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Quasicrystals: History in the Making

(l-r) John Cahn, Dan Shechtman, Ilan Blech,
and Denis Gratias, France 1995.
© CNRS Phototheque - Pierre Grumberg
By Amanda Jaffe-Katz

Dan Shechtman discovered quasiperiodic crystals in 1982 - a new form of matter. His findings, recorded from an aluminum manganese alloy which he had rapidly cooled after melting, demonstrated a clear diffraction pattern with fivefold symmetry.

Born in Tel Aviv in 1941, Nobel Laureate Dan Shechtman, who showed a precocious ability to view objects in a unique manner and a prodigious memory for detail, says, “Until the age of three, I lived on Dizengoff Street in a Bauhaus building. I remember looking out the porch fascinated at how people on the street below look from above.”

“In high school I was a sharpshooter, one of the best in the country. We were educated to become physically and mentally independent - if you threw us on a Desert Island we would survive. That’s the Israeli character,” says the world-renowned scientist who stood up for his discovery in the face of widespread disbelief.

The young Shechtman dreamed of being an engineer like his fictional hero, Cyrus Smith. And so, in 1962, he commenced his Technion studies in Mechanical Engineering. “When I graduated in 1966 there was a recession and no work, so I opted to continue for a master’s degree. After that, I was offered an excellent post as the chief engineer in a defenserelated industry. But I had already fallen in love with science. On the eve of starting the job, I notified them that I wasn’t coming, and began doctoral studies instead. This was a decisive crossroads in my career.”

“The most important thing about the quasicrystals is their meaning for fundamental science. They have rewritten the first chapter in the textbooks of ordered matter.”
- Prof. Sven Lidin, Member of the Nobel Committee for Chemistry
From 1975, Shechtman spent six years on the Technion faculty in the Department of Materials Engineering, studying rapid solidification in metal alloys. Then he went on his first sabbatical, to the National Bureau of Standards (now known as NIST), where he made the discovery that, for a while, made him one of the most unpopular scientists in crystallography and that nearly 30 years later won him the ultimate recognition - the Nobel Prize in Chemistry.

Shechtman had always loved microscopes: at his grade school he was the first pupil to show an interest in the apparatus. At Technion he fell under the spell of the electron microscope and mastered methods of using it. It was with such an instrument in 1982 that he first noticed fivefold symmetry - the Icosahedral Phase, the first structure in the field of quasiperiodic crystals.

Shechtman explains his discovery in the context of what was known at the time about crystals. “Modern crystallography started in 1912 with the seminal work of von Laue who originated the first x-ray diffraction experiment. The crystals von Laue studied were ordered and periodic, and all the thousands of crystals studied during the next 70 years were found to be ordered and periodic. Based on these observations, the ensuing definition of ‘crystal,’ namely, atoms in a crystal are ordered in a periodic way, was accepted by the community of crystallographers and by the scientific community in general,” he says. “Crystallography was a mature science and no one expected anything new. The allowed rotational symmetries in a crystal structure were 1, 2, 3, 4, and 6. Fivefold rotational symmetry, as well as any other symmetry beyond sixfold, was forbidden in periodic structures.”

Paul Steinhardt and Dov Levine, Technion, 2006
Shechtman’s work required him to sit alone in the dark, eyes glued to the screen of the electron microscope. As recorded in his logbook, what he saw on Plate 1724 was a pitch-black crystal which he thought was interesting and so he looked at the diffraction pattern. “I saw 10-fold symmetry - that later turned out to be fivefold. It was totally unexpected. It was ‘forbidden’ by the laws of crystallography. I worked all day to find what it could be. I thought it must be twinned crystals, and performed a series of experiments designed to find twins and I did not find them. So then I did micro-diffraction pattern experiments. There were no defects or ‘twins’ and so it was something unique in the atomic structure.”

Shechtman returned to Technion in 1983. Dr Ilan Blech was the only colleague who not only believed in him but who agreed to cooperate with him. Blech was able to decipher Shechtman’s experimental findings and offered an explanation, known as the Icosahedral Glass Model. Together, the researchers wrote an article that included the model and the experimental results, and submitted it to the Journal of Applied Physics in the summer of 1984. The paper was immediately rejected.

“I then submitted it to the journal Metallurgical Transactions where it was published months later, in 1985.” In the meantime, he gave the original Shechtman-Blech manuscript to John Cahn, who was later to receive Technion’s Harvey Prize in 1995. “Together with Denis Gratias, a French mathematical crystallographer, we rewrote it as a very concise, minimalist article just with the electron microscopy observations,” and, in November 1984, Physical Review Letters was the first to publish Shechtman’s discovery.

Back in the mid-1970s, mathematician Roger Penrose, of Oxford University, created an aperiodic mosaic, a pattern that never repeats itself, with just two tiles - a fat and a thin rhombus. And in 1982, Alan Mackay of Birkbeck College, London, showed that one can take a Penrose pattern, shine a laser beam through circles representing atoms in the mosaic, and create a fivefold diffraction pattern with sharp peaks.

“Immediately after our paper was published, Dov Levine and Paul Steinhardt published a paper offering a mathematical model to explain our findings,” Shechtman adds. Quasicrystals got their name in this article. Levine, now a professor in Technion’s Faculty of Physics and Steinhardt, now of Princeton University, made the connection between Mackay’s theoretical fivefold symmetry model and Shechtman’s diffraction pattern.

“When Ilan Blech, John Cahn, and Denis Gratias joined, I wasn’t alone anymore,” Shechtman states. “It was clear that we would win this battle because we knew we were right, due to my experimental evidence and the models of Blech and Steinhardt-Levine.”

Crystallographers, however, wanted x-ray results, and did not accept Shechtman’s findings. “The trusted tool of choice was x-ray diffraction; it is very precise but you cannot discover quasicrystals there,” he says. As large single quasicrystals became available in 1987, so did x-ray diffraction patterns that convinced the community of crystallographers that fivefold symmetry and quasiperiodicity can exist in crystals. Before, atomic order in crystals was synonymous with periodicity. Now, order could be either periodic or quasiperiodic. The old definition of crystal held until 1991, nearly a decade after Shechtman’s first observations of quasicrystals. The new definition, Shechtman says approvingly, “is so nice because it is humble and it is open. A humble scientist is a good scientist.”

Shechtman provides his explanation for why quasicrystals were not discovered before 1982. They are not rare, nor hard to make, nor costly, but they had to be discovered by transmission electron microscopy (TEM). “This is the triumph of TEM - they were too small for x-rays which require a sample the size of which you can feel between your fingers. Then, you need to be a professional. You also require tenacity; you must believe in what you are seeing and have courage.”

“Great discoveries come by serendipity. Usually you stumble upon it. In many cases, it will be an artifact but in other cases it will be something new.” He tells his students, “Here is a real test: if you believe in yourself, listen to others but don’t let them discourage you unless you are convinced they are right and you are wrong.”

The Technion quasicrystals legacy continues with research spearheaded by Distinguished Prof. Mordechai (Moti) Segev. His team was the first to demonstrate nonlinear photonic quasicrystals.

Nobel Laureate Dan Shechtman was made Distinguished Professor in 1998. He holds the Philip Tobias Chair in Material Sciences, and heads the Louis Edelstein Center for Quasicrystals and the Wolfson Centre for Interface Science.

Basic concepts

Electron diffraction pattern: When the electron microscope sends beams of electrons through material that hit the atoms within, they scatter and form a diffraction pattern.

Rotational symmetry: An image has rotational symmetry if there is a center point around which the object is turned a certain number of degrees and the object still looks the same. A square has 4-fold, a triangle has 3-fold, and a hexagon has 6-fold symmetry.

Twinning: Occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner, resulting in an intergrowth of two separate crystals that does not produce a simple diffraction pattern.
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