Boron and carbon – better together?

What does it look like?

The ideal arrangement of Boron Carbide, layers of green boron clusters with the black carbon atoms between them.

What is it?

Boron carbide is a material that was discovered by accident in the 19th Century, and is (aside from boron nitride and diamond) one of the hardest materials known. Being cheaper to produce means that it is used for armoring tanks and in bullet-proof vests.

Unlike the super hard materials diamond and boron nitride, it’s crystal structure actually quite complicated. It’s known to have a roughly 4:1 ratio of boron to carbon atoms, but closer examination revealed that in practice there is always less carbon than expected in the structure. This deficiency of carbon atoms leads to some complexities in the crystal structure, which have been debated since the 1940’s. Current thinking is that the structure is actually made up of a mixture of clusters of boron some associated with more carbon and some with less.

Where did the structure come from?

We’ve pictured the ‘ideal’ arrangement of Boron carbide, but you can get an example of how complex the determination of this structure is from entry #2235962 in the Open Crystallography Database.

All of the symmetry – Pm-3 Sr3C60

What does it look like?

Image from the space group project list https://crystalsymmetry.wordpress.com/230-2/

What is it?

Like the structures over the long weekend, today’s is inspired by the space group list project. Today is the last structure in this series, sampling though the list of 230 we arrived at number 200.

Buckyballs were a super discovery, a new form of carbon in the shape of a football. But to make them useful to society relies on understanding how they interact with other atoms. Here we have a structure of buckyballs which have been crystallised with strontium atoms – which reduces their symmetry (they usually form in Fm3m, spacegroup number 226).

Where did the structure come from?

This structure of strontium doped buckyballs came from work by Kortan et al. published in
Chemical Physics Letters in 1994 http://www.sciencedirect.com/science/article/pii/0009261494004951

Tetrahedral amorphous carbon: rough in the diamond

What does it look like:

The image shows a computer simulation of tetrahedral amorphous carbon grown on a substrate (at the right). The red atoms are diamond-like, the blue atoms are graphite-like and the green atoms. In this image the carbon atoms would have impacted the surface from the left hand side with an energy of 70eV (a velocity of about 85km/s-1). The green, blue and red atoms have two, three, and four neighbours respectively, where “neighbor” means “within 1.85A”.

What is it?

It’s another allotrope of carbon, this time tetrahedral amorphous carbon (ta-C). Tetrahedral amorphous carbon is not a natural material. It’s formed by firing carbon atoms at high speed onto a substrate using a plasma device such as a vacuum arc.

An amorphous material is one that has no long term regularly repeating structure: it can have local order- order that persists for a couple of unit cells- but no long range order. A good example of an amorphous material is window glass, which is mostly silicon dioxide (SiO₂) with some other stuff thrown in.

Now it may be a bit of a stretch to include ta-C on a blog about crystals, given that “amorphous” means “it’s not a crystal”. However, if we define crystallography in the most general sense as “what gets talked about at crystallography conferences, even if it’s hidden away in the dingy corner of the venue”, then we can use that as an excuse to talk about a fascinating range of materials.

Amorphous carbon (aC) as a structure type is hardly new. It can be found in natural carbon materials such as coal and Rosalind Franklin did pioneering measurements on aC in the 1950’s.

Even though amorphous materials are disordered (by definition!) you can still say things about their bonding and structure. Naturally occurring aC tends to be graphitic in nature: most of the bonds in the material are graphite-like (planar) sp² bonds, and each carbon is more or less surrounded by three other carbon atoms (like graphite).

In tetrahedral amorphous carbon, the majority of bonds are diamond-like (tetrahedral) sp³ bonds and the majority of the carbon atoms have four nearest neighbours (like diamond). Amorphous carbon can be characterized by the proportion of the two bond types.

In a crystal, we can characterise atomic distances to a certain degree of accuracy. In diamond, the distance between adjacent (“nearest-neighbour”) carbon atoms is 1.54A. This is true throughout the crystal.

In an amorphous material, there’s no longer a consistent, exact distance between atoms. Instead you have to ask the question, “for any given carbon atom, what is the probability I’ll find another carbon atom at a particular radius from that one?” This is called the radial distribution function (RDF). Let’s have a look at the following graph:

The important thing here is the black dots which are experimental data (the other lines are various models). At 1.5A is a broad hump which says that, for any individual carbon, its nearest neighbor is about 1.5A, but really it could be anywhere between 1.3 and 1.7A or so. The next nearest neighbor is about 2.5A. In contrast, you won’t find any carbon atoms that are 2A apart from each other. Finally there are no atoms closer to each other than about 1.2A. Otherwise, that would be called “cold fusion”.

So while the ta-C structure is disordered, it’s not entirely random. Each individual atom looks “a bit diamond-y”, and this idea of “order within chaos” is one of the interesting aspects of amorphous materials.

One of the main progenitors of the current interest in ta-C was a seminal 1991 paper, which introduced the idea of producing thin films of the material using vacuum arc. A vacuum arc is a plasma source that produces a stream of energetic, high speed ions. It’s quite different to conventional plasma sources used to make thin films. So the question arose: “what happens if we whack these high speed ions into a surface?” and the answer is “if you do it with carbon, you get a new allotrope”.

The field blossomed, and in the way of so much physics that started as a “what-if” question, is leading to applications such as this one. One of the fascinating aspects of physics (and arguably science in general) is that it doesn’t neatly divide into “pure” and “applied” research. The field began as a “relatively” pure physics project, albeit in an Applied Physics department. A great deal of the follow up modeling (so essential to understanding What Is Going On) could be called “pure” research. You might like to make some sort of inference from this. I couldn’t possibly comment.

Where does the structure come from?

The images are from the 2005 paper by Nigel Marks published in Diamonds and Related Materials, a journal with a surprisingly large research area with many unresolved scientific questions. For example it has not been resolved whether or not Related Materials are also a girl’s best friend.

A good starting point for amorphous materials is the book “Powder Diffraction: Theory and Practice” edited by Dinnebier and Billinge, which has a nice chapter introducing RDFs, G(r) and all the rest.

A. Marks, “Thin film deposition of tetrahedral amorphous carbon: a molecular dynamics study”, Diamond & Related Materials 14 (2005) 1223–1231

D.R. McKenzie et. al., “Compressive-Stress-Induced Formation of Thin-Film Tetrahedral Amorphous Carbon,” Phys. Rev. Lett., 67 (1991) 773

Fanxin Liu et al., “Released Plasmonic Electric Field of Ultrathin Tetrahedral-Amorphous-Carbon Films Coated Ag Nanoparticles for SERS”, Scientific Reports 4, Article number: 4494, doi:10.1038/srep04494. This one’s open access.

Peter J.F. Harris, “Rosalind Franklin’s work on coal, carbon, and graphite”, Interdisciplinary Science Reviews, 26 (2001) 204

April Birthstone: Unlucky for some…Diamond

Helen Brand outlines a downside to being born in April:

What does it look like?

—This picture was drawn using Diamond structure visualisation software

What is it?

If, like me, you are born in April then you learn to live with the fact that no one is going to give you jewellery made of your birthstone for your birthday. That’s because the birthstone of April is Diamond.

The cost and popularity of diamond has risen steadily since the 19^th Century thanks to improvements in technologies for cutting and finishing these stones as well as innovative advertising campaigns by diamond companies. Coloured diamonds occur naturally, e.g. the famous blue “Hope diamond”, thanks to impurities within the crystal structure. The Hope diamond is blue from boron impurities. As well as in jewellery, diamond is used as an abrasive, coating the edges of drill-bits and saws and other tools.

Images of both uncut (l) and cut (r) diamonds

Diamonds are rare commodities and they form at high pressures and temperatures at around 150 km below the surface of the Earth. They are brought close to the surface by deep volcanic eruptions.

Diamond comes from the Greek word for “unbreakable”, a reference to the extreme hardness of diamonds. Indeed, diamond has the highest hardness and thermal conductivity of any bulk material. The hardness of diamonds is all thanks to the crystal structure. As the structure picture shows, diamond is composed of only C atoms. These carbon atoms are bonded together into tetrahedra. (see below). The covalent carbon bonds in the diamond structure are very strong and require a lot of energy to break them.

 

Carbon tetrahedra in diamond

Synthetic diamonds have long been in demand for industrial applications of diamonds for tools. In the 1940s work began in earnest to try and develop a synthetic method of producing diamonds. In 1953 they succeeded and the first reproducible synthesis was designed. This synthesis can produce both industrial grade diamonds and gem-quality diamonds. In some cases, even a gem expert cannot tell the difference by eye between a natural and synthetic diamond!

Where did the structure come from?

Diamond was one of the first structures to be described by WL and WH Bragg, but the structure has been worked on alot subsequently.  The carbon-carbon bonds in diamond were described in 1944 by Riley and published in Nature but this particular structure was published by Wyckoff in 1963.

Rigidity in carbon and hydrogen – Adamantane

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

Adamantane is made of just two atoms, carbon (black) and hydrogen (white). Each adamantane molecule is constructed from four cyclohexane rings. 

What is it?

Adamantane is a member of the cycloalkane family of hydrocarbons. Unlike many members of the cycloalkanes its molecular structure is very stable but also very rigid.

This structural rigidity is a result of the carbon-carbon bonds which adopt the same arrangement as those found in diamond. This resemblance is the root of the name, which is derived from Greek adamantinos meaning ‘of diamond’.

The molecular structure of adamantane was first suggested by H. Decker in 1924, much to his surprise it had not yet been synthesised. (H 1924) The synthesis was attempted by several notable figures in organic chemistry and it was V. Prelog in 1941, building on the pioneering work of H. Meerwein, who finally succeeded.

Adamantane is one of the archetypal diamondoids; molecules that resemble diamond but typically of a fixed size or with other atoms substituting carbon. These structures are attractive possible building blocks for the construction of nanotechnology components; although due to their small size they remain relatively underused by jewellers.

Where did it come from?

The structure of adamantane was first determined by C. E. Nordman and D. J. Schmitkons in 1965, the data used in this article comes from work by J. P. Amoureux and M. Foulon. (P. and M. 1987) (E and J 1965)

Bibliography

E, Nordman C, and Schmitkons D J. Acta Crystallogr 18 (1965): 764.

H, Decker. “Versammlung deutscher Naturforscher und Ärzte. Innsbruck, 21–27 September 1924.” Angew. Chem. 37, no. 41 (1924): 795.

P., Amoureux J., and Foulon M. Acta Crystallogr 43 (1987): 470.

 

Kathleen Lonsdale and her mineral

What does it look like?

Crystal structure of lonsdaleite (hexagonal diamond) by Materialscientist

What is it?

Today is the birthday of one of the pioneers of crystallography, Kathleen Lonsdale (1903 to 1971).  Born in Ireland, she studied under WH Bragg at the Royal Institution and determined a number of fundamental structures – including the first determination of a molecular structure, hexamethylbenzene.

She was also very interested in the forms different form carbon and how diamonds could be synthesised, writing this review of the topic in Nature.  As a result, Lonsdaleite, which is another form of carbon discovered in 1967, was named after her.  Otherwise known as hexagonal diamond, Lonsdaleite, is very rare an only found naturally in meteorites.  It can be made in the laboratory, but only by subjecting carbon (in the form of graphite) to extreme high temperatures, and shock pressures – often with explosives.

There has been excitement that Lonsdaleite and a material that is structured (Wurtzite boron nitride) like it, could be tougher than the ‘normal’ diamond.   But although both Lonsdaleite and Wurtzite boron nitride are found naturally and can be made in the lab – we still haven’t made enough of them to test if they are stronger than diamond.  A challenge for future scientists?

Where did the structure come from?

The structure of Lonsdaleite was first reported by Bundy and Kasper in 1967, its structure is available on the American Mineralogical Database.

Buckminsterfullerene a.k.a. Buckyballs a.k.a. C60

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

What looks like a soccer ball but has a lot more in common with a diamond? That would be buckminsterfullerene, the carbon structure shown above. With a chemical formula of C60 and a catchy nickname (the bucky-ball) this carbon structure is made up of twenty hexagons and twelve pentagons, which form a truncated icosahedron. Since buckyballs are empty in the middle, they can be used as “atomic shrink wrap”, where a different atom is placed in the center of the C60 cage, which can then be “shrunk” by laser pulses to trap the atom. Like graphite, buckminsterfullerene is a sort mater, however, under pressure it transforms into a superhard structure which can even indent diamonds.

Where did the structure come from?

Buckyballs were first synthesized in the 1980’s by Robert Curl, Harold Kroto, and Richard Smalley. The structure was named after architect Buckminster Fuller, who was well-known for designing geodesic domes. Their work was awarded the 1996 Nobel Prize in Chemistry. The structure is #9011073 on the Crystallography Open Database.

Graphite – the makings of the future?

What does it look like?

Image generated by the VESTA (Visualisation for Electronic and STructual analysis) software http://jp-minerals.org/vesta/en/

What is it?

It’s what our ancestors (and some of us still) used to write with, and is only composed of one element carbon, but a feature of the structure of graphite could be shaping all of our futures.  It’s the more common form of carbon, and is made up of hexagonal sheets.   It is often classed a ‘semi metal’ as large lumps of it are quite metallic-looking and, more critically, it can conduct electricity along the layers.

The sheets in graphite are only loosely connected together, which means that they can be prised apart.  Perhaps the most successful pair to do this was Andre Geim and Kostya Novoselov, who used sticky tape to pull apart the layers of graphite.  What they created doing this was ‘Graphene’, and delved into a new and exciting world of two dimensional materials.  For this work they won the 2010 Nobel Prize in physics, which honored their ‘door opening’ of a brand new world of strange and exotic properties that other scientists could explore.    We’re yet to see everyday applications of graphene, but they are coming.

Where did the structure come from?

The structure of graphite was pondered for many years, with some incorrent structures published.  The hexagonal structure was revealed by Lipson and Stokes in 1942, it is structure #120018 in the open crystallography database.