Secrets of Plutonium Revealed
Researchers at Rutgers, The State University of New Jersey,
have unlocked some of the physical and chemical secrets of plutonium,
an element known for its use in atomic weapons and power plant fuel.
While the complex nuclear characteristics of plutonium are well-known,
it has properties as a metal or a chemical compound that have often
left scientists scratching their heads.
Writing in this week's issue of Nature, a prestigious international
scientific journal, Rutgers physicists report that the valence
electrons - those which control how atoms bond with each other -
fluctuate among different orbitals in solid plutonium metal on a very
short time scale. In contrast, earlier theories specified fixed numbers
of valence electrons in those orbitals. The Rutgers findings help
explain some contrary characteristics of plutonium: Unlike many metals,
plutonium is not magnetic and not a good conductor of electricity, and
it shows greater changes in volume under small changes in temperature
and pressure.
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.
http://www.rutgers.edu
Posted 29th March 2007