A strange new substance has unexpectedly emerged from a university lab in Germany: a two-dimensional quasicrystal, consisting of 12-sided, non-repeating atomic units.

The quasicrystalline film, described today in Nature, is the first example of a 2-D semi-ordered crystal – and the latest member of a family that already includes some of the most surprising forms of matter found either in nature or the lab.

Image: Wolf Widdra )

Scientists at Germany’s Martin Luther University produced the material by chance, coincidentally mimicking the circumstances under which the first lab-grown quasicrystals appeared. That discovery eventually earned Daniel Shechtman the 2011 Nobel prize in chemistry (a prize awarded to three scientists today for developing powerful computing models that can simulate complex chemical reactions).

Quasicrystals are a strange, semi-ordered form of matter, one that is neither repetitive in structure (as crystals are) nor disorganized (like a goopy protein soup). Instead, quasicrystal building blocks are all ever-so-slightly different from one another; their atomic arrangements, on large scales, are inconsistent. As a consequence, it’s impossible to find repeating structures within a quasicrystal, though it can be hard to identify the points where symmetry is broken.

For the last three decades, quasicrystals have both astounded and confounded scientists. The first sample, made in 1982, was so improbable that eventual Nobel prize winner Shechtman was ridiculed and ultimately asked to leave his lab. Then, for years, no one believed that quasicrystals could exist anywhere but the lab – assembling the strange, quasi-periodic structures was simply too tricky, requiring precise temperatures and strange conditions including vacuums and an argon atmosphere.

But in 2007, physicist Paul Steinhardt of Princeton University and geologist Luca Bindi from the University of Florence cracked open a strange-looking rock from Bindi's collection. And what did they find inside? Quasicrystals. Turns out, the rock was actually a meteorite – an extraterrestrial visitor that had been retrieved from the Koryak mountains in far eastern Russia in the late 1970s.

>"No simple explanation could explain the observation"

Bindi and Steinhardt eventually proved, in 2012, that the quasicrystals inside the rock had been forged in space, and were the natural result of an astrophysical process, and not the product of terrestrial furnaces or a consequence of the rock’s collision with Earth.

Meanwhile, two years ago, Wolf Widdra and his colleagues at Martin Luther University created the new, two-dimensional structure accidentally. The team had been scrutinizing the interface between two materials, with the goal of figuring out how to engineer properties not found in nature. In this case, they were studying how a certain kind of mineral called perovskite behaved when layered atop metallic platinum.

They heated the perovskite film to a high temperature. Suddenly, they spied a strange pattern glimmering at the materials’ interface: A sharp, simple pattern with 12-fold symmetry, thought to be an impossibility. When then-graduate student Stefan Forster tried to resolve the 12-fold pattern into two groups with six-fold symmetry – an arrangement permitted in crystal structures – he couldn’t do it.

“No simple explanation could explain the observation,” Widdra said.

Unexpectedly, the team had created a thin, two-dimensional quasicrystalline layer.

“We were very surprised,” Widdra said. “It took quite a while until we were convinced that we had a new form of two-dimensional quasicrystal.”

Oxides minerals, like perovskite, don't ordinarily form quasicrystalline structures; normally, these compounds live in crystal form, made from ordered, repetitive building blocks with 2-, 3-, 4-, or 6-fold rotational symmetries (think of dividing a triangle, square, or hexagon into symmetrical parts). No one thought a perovskite could assume a semi-ordered, aperiodic structure.

Image: Wolf Widdra )

Somehow, though, the perovskite and platinum had interacted and grown a thin, nanometers-thick, quasicrystalline layer. Its building blocks were 12-sided, dodecagonal arrangements with internal patterns of squares, triangles, and rhomboids. “They have a perfect order, but never repeat themselves,” Widdra said.

Laying the dodecagons side-by-side produced the thin-film quasicrystal.

“This is another beautiful example of just how commonly quasicrystalline structures form,” said physicist Alan Goldman of Iowa State University and the U.S. Department of Energy’s Ames Laboratory, who was not involved in this study. “The number of examples continues to grow and continues to surprise us.”

And it will likely continue to grow. Widdra suspects that many perovskite structures will produce quasicrystals under the right conditions, and that these strange films will find a place in electrical coatings and thermal insulators. The question now is, why can some materials be coaxed into forming quasicrystalline structures, while others choose to assume more conventional forms? “We really don’t understand why,” Goldman said. “Each new system provides us with some clues, and the more examples we find, the closer we come to answering that question.”