Chemistry Coursework
By Tejas Ravi Lahir
IM-16-0181
Dan Shechtman Nobel Prize in 2011 for Discovery of Quasicrystals.
The science of crystallography is generally assumed to have started in 1912 by the experiment conducted by Von Laue where he used X rays on crystals to study the nature of the short wavelength X rays by diffracting them, but ended up determining the regular order and symmetry of crystals themselves. The early pioneer scientists in crystallography received Nobel Prizes for determining the structure of crystals.
The definition of a crystal emerged from these studies was as follows:
“Atoms in a crystal are arranged in a pattern that repeats itself in three dimensions throughout the interior of the crystal.”
Seventy years later, in 1982, Dan Shechtman, while studying the diffraction of X rays from crystals came across a set of crystals that were arranged in a pattern that did not repeat itself (called quasicrystals).
His work changed the definition of crystals.
Being meticulous in his experiments, standing ground in face of relentless criticism for over a decade from his peers (including extreme opposition from a two time Nobel Laureate) and belief in his work makes him a hero who changed the definition of crystals and won the Nobel Prize in 2011 for “Discovery of Quasicrystals”.
The aim of the essay is to understand the structure of Quasicrystals and their relevance in our daily lives.
Till 1982, the common wisdom of the science of crystallography was that crystals could be made from molecules having 1, 2, 3, 4 and 6 folds of rotation. Crystals could also be made from molecules which have an individual 5-fold rotation but it was expected that the lattice of a crystal would not have a 5-fold rotation. Since crystallography was a mature science by then nobody expected anything new. Even if a new crystal was discovered by X rays or microscopy they could decipher the structure and its symmetry and slot it accordingly.
Diagrams of Crystal Patterns
As seen from an electron microscope, the order of atoms in a diamond are periodic. The allowed rotational symmetries are 1, 2, 3, 4 and 6. (Five-fold rotational symmetry or any other symmetry beyond 6 is forbidden in periodic structures)
Periodicity of diffraction peaks in the reciprocal space
Rotational Symmetry: 1, 2, 3, 4 and 6. No 5 fold or beyond 6 rotational symmetry.
In 1982, Dan Shechtman was taking x ray diffraction of crystals when he came across a crystal which gave an image shown below. Thinking it was an experiment gone wrong, he took the diffraction image again. The results repeated themselves. As Dan started eliminating any possible errors, Dan looked at the possibility that the sample was a twin crystal (i.e. five crystal atoms which have a special boundary between them). He did a series of dark field experiments but the results did not prove the sample to be twin crystals.
Readings when Dan Shechtman made the discovery
Unable to solve the mystery of these crystals, Dan performed a series of micro diffraction experiments. In micro diffraction, Dan converged the electron beam into a spot and then observed the diffraction pattern. The experiment conclusive proved the absence of twin crystals and made Dan realize that he had stumbled upon something very special.
This is the point at which Dan thought it was a ’10-fold’ rotational symmetry. Subsequent experiments by him proved that it was a five-fold symmetry.
The conclusions reached by Dan were:
1. Electron Diffraction has a five-fold rotational symmetry and it is not periodic.
2. The ratio of distances between the central spot and other spots is a Fibonacci number ƭ (also known as the “Golden Mean”)
Further experiments with high resolution microscopy proved that the material was indeed a crystal. Taking this research further with Blech, Dan and Blech came out with a research paper which was initially sent to a journal of physicists for publication. However they did not publish it but suggested it be sent to a metallurgical journal, who happily published it.
Shortly afterwards Dan and Blech joined hands with John Cahn and Denis Gratias and published a focused concise paper suggesting quasi periodicity in some crystals.2 A Year later another group of mathematical crystallographers did the same experiments with Penrose tiles and proved that 5 fold symmetry was present in the Penrose tiles and it was this group that coined the term quasi periodic crystals.
The Icosahedron is the basic structure of many quasi periodic crystals. Thus many quasi crystals have icosahedral symmetry with
a. Six five-fold axes
b. Ten three fold axes
c. Fifteen two fold axes
With a tough period from 1984-87 to find acceptance for his findings to ultimately the definition of a crystal being changed by the crystallographic society in 1994 was a long and tough battle for Dan. Eventually the definition was changed to:
A crystal is any solid having an essentially discrete diffraction diagram, and by an aperiodic crystal we mean any crystal in which three dimensional lattice periodicity can be considered to be absent.
Ni-Cr particles, V-Ni-Si alloys and Cr-Ni-Si alloys are examples of quasi periodic crystals. A study of Khatyrka meteorites revealed that their chemical composition is Al71Ni24Fe5 and that they are quasi crystals. They are stable in a temperature of around 1200K at natural pressure, suggesting natural quasicrystals are formed by rapid quenching of meteorite heated during impact induced shock.
Quasicrystals are classified into three groups depending on thermal stability:
a. Stable quasicrystals grown by slow cooling or casting with subsequent annealing,
b. Metastable quasicrystals prepared by melt spinning
c. Metastable quasicrystals formed by crystallization of amorphous phase.
To find out the applications of quasicrystals Dan suggested taking advantage of the properties of quasicrystals:
a. Low Coefficient of friction
b. Hardness at very high temperatures
c. Non Corrosive
Thus, quasicrystals are used primarily for heat insulation, broad wavelength reflectors and bone repair and prostheses applications, where biocompatibility, low friction and corrosion resistance are important. Surprisingly, while manmade quasicrystals are used in many commercial applications, practical applications for naturally occurring quasicrystals have not yet been found.
References:
Bragg, W.H. and Bragg, W.L., 1913. The reflection of X-rays by crystals. Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, 88(605), pp.428-438. [Online].
Available at:http://www.jstor.org/stable/93501?seq=1#page_scan_tab_contents
(Accessed 25 March 2017)
Hammond, C., 2009. The basics of crystallography and diffraction (Vol. 12). Oxford: Oxford University Press. [Online].
Available at:http://oldwww.iucr.org/iucr-top/journals/bookreviews/by0141.html
(Accessed 24 March 2017)
Jha, A., 2013. Dan Shechtman:’Linus Pauling said I was talking nonsense’: The Israeli Nobel laureate discusses the discovery that caused a furore among fellow scientists. The Guardian. [Online].
Available at:https://www.theguardian.com/science/2013/jan/06/dan-shechtman-nobel-prize-chemistry-interview
(Accessed 25 March 2017)
Kittel, C. and Holcomb, D.F., 1967. Introduction to solid state physics. American Journal of Physics, 35(6), pp.547-548. [Online]. Available at:http://mse.fudan.edu.cn/NanoMembrane/ISSP/Reference/Introduction_To_Solid_State_Physics_8Th_Edition_-_Solution_Manual-Kittel__Charles.pdf
(Accessed 25 March 2017)
Nobel Lecture by Dan Shechtman 2011, Video Recording, Nobel Media AB 201, Stockholm University Sweden, Available at: http://www.nobelprize.org/mediaplayer/index.php?id=1731
(Accessed 19 March 2017)
Röntgen, W.C., 1896. On a new kind of rays. Science, pp.227-231. [Online].
Available at:http://www.jstor.org/stable/1623595?seq=1#page_scan_tab_contents
(Accessed 24 March 2017)
Sunday, T.U.T.O.R.I.A.L.S., 2012. Dan Shechtman to give plenary address on quasicrystals at 2012 MRS Fall Meeting. MRS BULLETIN, 37.[Online]. Available at:https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769412002606
(Accessed 20 March 2017)