How Fullerite Becomes Harder Than Diamond, So Hard It Scratches Diamond

Alexander Kvashnin, Candidate of Physics and Mathematics, the main author, remarked, “When we started to discuss this idea, I was working at TISNCM. There, in 1998, a group of scientists headed by Vladimir D. Blank obtained a new material based on fullerenes — ultrahard fullerite, or «tisnumit». According to the measurements, this new material could scratch diamond, that is, it was in fact harder than diamond.”

Fig.: (left) molecule of fullerene, (middle) fullerite, (right) polymerized fullerite (SH-phase)

Credit: Moscow Institute of Physics and Technology

The obtained substance was not single crystal material; it contained amorphous carbon and 3D-polymerized molecules of С60. Still, its crystal structure is not completely determined. The fullerene molecule has excellent mechanical rigidity. At the same time, the fullerite crystal is quite a soft material under normal conditions, but becomes harder than diamond under pressure (due to the 3D polymerization). Although this material has been synthesized and studied for more than 20 years now, the reason why it becomes ultrahard is still unknown. There is a number of models that have been developed to explain how fullerenes can be polymerized into fullerite.

One of the models was proposed by Prof. Leonid A. Chernozatonskii. The X-ray diffraction pattern of the model perfectly agrees with experimental data and should have high volumetric bulk modulus, several times higher than the diamond value. But the relaxed structure of the model does not display such fascinating properties.

Alexander Kvashnin remarks, “We based our analysis on that model and the experimentally known fact that if you apply high pressure, more than 10 GPa to fullerene powder, and heat it above 1800 К, you obtain a polycrystalline diamond. The idea was to combine these two facts. On the one hand, a super-hard fullerite material, and on the other hand, under pressure, fullerenes turn into a polycrystalline diamond.”

The scientists suggested that under pressure, part of the fullerite turned into diamond, while the other part remained as fullerite, but in a compressed state within the diamond. To simplify the model, the fullerite crystal structure proposed by Prof. Chernozatonskii was taken and placed inside a single crystal diamond. Then this composite material was studied. The idea was that fullerite inside diamond should be compressed. It is known that in the compressed state, the elastic and mechanical properties of the material would increase. And diamond would act as a shell, which keeps the compressed fullerite inside to preserve all those properties. 

“The developed model will help us to understand the nature of its unique properties and to help to systematically synthesize the new ultra-hard carbon materials, as well as to contribute to the further development of this promising field of science”, said Pavel Sorokin, Doctor of Physical and Mathematical Science, head of the project (TISNCM, MISIS, MIPT).

Fullerite itself is not very hard; its bulk modulus is 1.5 times less than that of diamond. But when it’s compressed, its bulk modulus increases dramatically. To preserve this enhanced bulk modulus, the fullerite should always remain in such a compressed state. Using the results of simulations, the scientists can conduct targeted experiments to obtain an ultra-hard material.

Contacts and sources:
Ilyana Shaybakova
Moscow Institute of Physics and Technology