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Allotropes of plutonium

 

 
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Allotropes of plutonium
 
 
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table class="wikitable">
Phase Crystal Structure Density (g/cm3)
alpha (a) simple monoclinic 19.86
beta (ß) body-centered monoclinic 17.70
gamma face-centered orthorhombic 17.14
delta  Even at ambient pressure, plutonium occurs in a variety of allotropes.






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Pu Phases
table class="wikitable">
Phase Crystal Structure Density (g/cm3)
alpha (a) simple monoclinic 19.86
beta (ß) body-centered monoclinic 17.70
gamma face-centered orthorhombic 17.14
delta  Even at ambient pressure, plutonium occurs in a variety of allotropes. These allotropes differ widely in crystal structure and density; the a and d allotropes differ in density by more than 25% at constant pressure.

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

In thermodynamics, the internal energy of a thermodynamic system, or a physical body with well-defined dimension, denoted by U, or sometimes E, is the total of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational and electricity energy of atoms within molecules or crysta...
s but significantly varying densities
Density

The density of a material is defined as its mass per unit volume. The symbol of density is ....
 and crystal structure
Crystal structure

In mineralogy and crystallography, a crystal structure is a unique arrangement of atoms in a crystal. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice....
s. This makes plutonium very sensitive to changes in temperature, pressure, or chemistry, and allows for dramatic volume changes following phase transition
Phase transition

In thermodynamics, a phase transition is the transformation of a thermodynamic system from one phase to another.At phase-transition point, physical properties may undergo abrupt change- for instance, volume of the two phases may be vastly different....
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.

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

Cast iron usually refers to Gray iron, but also identifies a large group of ferrous alloys, which solidify with a eutectic. The color of a fractured surface can be used to identify an alloy....
 but changes to the plastic and easy to work ß 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 a phase has a low-symmetry monoclinic
Monoclinic crystal system

In crystallography, the monoclinic crystal system is one of the 7 lattice point groups. A crystal system is described by three Vector . In the monoclinic system, the crystal is described by vectors of unequal length, as in the orthorhombic system....
 structure, hence its poor conductivity, brittleness, strength and compressibility.

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

An alloy is a partial or complete solid solution of one or more chemical element in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history....
ed with a small percentage of gallium
Gallium

Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the Ga salt, in trace amounts in bauxite and zinc ores....
, aluminium
Aluminium

Aluminium or aluminum is a silvery white and ductile member of the boron group of chemical elements. It has the symbol Al; its atomic number is 13....
, or cerium
Cerium

Cerium is a chemical element with the symbol Ce and atomic number 58....
, enhancing workability and allowing it to be welded
Welding

Welding is a fabrication or sculpture process that joins materials, usually metals or thermoplastics, by causing coalescence . This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself,...
 in weapons applications. The delta phase has more typical metallic character, and is roughly as strong and malleable as aluminium.