Researchers Study Structures of Materials to Understand Elemental Behavior

Researchers are studying the motion of over 500 atoms, to find out the forces on each atom and the total energy through density functional calculations. To achieve this end the team is running structural studies with numerical simulations on a supercomputer.

The researchers are using this supercomputer, JUQUEEN, in Jülich, Germany, to run extensive numerical simulations. (Credit: Forschungszentrum Jülich)

As the great British scientist Francis Crick said, "If you want to understand function, study structure."

The two forms of carbon – graphite and diamond – are clear examples of this in the field of chemical physics. While graphite and diamond only differ in the atomic arrangement of atoms of a single element, there is considerable difference in their properties.

Differences in the properties of near-similar elements belonging to a “family” can be very interesting. Silicon, tin, carbon, lead, and germanium all belong to a family that have the same structure of outermost electrons, but act as semiconductors (germanium and silicon), insulators (carbon) and metals (lead and tin).

Can one gain a better insight into these and other tendencies in elemental families? A team of researchers from Tampere University of Technology, Aalto University in Finland and Peter Grünberg Institute (PGI) in Germany have reported their study of the relationship between the function (physical properties) and structure (arrangement of atoms) of a liquid metal form of bismuth. The details of the study have been published in The Journal of Chemical Physics, from AIP Publishing.

There are relatively few -- less than 100 -- stable elements, which means that their trends are often easier to discern than for those of alloys and compounds of several elements.

Robert O. Jones, a scientist at PGI

The major motivation for the team’s project was the high-quality experimental data on neutron diffraction and inelastic x-ray scattering (IXS) that was available, and the opportunity it presented for comparison with results for other Group 15 nitrogen family liquids such as arsenic, bismuth, antimony and phosphorus. There seems to be two liquid phases of phosphorus, and cooling liquid antimony results in the formation of its amorphous form, which crystallizes immediately and explosively.

Their structures were studied using extensive numerical simulations that were run on one of the most powerful supercomputers in the world – JUQUEEN, in Jülich, Germany.

We're studying the motion of more than 500 atoms at specified temperatures to determine the forces on each atom and the total energy using density functional calculations. This scheme, for which Walter Kohn was awarded the 1998 Nobel Prize in chemistry, doesn't involve adjustable parameters and has given valuable predictions in many contexts.

[The velocities and positions of each atom, for instance, are] stored at each step of a 'molecular dynamics' simulation, and we use this information to determine quantities that can be compared with experiment. It's important to note that some quantities that are given directly by the simulation, such as the positions of the atoms, can only be inferred indirectly from the experiment, so that the two aspects are truly complementary.

One of the team’s more unexpected yet pleasing results was the excellent agreement with recent IXS results. One of the experimentalists involved noted that the agreement of our results with the IXS 'is really quite beautiful,' so that even small differences could provide additional information. In our experience, it's unusual to find such detailed agreement."

The team’s work provides further confirmation that simulations and experiments complement each other and that the level of agreement can be remarkably good -- even for 'real' materials. However, it also shows that extensive, expensive, and time-consuming simulations are essential if detailed agreement is to be achieved.

To understand the “explosive” nature of crystallization in amorphous antimony (Sb), Jones and team have expanded their approach to yet longer simulations in liquid antimony at eight different temperatures.

"We've also run simulations of the crystallization of amorphous phase change materials over the timescale -- up to 8 nanoseconds -- that is physically relevant for DWD-RW and other optical storage materials," he added, emphasizing that these types of simulations on computers today typically require many months. "They show, however, just how valuable they can be, and the prospects with coming generations of computers -- with even better optimized algorithms -- are very bright."

There are great potential prospects for applications in other fields of materials science, but the researchers are currently looking at memory materials of a different form, where the formation and deformation of a metallic filament or a conducting bridge between two electrodes in a solid electrolyte can be the foundation of storage materials in the future.

"Details of the mechanism of bridge formation are the subject of speculation, and we hope to provide insight into what really happens," Jones said.

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