Electronic structure of uranium compounds is revealed

Laboratory experiment performed in Brazil elucidates how outermost layers of radioactive elements known as actinides are ordered and represents new step in search for room-temperature superconductors.

By José Tadeu Arantes  |  Agência FAPESP – The actinides are a group of 15 radioactive elements that are part of the seventh row of the periodic table. This means their atoms have electrons in all seven possible energy levels. In ascending order of atomic numbers, they start with actinium (89 protons and 89 electrons) and end with lawrentium (103 protons and 103 electrons). Uranium (92) and thorium (90) are the most abundant actinides in the Earth’s crust. 

Generally speaking, all macroscopic properties of materials – and hence their technological applications – depend on electron distribution in the outermost layers of their atoms. This is what determines whether a material is malleable or brittle, how well it conducts electricity, and whether it responds strongly or weakly to a magnetic field.

The outermost orbitals can also combine to form hybrid orbitals with different shapes and energy levels. Orbital hybridization modifies the properties of atoms, how they bond to other atoms, and even the structure of the molecules formed.

An experimental and theoretical study conducted at the National Synchrotron Light Laboratory (LNLS) used X-rays to investigate the configuration of the outermost orbitals and their hybridization in uranium compounds, providing new knowledge about actinides. The properties of these materials have remained relatively unknown compared with those of lighter elements owing to the difficulty of handling them safely. This has changed thanks to the new study.

An article on the study by Narcizo Marques de Souza Neto, first author Ricardo dos Reis and collaborators in Brazil and abroad has been published in the journal Nature Communications. Souza Neto designed the study and acted as principal investigator.

“Because actinides are heavy elements with many electrons, they display a unique energy distribution in their last orbitals, 5f and 6d. Our main contribution was to establish experimental techniques capable of probing these outer levels. We succeeded in selectively observing the properties of layers 5f and 6d, as well as their hybridization. Previously, this hadn’t been possible. Most of these materials’ macroscopic properties are due to interactions between levels 5f and 6d,” Souza Neto told Agência FAPESP.

The LNLS group used a technique called X-ray magnetic circular dichroism (XMCD), with relatively high-energy X-rays in the range of 17 keV, to investigate orbitals 5f and 6d and their hybridization in uranium compounds.

The XMCD technique is based on the differences between two X-ray absorption spectra in a magnetic field, one with left-polarized light and the other with right-polarized light. An analysis of these differences yields a great deal of information about the properties of the atom concerned.

“This experimental technique was previously used to study other materials but had never been applied to actinides because of various operational difficulties,” Souza Neto said.

The study was supported by FAPESP through its Young Investigator Grant Program and Multiuser Equipment Program, as well as a PhD scholarship and a Scientific Initiation scholarship.

Superconductors at room temperature

The LNLS group worked with two silicate compounds: uranium-manganese and uranium-copper. The previously existing idea was that the compound’s magnetism was entirely due to the manganese in the case of uranium-manganese and to the uranium in the case of uranium-copper.

The researchers found that at room temperature, uranium-manganese also displays a degree of magnetism in the uranium, induced by the manganese, and that at low temperatures, in addition to induced magnetism, the uranium lattice is ordered independently and therefore displays even stronger magnetism.

In the case of uranium-copper, the group discovered that the copper also displays magnetism induced by the uranium. All this is new knowledge produced by the study.

“In addition to the behavior of uranium as a totality, we also observed what happens in its layers and sublayers, in this case 5f, 6d and the hybrid, at low and high temperatures. And we explained why it happens,” Souza Neto said.

“The Young Investigator grant I received from FAPESP was for the development of X-ray techniques under extreme conditions. This involved both low temperatures and high pressures with very low magnetic signals. The study we performed, which resulted in the article in Nature Communications, was one of the outcomes of the project. We developed the technique and succeeded in deploying several strategies to measure low signals efficiently. As a result, we were able to measure signals 50 times lower than would be measured at the other absorption edge, in this case the uranium. Thanks to the study, we now enjoy very good conditions in which to perform measurements not only in actinides but also in several other kinds of material.”

There are uranium compounds that are some of the few examples of unconventional superconducting materials with two antagonistic properties: ferromagnetism and superconductivity. Until now, superconductivity, the property belonging to certain materials whereby they conduct electricity without resistance or losses, has been achieved only by cooling these materials to extremely low temperatures.

A deeper understanding of these unconventional uranium compounds could be a key step toward obtaining a material that is a superconductor at room temperature. This would have an extraordinary technological and social impact.

“Although our research was conducted strictly as basic science, technological developments as interesting as this are by no means beyond its horizons,” Souza Neto said.

The article “Unraveling 5f-6d hybridization in uranium compounds via spin-resolved L-edge spectroscopy” by R. D. dos Reis, L. S. I. Veiga, C. A. Escanhoela Jr., J. C. Lang, Y. Joly, F. G. Gandra, D. Haskel and N. M. Souza-Neto can be read at: nature.com/articles/s41467-017-01524-1.