|
While the authors’ findings and study methods are
mainly of interest to other researchers seeking clear explanations of
complex materials, the knowledge may someday help scientists create
safer and more versatile nuclear materials for energy, industry and
medicine.
"Previous theories about plutonium’s makeup placed
a fixed number of valence electrons in the particular orbital we
examined, known as the 5f orbital," said Kristjan Haule, an assistant
professor of physics and astronomy at Rutgers. "Different theories
assigned different numbers of electrons to that orbital – some four,
others five and yet others, six. But whatever number the theory
prescribed, it remained constant. Each theory could explain some of
the element’s characteristics, but none could account for all the
experimental evidence."
The Rutgers approach abandoned the idea of a fixed
or unique number of valence electrons in the 5f orbital. "We revisited
the notion of valence in a solid," Haule said. "While it happens
rarely in nature, we thought it should be possible for the number of
valence electrons to fluctuate among orbitals in atoms that are part
of a solid structure."
It turns out that plutonium is especially suited to
exhibiting this behavior. The Rutgers physicists determined that
almost 80 percent of the time, there are five electrons in the 5f
orbital. Almost 20 percent of the time, there are six, and less than 1
percent of the time, there are four.
"A theory that permits fluctuating valence
electrons consistently explains properties that scientists observe in
laboratory experiments," Haule said, citing recent results using X-ray
absorption and electron energy-loss spectroscopy. "In addition, the
theory accurately predicts the properties of two neighboring elements,
americium and curium, which have similar atomic structures but show
greatly differing magnetic and electric properties."
The new approach involves a merger of two existing
theories, known as local density approximation and dynamical mean
field theory, or LDA+DMFT. Taken separately, they and others fell
short in accounting for all of plutonium’s observed physical
characteristics.
The work done by Haule and his colleagues is in a
branch of physics known as condensed matter physics, which deals with
the physical properties of solid and liquid matter. In particular,
their work focuses on strongly correlated materials, which have
strongly interacting electrons and, therefore, can’t be described
using theories that treat electrons as largely independent entities.
The radioactive metals, such as plutonium, and their periodic table
neighbors, known as rare earth elements, are examples of strongly
correlated materials, with highly localized electrons in their f
orbitals. In these elements, most of the physical properties such as
electrical resistivity and magnetic characteristics depend on the
f-orbital electrons. The findings reported in Nature strengthen
methods for predicting characteristics of all of these complex
materials. |
