Une équipe de recherche internationale a acquis de nouvelles connaissances sur les propriétés chimiques de l'élément superlourd flerovium - l'élément 114 - dans les installations de l'accélérateur du GSI Helmholtzzentrum für Schwerionenforschung à Darmstadt. Les mesures montrent que le flerovium est le métal le plus volatil du tableau périodique. Flerovium est donc l'élément le plus lourd du tableau périodique qui a été étudié chimiquement.

Avec les résultats, publiés dans la revue Frontiers in Chemistry , le GSI contribue à l'étude de la chimie des éléments superlourds et ouvre de nouvelles perspectives pour l'installation internationale FAIR (Facility for Antiproton and Ion Research), actuellement en construction.

Sous la direction de groupes de Darmstadt et de Mayence, les deux isotopes de flerovium à durée de vie la plus longue actuellement connus, le flerovium-288 et le flerovium-289, ont été produits à l'aide des installations de l'accélérateur du GSI/FAIR et ont fait l'objet d'études chimiques dans le dispositif expérimental TASCA. Dans le tableau périodique, le flerovium est placé sous le plomb, un métal lourd.

Cependant, les premières prédictions avaient postulé que les effets relativistes de la charge élevée dans le noyau de l'élément superlourd sur ses électrons de valence conduiraient à un comportement de type gaz noble, tandis que les plus récentes avaient plutôt suggéré un comportement faiblement métallique. Deux expériences de chimie menées précédemment, dont l'une au GSI à Darmstadt en 2009, ont conduit à des interprétations contradictoires.

Alors que les trois atomes observés dans la première expérience ont été utilisés pour déduire un comportement de type gaz rare, les données obtenues au GSI ont indiqué un caractère métallique basé sur deux atomes. Les deux expériences n'ont pas permis d'établir clairement le caractère. Les nouveaux résultats montrent que, comme prévu, le flerovium est inerte mais capable de former des liaisons chimiques plus fortes que les gaz nobles, si les conditions le permettent. Le flerovium est par conséquent le métal le plus volatil du tableau périodique.

Le flerovium est donc l'élément chimique le plus lourd dont le caractère a été étudié expérimentalement. Avec la détermination des propriétés chimiques, GSI/FAIR confirme sa position de leader dans la recherche des éléments superlourds.

"L'exploration des frontières du tableau périodique a été un pilier du programme de recherche au GSI depuis le début et le sera à FAIR dans le futur. Le fait que quelques atomes puissent déjà être utilisés pour explorer les premières propriétés chimiques fondamentales, donnant une indication de la façon dont de plus grandes quantités de ces substances se comporteraient, est fascinante et possible grâce à la puissante installation d'accélérateur et à l'expertise de la collaboration mondiale », déclare le professeur Paolo Giubellino, directeur général scientifique de GSI et FAIR. "Avec FAIR, nous faisons entrer l'univers dans le laboratoire et explorons les limites de la matière, mais aussi des éléments chimiques."

Six semaines d'expérimentation

Les expériences menées au GSI/FAIR pour clarifier la nature chimique du flerovium ont duré au total six semaines. For this purpose, four trillion calcium-48 ions were accelerated to ten percent of the speed of light every second by the GSI linear accelerator UNILAC and fired at a target containing plutonium-244, resulting in the formation of a few flerovium atoms per day.

The formed flerovium atoms recoiled from the target into the gas-filled separator TASCA. In its magnetic field, the formed isotopes, flerovium-288 and flerovium-289, which have lifetimes on the order of a second, were separated from the intense calcium ion beam and from byproducts of the nuclear reaction. They penetrated a thin film, thus entering the chemistry apparatus, where they were stopped in a helium/argon gas mixture.

This gas mixture flushed the atoms into the COMPACT gas chromatography apparatus, where they first came into contact with silicon oxide surfaces. If the bond to silicon oxide was too weak, the atoms were transported further, over gold surfaces—first those kept at room temperature, and then over increasingly colder ones, down to about –160 °C.

The surfaces were deposited as a thin coating on special nuclear radiation detectors, which registered individual atoms by spatially resolved detection of the radioactive decay. Since the decay products undergo further radioactive decay after a short lifetime, each atom leaves a characteristic signature of several events from which the presence of a flerovium atom can unambiguously be inferred.

One atom per week for chemistry

"Thanks to the combination of the TASCA separator, the chemical separation and the detection of the radioactive decays, as well as the technical development of the gas chromatography apparatus since the first experiment, we have succeeded in increasing the efficiency and reducing the time required for the chemical separation to such an extent that we were able to observe one flerovium atom every week," explains Dr. Alexander Yakushev of GSI/FAIR, the spokesperson for the international experiment collaboration.

Six such decay chains were found in the data analysis. Since the setup is similar to that of the first GSI experiment, the newly obtained data could be combined with the two atoms observed at that time and analyzed together.

None of the decay chains appeared within the range of the silicon oxide-coated detector, indicating that flerovium does not form a substantial bond with silicon oxide. Instead, all were transported with the gas into the gold-coated portion of the apparatus within less than a tenth of a second.

The eight events formed two zones:a first in the region of the gold surface at room temperature, and a second in the later part of the chromatograph, at temperatures so low that a very thin layer of ice covered the gold, so that adsorption occurred on ice.

From experiments with lead, mercury and radon atoms, which served as representatives of heavy metals, weakly reactive metals as well as noble gases, it was known that lead forms a strong bond with silicon oxide, while mercury reaches the gold detector. Radon even flies over the first part of the gold detector at room temperature and is only partially retained at the lowest temperatures. Flerovium results could be compared with this behavior.

Apparently, two types of interaction of a flerovium species with the gold surface were observed. The deposition on gold at room temperature indicates the formation of a relatively strong chemical bond, which does not occur in noble gases. On the other hand, some of the atoms appear never to have had the opportunity to form such bonds and have been transported over long distances of the gold surface, down to the lowest temperatures.

This detector range represents a trap for all elemental species. This complicated behavior can be explained by the morphology of the gold surface:it consists of small gold clusters, at the boundaries of which very reactive sites occur, apparently allowing the flerovium to bond. The fact that some of the flerovium atoms were able to reach the cold region indicates that only the atoms that encountered such sites formed a bond, unlike mercury, which was retained on gold in any case.

Thus, the chemical reactivity of flerovium is weaker than that of the volatile metal mercury. The current data cannot completely rule out the possibility that the first deposition zone on gold at room temperature is due to the formation of flerovium molecules. It also follows from this hypothesis, though, that flerovium is chemically more reactive than a noble gas element.

The exotic plutonium target material for the production of the flerovium was provided in part by Lawrence Livermore National Laboratory (LLNL), U.S.. In the Department of Chemistry's TRIGA site at Johannes Gutenberg University Mainz (JGU), the material was electrolytically deposited onto thin titanium foils fabricated at GSI/FAIR.

"There is not much of this material available in the world, and we are fortunate to have been able to use it for these experiments that would not otherwise be possible," said Dr. Dawn Shaughnessy, head of the Nuclear and Chemical Sciences Division at LLNL. "This international collaboration brings together skills and expertise from around the world to solve difficult scientific problems and answer long-standing questions, such as the chemical properties of flerovium."

"Our accelerator experiment was complemented by a detailed study of the detector surface in collaboration with several GSI departments as well as the Department of Chemistry and the Institute of Physics at JGU. This has proven to be key to understanding the chemical character of flerovium. As a result, the data from the two earlier experiments are now understandable and compatible with our new conclusions," says Christoph Düllmann, professor of nuclear chemistry at JGU and head of the research groups at GSI and at the Helmholtz Institute Mainz (HIM), a collaboration between GSI and JGU.

How the relativistic effects affect its neighbors, the elements nihonium (element 113) and moscovium (element 115), which have also only been officially recognized in recent years, is the subject of subsequent experiments. Initial data have already been obtained as part of the FAIR Phase 0 program at GSI. Furthermore, the researchers expect that significantly more stable isotopes of flerovium exist, but these have not yet been found. However, the researchers now already know that they can expect to find a metallic element. + Explorer plus loin

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