Plutonium is a metal of great interest because of its most unusual metallurgical, electronic, and nuclear properties. In the Periodic Table, it sits in the middle of the actinide series, and there it occupies a unique position, unlike any other element. It exhibits six solid allotropic changes, at ordinary pressure, and is strikingly unstable with temperature, pressure, chemical additions, and time. It undergoes virtually every type of phase transformation known. In the solid state, the 5f electrons of Pu are caught in an abrupt transition between being localized and contributing to bonding. The proximity of the 7s, 6d, and 5f electrons in the valence orbitals leads to many complex electronic interactions and physical properties.
The intepretation of many of the above features is a challenge to both physics and materials science.