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Chemistry Chat
New Allotropes of Main Group Elements (3)
Kentaro Sato
Sulfur
(1) S8
When it comes to allotropes, sulfur is one of the most versatile of all elements. High school textbooks list a few of sulfur allotropes, such as orthorhombic sulfur, monoclinic sulfur, and amorphous sulfur. The first two allotropes both consist of eight-membered ring molecules of sulfur (S8) as a basic unit and differ in the crystal packing pattern. The yellow crystal we associate with “sulfur” is the orthorhombic sulfur (aka. α-sulfur), which is the only stable allotrope at room temperature.
The eight-membered ring sulfur (S8) (Wikipedia)
The molecule of S8 takes a crown-like shape in which the sulfur atoms are connected in zigzag-fashion. The bond angle is roughly 108° and not so apart from that of carbon (109.5°), yet it is interesting that sulfur is most stable as an eight-membered ring unlike carbon.
(2) Amorphous Sulfur
When this orthorhombic sulfur is heated, it melts into a dark red liquid after passing through monoclinic sulfur state. Fast cooling of the melt in cold water leads to the formation of amorphous sulfur (or “rubbery” sulfur). It is the product of ring-opening polymerization of S8 and very elastic as its nickname suggests. Upon long storage, it spontaneously returns to orthorhombic form.
The color of rubbery sulfur had been considered either black or dark brown, but in 2009, a seventeen-year-old boy experimentally proved it inaccurate and textbooks had to be rewritten. In the experiment, when he converted a sample of orthorhombic sulfur with 99% purity into rubbery sulfur, he obtained a brown product as expected. However, when he used 99.5% pure orthorhombic sulfur, he obtained a surprisingly light yellow product.
It should be noted that the composition of amorphous sulfur changes depending on how it is prepared and so does its color. Therefore, the yellow color the boy observed was probably not the outcome of purity factor alone. Nevertheless, it was a remarkable finding made thanks to his careful analysis and critical thinking.
(3) Other cyclic sulfur allotropes
Sulfur can exist in shapes other than eight-membered ring and many of them have been created by chemical synthesis. For example, the six-membered sulfur (S6) was synthesized by reacting H2S4 and S2Cl2. On molecular level, S6 takes a chair conformation similar to cyclohexane, and on macroscopic level, it forms a red-orange rhombohedral crystal.
The twelve-membered ring sulfur is fairly stable and has been studied in detail. The molecule of S12 has a three-fold rotational axis and looks like a compressed cuboctahedron. Interestingly, this structure is unique and different from other molecules such as cyclododecane and crown ether.
The twelve-membered ring sulfur (S12) (Wikipedia)
In addition, various sulfur allotropes such as 7, 9, 10, 11, 13, 14, 15, 18, and 20-membered rings have been synthesized.
(4) Additional sulfur allotropes
The boiling point of sulfur is 445 degrees Celsius. The composition of the vapor is S8 near the boiling point, but S2 starts to form at around 750 degrees. The purple-colored gas of diatomic sulfur is, like oxygen, in the state of triplet radical.
Other sulfur allotropes include fibrous and sheet sulfurs. It is said that sulfur has more than 30 allotropes in total. Sulfur does not make nanotube- or graphene-equivalent structures unlike elements such as phosphorus, but instead its allotropes form an impressively diverse chemical space. It is very well possible that new allotropes of sulfur will be discovered in future.
Selenium
Let us touch on selenium, which is positioned below sulfur in the periodic table. Selenium has many allotropes too, and the most stable one at room temperature and under normal pressure is called gray selenium or metallic selenium. In gray selenium, the atoms are bonded to form a long helical structure.
On the other hand, selenium also makes an eight-membered ring like sulfur, and the crystalline form of this is known as α-selenium. Six- and seven-membered ring allotropes and amorphous selenium are also known.
Selenium is, however, highly toxic and the compounds often have a strongly bad smell. It was once used in rectifiers but is diminishing now after having been replaced by silicon. Perhaps for these reasons, the research on selenium is not as active as other elements. Selenium may get more attention if a new unique allotrope is discovered, but the possibility is uncertain.
Germanium
Germanium is below silicon in the periodic table and it takes a diamond-like structure like silicon at ambient temperature and pressure. Having semi-conducting property, this α-germanium used to be found in transistors.
Germanium has another allotrope called β-germanium. When a high pressure (higher than 120 kilobar) is applied to α-germanium, it undergoes phase transition to become β-germanium, which has metallic properties.
In previous columns, we have covered several examples of flat or almost flat two-dimensional honeycomb-structured allotropes of boron, silicon, phosphorus, and other elements. Like graphene, these substances exhibit unique physical properties and are attracting attentions.
It is then natural to wonder whether germanium can do the same. The carbon-based graphene and the silicon-based silicene were synthesized on copper and silver surfaces, respectively. If copper and silver has been used already, why not trying gold next? So, in 2014, when the vapor of germanium was deposited on a gold surface using a method called germanium molecular beam epitaxy, an impressive single-layered “germanene” was obtained. This is one of the new promising materials for which further progress can be expected.
Tin
The element that sits below germanium is tin, which is one of the metallic elements we humans have been using for a long time. There is a famous episode about the allotropes of tin. In 1812, when the Napoleon-led French army went deep into Russia, the soldiers were wearing uniforms equipped with buttons made with tin. And the freezing temperature of the Russian winter caused the tin to undergo phase transition, from metallic β-tin to nonmetallic α-tin. The later has lower density and is weaker, so the buttons crumbled and the soldiers couldn’t protect themselves from the cold.
However, this story is generally considered to be doubtful by historians. The actual temperature needed for the phase transition of tin is -30 degrees Celsius and it takes a long time too, so it was unlikely that even the Russian winter was that harsh. The story was probably made up to explain the nightmarish defeat of the heroic figure.
In the same way as germanium, α-tin is a non-metal with diamond-like structure and β-tin is a metal with tetragonal crystalline structure. However, unlike germanium, the β-tin is the stable form at ambient temperature and pressure. Additionally, the allotropes of tin include γ-tin, which exists at higher than 161 degrees Celsius, and σ-tin, which can exist only under high pressure conditions.
The tin-equivalent of graphene, silicene, and germenene as yet another two-dimensional material had been predicted for some time. In 2015, it was actually synthesized in lab using molecular beam epitaxy, the same way germanene was produced.
The newly obtained “stanene” is expected to possess “perfect conductivity” at room temperature, meaning that it can transfer electricity without wasting any energy as heat. Also, stanene is predicted to react quickly with oxides of nitrogen and sulfur, which could be developed into a technology for the removal of air pollution substances.
The exciting thing about new materials is that they open up a new path that expands the possibility of science. The new allotropes we have just covered will likely trigger further discoveries and push the scientific research to higher levels.
Introduction of the author
Kentaro Sato
[Brief career history] He was born in Ibaraki, Japan, in 1970. 1995 M. Sc. Graduate School of Science and Engineering, Tokyo Institute of Technology. 1995-2007 Researcher in a pharmaceutical company. 2008- Present Freelance science writer. 2009-2012 Project assistant professor of the graduate school of Science, the University of Tokyo. 2014-present Publicist for π-system figuration, scientific research on innovative areas.
[Specialty] Organic chemistry
[Specialty] Organic chemistry