Researching Something That Does Not Exist

Chemistry, as you probably already know, is that branch of the natural sciences that deals with the study of matter. Researchers within this field are looking at the properties of individual atoms, how they bond to one other, how they interact and react, and how they are affected by other forces. At CAS Oslo, the research group Molecules in Extreme Environments has taken this to the next level.

Making matter from single atoms

Elke Pahl and Krista G. Steenbergen, faculty members at Massey University Auckland, New Zealand, are participating in the project. They are working with theoretical and computational modeling of atoms to get an understanding of what a substance, or a ‘bulk solid,’ made up of billions of such atoms would look like.

‘When we have information about an atom’s composition and properties, we get an idea of how it would interact and react with other atoms. With information from a couple of observations of one atom, we can model how a billion of them would bond and form matter.’ Steenbergen explains.

One of the elements Pahl and Steenbergen have been working with lately, together with CAS Oslo fellow Peter Schwerdtfeger, is element number 112, Copernicium (Cn) -- more specifically the isotope 283Cn.

Copernicium is a ‘superheavy’ element, meaning it has a large number of protons in its nucleus. All superheavy elements are highly unstable and only exists briefly under highly controlled circumstances here on Earth. In fact, scientists have never been able to observe more than one atom of Copernicium at a time.

‘Copernicium does not occur naturally here on Earth, and we have yet to discover it elsewhere in the universe,’ Steenbergen says. ‘This is not to say that Copernicium does not or cannot exist at all, only that the conditions under which Copernicium would be stable are not here on Earth, but could be, as an example, inside a star where it is hard for us to detect.’

Creating atoms through collisions

Copernicium is created in a laboratory by crashing one atom into another. In the case of 283Cn, a heavy calcium atom (48Ca) is accelerated to an extremely high speed in a large cyclotron before being slammed into a plutonium atom (242Pu).

‘When the calcium hits the plutonium target, they merge and become element 114, Flerovium. This is another ‘superheavy’ element, which decays after half a second,’ Pahl explains. ‘When it decays, it emits an alpha particle, which is two protons and two neutrons, from its nucleus and becomes Copernicium.’

These experiments were carried out at Joint Institute of Nuclear Research in Dubna, Russia, by a team of experimental physicists.

‘But it does not work every time,’ Steenbergen adds. ‘You need a bit of luck as well when you are hoping to create an element that does not exist naturally by forcing two atoms to become one and hoping it decays in a specific way.’

As of today, 283Cn has been successfully created and examined only six times.

Energy and patterns of a solid

283Cn is not a particularly stable isotope, and it decays within four seconds. This means that researchers have very limited time to study the atom before it turns into another element.

‘The experimental physicists hurry to transfer the atom they hope is Copernicium from the chamber it was created, down a capillary, and into a chamber with an array of gold-coated detectors set along a temperature gradient,’ Steenbergen explains. ‘The temperature at which the atom lands gives researchers an idea of how favorable it is for the atom to bond with gold, which we refer to as its adsorption enthalpy.’

‘This kind of testing has been done with a lot of nonsuperheavy elements earlier, and researchers have found that where the atom lands on the gold detectors (i.e., adsorption enthalpy) is mostly linearly related to the element’s cohesive energy,’ Pahl adds.

Cohesive energy (Ecoh) is the amount of energy that would be released if a bulk solid was completely vaporized into individual atoms. Elements that exist as metals at room temperature and atmospheric pressure, like iron (Fe), are very strongly bound (i.e., higher Ecoh), while gases, like Radon (Rn), are more weakly bound.

A bulk solid’s cohesive energy is integrally tied to the pattern in which the atoms bond together. This pattern is called a lattice structure, and is repeated over and over again in the substance. Different atoms bond differently to each other, creating different patterns with different ‘strengths.’

‘As theoreticians, we can actually model how much cohesive energy an element would have if it had different lattice structures.’ Steenbergen tells. ‘Zinc, which usually has a ‘hexagonal close-packed’ (hcp) lattice, we can model with a rhombohedral lattice. In this instance, the hcp lattice has more energy, meaning it is more stable.’

For Copernicium, both the cohesive energy and lattice structure has long been unknown.

From observations to simulations

‘From the observations the experimental physicists did, they estimated that the cohesive energy of a bulk solid of Copernicium would be 0.39, plus/minus 0.1 eV.’ Steenbergen says, adding: ‘Note the large error bars. It is 25 % of the total cohesive energy.’

The error bars, Steenbergen says, are so large because only six atoms have been created in a cyclotron experiment where they have measured the adsorption enthalpy on gold.

‘Four of the atoms landed on the gold array within the same temperature range, while one landed on a higher temperature and another on a lower,’ Steenbergen explains. ‘Because of this, the experimental physicists could not give a definite conclusion.’

The experiments necessary to create Copernicium are very expensive, and they do not guarantee the creation of a Cn-atom. The experimental physicists therefore enlisted the help of Pahl and Steenbergen’s team to see if they could validate their findings.

Pahl and Steenbergen used highly sophisticated quantum chemical methods in order to model the lattice structure and cohesive energy of bulk solid Copernicium computationally.

Their findings show that the cohesive energy of Copernicium would be between 0.376-0.410 eV, and that it is most likely to have an hcp lattice structure.

‘We maxed out computational time, processors, and memory, and had to introduce some approximations in order to make the study feasible, but we managed to highly validate the experimental cohesive energy,’ Steenbergen says.

‘We also figured out that Copernicium would most definitively be a liquid, if not even a gas, under earthly conditions if it were stable enough to exist,’ Pahl adds.

Understanding the universe

Even though we will most likely never benefit from Copernicium itself here on Earth, the research on the element has given us new knowledge to add to the periodic table.

‘It is interesting for us to see the continuation of trends within groups of elements, or deviations,’ Pahl says. ‘To be able to add more elements to a group and to the periodic table, they have to be discovered. As superheavy elements do not occur naturally here on Earth, we can only observe and study them through experiments and simulations.’

Validating experimental results through computational simulations can also verify research methods. Both experimental physicists and computational chemists can then be more confident that their method will lead to correct results in future experiments and simulations.

‘With our simulations, we can see what would happen to all kind of matter under different conditions. As part of the CAS Oslo project, we are looking at the effects of extreme pressure on different kinds of elements,’ Pahl says. ‘Hopefully, our results can help astronomers better understand the universe.’

By Camilla Kottum Elmar

Update: Their findings were pubished in Physical Chemistry Chemical Physics. You can read their article, entitled The cohesive energy of superheavy element copernicium determined from accurate relativistic coupled-cluster theory, here: 

Phys. Chem. Chem. Phys., 2017, 19, 32286-32295.