to highlight the 12-fold symmetry. | Photo Credit: https://doi.org/10.1073/pnas.2215484119
Bindi et al. wrote in their paper: “Just as the discovery of natural quasicrystals in the Khatyrka meteorite pointed to the idea that shock synthesis may be an effective means of searching for new elemental compositions that form … quasicrystals, the discovery of a dodecagonal quasicrystal formed by a lightning strike or downed power line suggests that electric discharge experiments may be another approach to be added to our arsenal of synthesis methods.”
At least one effort to make a quasicrystal (with manganese) with 12-fold symmetry had succeeded in the lab, in 2015, but it didn’t have aluminium. The attempt also required a very complicated process, with several right interventions at just the right time, a far cry from the mess of a large electric current plunging into millennia-old dunes. Yet in the evanescent chaos that followed, several million atoms negotiated their place, crystallographers’ rules be damned, to create a grain of incandescent beauty.
Published - February 07, 2023 09:00 am IST
