Plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...

occurs in a variety of

allotropes Allotropy or allotropism () is the property of some chemical elements to exist in two or more different forms, in the same physical state, known as allotropes of the elements. Allotropes are different structural modifications of an element: th ...

, even at ambient pressure. These allotropes differ widely in crystal structure and density; the α and δ allotropes differ in density by more than 25% at constant pressure.

Overview

Plutonium normally has six allotropes and forms a seventh (zeta, ζ) under high temperature and a limited pressure range. These allotropes have very similar

energy level A quantum mechanical system or particle that is bound—that is, confined spatially—can only take on certain discrete values of energy, called energy levels. This contrasts with classical particles, which can have any amount of energy. The te ...

s but significantly varying

densities Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek language, Greek letter Rho (letter), rho), although the Latin letter ''D'' ca ...

and

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

s. This makes plutonium very sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following

phase transition In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...

s. Unlike most materials, plutonium ''increases'' in density when it melts, by 2.5%, but the liquid metal exhibits a linear decrease in density with temperature. Densities of the different allotropes vary from 16.00 g/cm³ to 19.86 g/cm³.

Machining plutonium

The presence of these many allotropes makes machining plutonium very difficult, as it changes state very readily. For example, the α phase exists at room temperature in unalloyed plutonium. It has machining characteristics similar to

cast iron Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...

but changes to the β phase (''beta phase'') at slightly higher temperatures. The reasons for the complicated phase diagram are not entirely understood; recent research has focused on constructing accurate computer models of the phase transitions. The α phase has a low-symmetry

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

structure, hence its poor conductivity, brittleness, strength and compressibility.

Stabilization

Plutonium in the δ phase (''delta phase'') normally exists in the 310 °C to 452 °C range but is stable at room temperature when

alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...

ed with a small percentage of

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

, or

cerium Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 o ...

, enhancing workability and allowing it to be

welded Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as braz ...

in weapons applications. The delta phase has more typical metallic character, and is roughly as strong and malleable as aluminium. In fission weapons, the explosive

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...

s used to compress a

plutonium core The pit, named after the hard core found in fruits such as peaches and apricots, is the core of an implosion nuclear weapon – the fissile material and any neutron reflector or tamper bonded to it. Some weapons tested during the 1950s used pit ...

will also cause a transition from the usual delta phase plutonium to the denser alpha phase, significantly helping to achieve

supercriticality In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissio ...

.Plutonium Crystal Phase Transitions
Globalsecurity.org (27 April 2005). Retrieved 2010-02-08. The plutonium-gallium alloy is the most common δ-stabilized alloy.

Gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group (aluminiu ...

americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...

scandium Scandium is a chemical element with the symbol Sc and atomic number 21. It is a silvery-white metallic d-block element. Historically, it has been classified as a rare-earth element, together with yttrium and the Lanthanides. It was discovered in ...

and

can stabilize the δ phase of plutonium for room temperature.

Silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts p ...

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

and

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...

allow formation of metastable δ state when rapidly cooled. High amount of

hafnium Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dmitri M ...

holmium Holmium is a chemical element with the symbol Ho and atomic number 67. It is a rare-earth element and the eleventh member of the lanthanide series. It is a relatively soft, silvery, fairly corrosion-resistant and malleable metal. Like a lot of othe ...

and

thallium Thallium is a chemical element with the Symbol (chemistry), symbol Tl and atomic number 81. It is a gray post-transition metal that is not found free in nature. When isolated, thallium resembles tin, but discolors when exposed to air. Chemists W ...

also allows retaining some of the δ phase at room temperature.

Neptunium Neptunium is a chemical element with the Symbol (chemistry), symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. Its position in the periodic table just after uranium, named after ...

is the only element that can stabilize the α phase at higher temperatures.

Titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

and

stabilize the β phase at room temperature when rapidly cooled. Phase diagram of plutonium (1975) 150 kbar region

References

{{DEFAULTSORT:Allotropes of Plutonium

Plutonium