Researchers from Fudan University and the University of Shanghai have finally uncovered the science behind a phenomenon that has left many baffled: when subjected to somewhat high pressures, graphite transforms into hexagonal diamond and not the more precious cubic diamond, despite what theory predicts. The mystery, according to the team, lies in the speed of the reaction, also known as reaction kinetics.
With the help of a new and advanced type of simulation, the scientists have managed to identify the specific pathways in the graphite-to-diamond reaction that use the lowest amount of energy. As revealed during the study, the reason behind this strange behavior is actually quite simple: the transition from graphite to hexagonal diamond takes place nearly 40 times faster than the change to cubic diamond.
The latter usually occurs at higher pressures, though even then a large quantity of hexagonal diamond invariably finds its way into the final product. Speaking about the findings, recently published in the Journal of American Chemical Society, Zhi-Pan Liu of Fudan University said:
This work resolves the long-standing puzzle of why hexagonal diamond is preferentially produced from graphite instead of the cubic diamond at the onset of diamond formation. Considering that graphite-to-diamond is a prototype solid-to-solid transition, the knowledge learned from this work should greatly benefit the understanding of high-pressure solid physics and chemistry.
Although all of them are allotropes of carbon, the primary difference between graphite, hexagonal diamond and cubic diamond lies in the way their atoms are arranged. Graphite, for instance, is made up of multiple layers of graphene containing honeycomb-shaped lattices. Because atoms in graphene have less than four bonds, graphite possesses a general flakiness that makes it ideal as lead for pencil.
In case of diamond, both hexagonal and cubic, each of the constituent carbon atoms is fully-bonded; a feature that in turn accords it its characteristic brittleness. Cubic diamond (the type commonly used in jewelry) is comprised of layers that are arranged in the same direction. By contrast, hexagonal diamond boasts what is known as hexagonal symmetry, in which two consecutive layers are oriented in opposite directions.
When subjected to pressures greater than 20 gigapascals, graphite changes into cubic diamond with few traces of hexagonal diamond present in it. As explained by the researchers, this is one point where both theory and experiment meet. The complication arises when the pressure is less than 20 gigapascals. Under such conditions, computer simulations have long predicted that cubic diamond would be the final product.
The reality, however, is quite different. The disparity, the team believes, is because simulations wrongly assume that formation of cubic diamond nucleus requires less energy than the other kind. What they fail to take into account is the strain energy that is usually associated with a mismatch of orientation between the two interfaces: graphite and diamond.
For the new research, the team relied on a new simulation technique called stochastic surface walking. This in turn allowed them to analyze all combinations of interfaces, identifying the seven interfaces with the lowest-energy pathways. So far, the experiments have revealed that the transformation from graphite to hexagonal diamond is significantly more stable and less strained than the transition to the more valuable type of diamond.
The scientists are currently trying to enhance the simulation technology by integrating features of neural networks. It is important to note that while cubic diamond may appear more precious and desirable to us, both kinds have their own set of unique properties and advantages. Liu added:
While cubic diamond is familiar in everyday life and is a highly useful material, hexagonal diamond could also be very useful. For example, it was predicted by theory to be even harder than cubic diamond. While the hexagonal diamond (lonsdaleite) can be found in meteorites, the production of large hexagonal diamond crystals has not been achieved in experiment. One would therefore expect that large hexagonal diamond crystals, if produced, would be even more precious than cubic diamond.