The detonation of the first atomic bomb during the 1945 Trinity Test produced temperatures and pressures so extreme that the surrounding sand fused into a glassy material called trinitite. Physicists have now discovered a rare material known as a quasicrystal in one of the trinitite samples. According to a new paper published in the Proceedings of the National Academy of Sciences, that makes the discovery the oldest anthropogenic quasicrystal yet known.
The very definition of a crystal assumes a precisely symmetrical ordering of atoms in periodic patterns that repeat over and over in a 3D lattice. The patterns look the same no matter which direction you look at them, but quasicrystals are different. They clearly follow mathematical rules, but each cell has a slightly different configuration of cells nearby rather than repeating in an identical pattern. It’s that unique structure that gives quasicrystals their unusual properties.
Think about tiling a bathroom floor. The tiles can only be in certain symmetrical shapes (triangles, squares, or hexagons); otherwise, you won’t be able to fit them together without leaving gaps or overlapping tiles. Pentagons, icosahedrons, and similar shapes with different symmetries that never precisely repeat just won’t work—except in the case of quasicrystals, where nature decided they could work. The trick is to fill the gaps with other kinds of atomic shapes to create the unlikely aperiodic structure.
The discovery of quasicrystals makes for a cracking good tale. You have your scientific underdog, an Israeli physicist named Daniel Shechtman, who was examining samples of an aluminum-manganese alloy with an electron microscope in 1982. This experiment involved bouncing electrons off the atoms in a sample, creating bright and dark regions indicating the positions of the atoms themselves. Shechtman noticed an odd, aperiodic diffraction pattern: a seemingly impossible tenfold symmetry. As the story goes, he muttered to himself, “Eyn chaya kao” (Hebrew for “there can be no such creature”) because it was in clear violation of the known rules of crystallography established over 150 years before.
Shechtman’s colleagues were understandably skeptical; the mocking he endured was perhaps less understandable (the head of his laboratory at the time sarcastically advised him to reread his crystallography textbook). But Shechtman persevered and ended up revolutionizing the field, redefining the scientific consensus on what constitutes a crystalline solid. These days, quasicrystals are practically commonplace, with over 100 varieties regularly synthesized in the laboratory and used in surgical instruments, LED bulbs, and nonstick frying pans (they are excellent insulators because they exhibit such poor heat conductivity). And Shechtman received the 2011 Nobel Prize in Chemistry.
Quasicrystals caused a stir again in 2008 with the discovery of the first known naturally occurring quasicrystals. Princeton physicist Paul Steinhardt was studying a museum rock collection belonging to Luca Bindi of the University of Florence and noticed the telltale aperiodic structure indicative of a quasicrystal in one of the samples. The sample, collected in 1979, came from a meteorite that had landed in the Koryak Mountains in Russia. Steinhardt organized an expedition to the region and combed the frozen tundra in tractor vehicles. That’s where he found even more meteorite fragments containing quasicrystals.
As I wrote at Gizmodo in 2016, Caltech’s Paul Asimow, Steinhardt, and others found a likely mechanism for the formation of quasicrystals in the so-called Khatyrka meteorite. They subjected certain rare materials to extremely strong shock waves, and the results suggested that quasicrystals may form in rocky bodies during collisions in the asteroid belt before falling to Earth as meteorites.
Scientists had already determined that the Khatyrka meteorite had undergone some kind of shock event long before it fell to Earth—most likely from a collision with another object in the asteroid belt in the early days of our Solar System. So Asimow et al. took a sample of copper-aluminum alloy—similar in composition to the icosahedrite found in the meteorite—put it into the chamber, and shocked it with a tantalum capsule to produce the equivalent of 200,000 atmospheres.
Now Steinhardt, Asimow, Bindi, and several other colleagues are back with a new paper announcing the discovery of a previously unknown quasicrystal in red trinitite from the first detonation of an atomic bomb. And in this case, we know exactly where and how the quasicrystal formed thanks to the historical records from the Manhattan Project.
Just before sunrise on July 16, 1945, at the secluded Alamogordo Bombing Range in the Central New Mexican desert, a prototype nuclear bomb nicknamed “Gadget” was hoisted to the top of a 100-foot tower and detonated. The blast vaporized the steel tower and produced a mushroom cloud rising to more than 38,000 feet. The heat from the explosion melted the sandy soil around the tower into a mildly radioactive, glassy crust now known as trinitite. The shock wave was powerful enough to break windows 120 miles away.
Much of the trinitite that formed was from sand primarily consisting of quartz and feldspar, giving it a classic greenish color. But there are rarer samples, with reddish hues, that are also rich in metals, since the sand fused with metals from the test tower and recording equipment, most notably the copper oxide in the vaporized transmission lines. It was these samples that were of most interest, since the known quasicrystals to date have been metal-like alloys.
The team first used backscattered electron microscopy on the samples to find various metallic blobs that might house quasicrystals. The researchers then placed the isolated metallic blobs under an electron microprobe and also subjected their samples to single-crystal X-ray diffraction analysis. The result: the identification of a quasicrystal with fivefold, threefold, and twofold symmetries, made of iron, silicon, copper, and calcium. Producing such a structure in silicon would require the extreme heat and pressure of a nuclear shockwave, although quasicrystals could possibly form in other extreme conditions, such as lightning striking rock or sediments to produce fulgurite.
“The dominance of silicon in its structure is quite distinct,” Valeria Molinero, a theoretical chemist at the University of Utah, who is not a co-author on the paper, told Nature. “However, after many quasicrystals have been synthesized in the lab, what I find truly intriguing is that they are so scarce in nature.” Steinhardt proffered a possible explanation, suggesting that the unusual combination of elements and arrangements could account for their rarity.
In the future, it may be possible to create quasicrystals with custom magnetic or electric properties. University of Utah scientists have demonstrated that ultrasound waves can be used to organize carbon nanoparticles in water into the same aperiodic pattern found in quasicrystals, according to a paper published last month in Physical Review Letters.
Co-author Fernando Guevara and his collaborators set up four pairs of ultrasound transducers in an octagonal shape, then placed carbon nanoparticles suspended in water within the experimental octagon. Then they turned on the transducers, and the ultrasonic waves guided the nanoparticles into the quasiperiodic arrangement. It should be possible to create an actual material with that pattern by suspending the nanoparticles in a liquid polymer, which could be cured and hardened once the pattern was in place.
“[Quasicrystals] have been shown to be stiffer than similar periodic or disordered materials. They can also conduct electricity, or scatter waves in ways that are different from crystals,” said Guevara. “Crucially, with this method, we can create quasiperiodic materials that are either 2D or 3D and that can have essentially any of the common quasiperiodic symmetries by choosing how we arrange the ultrasound transducers and how we drive them.”
DOI: PNAS, 2021. 10.1073/pnas.2101350118.