Some elements are incredibly common and have been studied for hundred of years in one form or another. Abundant elements like oxygen, nitrogen and hydrogen are easy to find as they are literally all around us. Other elements like gold, lead and copper are unreactive and so are easily found lying around most of the time in their elements form or in an easily extractable ore. Elements near the bottom of the periodic table, in the radioactive set, are not only rare as they are constantly decaying but also are quite reactive. The example under examination today is one that most people will have heard of as it is that of plutonium. Samples of plutonium have over recent years been subject to a series of experiments and associated theoretical treatment from X-ray spectroscopy to neutron scattering.
The aim has been to try and prove one of two contradicting theoretical predictions correct. The first is that the outer electrons of plutonium are very metallic with large delocalisation over the structure according to density-functional theory (DFT); while the other is that the electrons are actually held quite locally to the plutonium atoms and intraatomic movement is much less than most metals which is based on dynamical mean-field theory (DMFT). Now these studies have been performed on the most common lattice structure, that being δ-plutonium, and this allotrope is certainly still a mystery. But both the theoretical predictions become even worse when applied to ε-plutonium which shouldn’t even be physically held together according to these theories.
This calls into question whether these theories are accurate at all as well as the fact neither theory can accurately model the thermodynamics of plutonium as a nuclear fuel. Experimentally ε-plutonium is hard to examine as it only can be created at high temperatures and their has been very little research done on it. This paper has calculated the phonon structure of the plutonium at 900 K and used it to predict the mechanical behaviour of plutonium in these conditions. This will be the first of research to come to reasonable conclusions about the qualities of ε-plutonium which is a considerably important state at high temperatures and hopefully this will allow further research of both the experimental and the theoretical to be performed in the future.
Paper links: Lattice dynamics and elasticity for ε-plutonium