With less than 200 nanograms of einsteinium – half the world’s stock at the time – scientists first discovered the binding and spectroscopic behavior of the synthetic element.
Einsteinium, discovered in the rubble after the explosion of the first hydrogen bomb in 1952, is a highly radioactive actinide. Since it does not occur naturally on earth, little is known about its chemistry except that it forms some halide and oxide salts. Making more than just trace amounts of it means bombarding lighter elements with neutrons for a long time – a process that can only be done in one place in the world, the high-flux isotope reactor at Oak Ridge National Laboratory in Tennessee, USA
The laboratory’s latest efforts yielded only 400 ng of element 99, half of which was led by a team led by Rebecca Abergel of the University of California, Berkeley, Corwin Booth of the Lawrence Berkeley National Laboratory and Stosh Kozimor of the Los Alamos. National Laboratory. Despite working with less than 200 ng of the element, the researchers were able to place einsteinium through x-ray absorption measurements, revealing the coordination chemistry and spectroscopic behavior for the first time.
In some ways, einsteinium behaves similarly to its lighter neighbors on the periodic table and assumes a +3 oxidation state in a complex with an octadentate hydroxypyridinone ligand. However, the compound’s short einsteinium oxygen bond length was a surprise. Measurement of luminescence spectroscopy yielded another unexpected result. ‘The way [einsteinium] changing to complexity is in the opposite direction in terms of shifting wavelength and energy than what happens with the other actinides, ‘explains Abergel. The team is now confirming why einsteinium’s behavior is so different from other actinides.
Normal rules no longer apply
“One thing we are talking about here is that we are not dealing well with the effects of relativistic effects on the chemistry of these elements,” said Jenifer Shafer, an expert in heavy actinide chemistry at the Colorado School of Mines, USA. “The normal rules of quantum mechanics and electronic ordering – things like Hund’s rule – seem to solve when you get to this part of the periodic table.”
One of the most exciting things about doing actinide chemistry is that there are elements for which we can write the textbook.
Stosh Kozimor, National Laboratory in Los Alamos
Shafer was impressed with the Berkeley team’s ability to make the experiments happen in the first place. “I work a bit with Einsteinium, but I can not imagine the challenge of trying to coordinate a few hundred nanograms and get important chemical data,” she says.
‘You can not really see [the material when it arrives], ‘recalls UC Berkeley researcher Katherine Shield, who did a lot of banking chemistry. “It comes in a small bottle, but the only way you know you’re working with it is by using radiation detectors.” Apart from the minimal amount that the researchers had to work with, Einsteinium’s half-life of 275 days meant that they lost material over time. “You really hope you do not accidentally drop that one drop while doing chemistry,” Shield says.
Before the team even received the einsteinium, the team thoroughly outlined the experiments they wanted to perform and in what order, to minimize material loss. “We planned a lot in advance and nothing went according to that plan,” laughs Abergel. Discovering that their sample contained a lot of californium, they had to abandon the planned experiments with x-ray diffraction and turn to the X-ray absorption spectroscopy, which enabled them to ignore the pollution.
After preparing the einsteinium complex in Berkeley, team member Korey Carter then drove the precious – and dangerous – monster for an hour to the Stanford Synchrotron Radiation Lightsource. Once there, handling half of the world’s supply of einsteinium was ‘absolutely terrifying’, Kozimor recalls. “It takes a well-trained team and steady nerves.”
“It’s also a privilege to do this kind of science,” says Abergel. Understanding chemical behavior of einsteinium can help scientists produce and purify it to make even heavier elements. “Einsteinium targets will make it more plausible to identify the island of stability, which is a big deal for nuclear physicists,” explains Shield. The Island of Stability is a predicted set of super-heavy isotopes that can have much longer half-lives than their close neighbors.
For now, the team will continue to work with the remaining einsteinium sample, using electron microscopy, radioanalysis and separation experiments. ‘One of the most exciting things about doing actinide chemistry is that there are elements for which we can write the textbook. It’s really exploratory, ‘says Kozimor.