The diamond is a fascinating material, to some degree “the king of crystals”. Not only is it the most expensive gem and the hardest known material but it is also part of a family of crystalline structures formed exclusively by carbon atoms and with properties that are as amazing and valuable as the diamond itself. This family of polymorphs – crystals with the same chemical composition but different structures – includes graphite and also the fullerenes, carbon nanotubes and graphene. All of them are different forms of ordering carbon atoms, and some of them can play a very important role in the history of humanity.
Do you know what the relationship is between the different carbon polymorphs? Have you ever wondered how the quality of a diamond is assessed? Did you know that a square-metre two-dimensional sheet of graphene is capable of supporting four kilograms in weight?
The importance of the crystalline structure of a compound is crucial for its properties and there is nothing better than diamond and graphite to demonstrate this. Both are pieces of carbon – that is, they are both formed by carbon atoms. However, one is a cheap material that we use for, among other things, drawing: graphite is the material that nowadays is the lead of our pencils. Whereas diamond is the most prized, unbreakable as per the etymology of its name – adámas – the hardest known material, the mineral that none other is capable of scratching.
|Photograph of brilliant-cut diamond
Photo: Mario Sarto
Evocative picture of the crystallography of a pencil. From the “lead” other pencils are born that look like hexagonal crystals of graphite.
Photo: Christopher White
The difference between diamond and graphite is the structure, and hence in the form in which the carbon atoms are bound. In the diamond this structure is extremely compact. For crystallographers, it belongs to the cubic system. In this structure all the carbon atoms are linked by strong covalent bonds, which makes it a very hard material, the hardest known.
Cubic structure of diamond in which all the carbon atoms are united between each other by strong covalent bonds.
The structure of graphite is very different to diamond. In graphite, the carbon atoms are distributed in layers. Within each layer, each carbon atom is bonded to another three, forming hexagons. These three bonds are strong covalent bonds of the same type as those of the structure of diamond. But between each layer, the carbon atoms are bonded more weakly, by what are known as Van der Waals forces.
Lamellar structure of graphite. The atoms in the layers are joined by strong covalent bonds but the forces that join the carbon atoms between layers are weak Van der Waals forces.
This structure is ideal for understanding the concept of anisotropy in crystals – in other words, the variation of crystal’s properties with the direction in which it is measured. The cohesion of the atoms in any of the directions contained in the layers of hexagons is extraordinary, making it very difficult to separate them from each other. But the cohesion between layers, that is, in any direction outside of the hexagonal layers, is much weaker. The layers separate very easily, so much so that if we apply pressure with a graphite crystal upon paper the layers separate and remain stuck to the paper. This is the graphite pencil.
As well as graphite and diamond, carbon atoms can form other structures that have great scientific and technological importance. A few years ago two Russian researchers, Andre Geim and Konstantin Novoselov, were playing about separating the hexagonal sheets of graphite with sticky tape. They wanted to see if they could manage to separate just one single sheet. With patience, they succeeded, and they gave it the name of graphene. These layers of hexagonally distributed carbon atoms have surprising properties and these are the focus of much expectation for future technologies.
The discovery of graphene won the researchers the Nobel Prize. If you want to learn more about graphene, we recommend the Nobel Committee’s website, where you will find both basic and advanced information on graphene.
You could say that graphene is a two-dimensional diamond. In other words, it is an ultrafine material with the properties of diamond: extremely hard, a better electrical conductor than copper, better heat conductor than any other material, so dense that no gas can pass through it, and it is also flexible.
|Model of the structure of graphene.|
Other interesting structures that carbon atoms form are fullerenes and nanotubes.
Diamond is the jewel that has been most resistant to synthesising, but how to do so has been known since the second half of the last century. At first, methods of very high pressure were used but today methods that combine high temperatures, a certain pressure, and the use of a seed upon which the growth is started using carbon dissolved in a flux. If you want to know how synthetic diamonds are manufactured in the United States then we recommend the following video:
Although when diamonds are talked about everybody thinks of the hugely expensive jewels, diamond crystals have many other applications, such as abrasives, and for cutting tools. Most synthetic diamonds are very small and are used in industry. But recently jewel-quality diamonds of several karats in size have been produced, for use in jewellery. The technique has been developed particularly in Russia, in the Novosibirsk technological area, but it is already very widespread.
Although it may seem incredible, diamonds can be – and are – industrially manufactured at atmospheric pressure. The discoverers of the method, some Russian scientists, were considered dreamers, as it was believed that it was only possible to make diamonds at high temperatures and pressures. But they were right, and today diamonds are made from methane with the aid of microwaves that create an atmosphere of hydrogen plasma. In reality, little diamond is produced and a lot of graphite, but the hydrogen dissolves the graphite leaving extra-fine films a few hundred microns thick. These films are used in the making of speakers, in coatings for covering supermaterials with a chemical and mechanically resistant layer. Diamond is an excellent heat conductor, hence the effort to create semiconductor diamonds as an alternative to silicon.