There are six allotropes of plutonium. Given the melting temperature of just ~640 °C, it means the various allotropes are stable only in limited temperature ranges. In plutonium, the narrow conduction bands and high density of states of 5f electrons make it energetically favorable for the ground-state crystal structure to distort to a low-symmetry monoclinic lattice at lower temperatures. Plutonium adopts more typical symmetric structures only at elevated temperatures or with suitable alloying.

Up to ~122 °C, plutonium is known as alpha phase (a-Pu) and has a simple monoclinic crystal structure. The density of this phase at 21 °C is 19.816 gcm~3, and the lattice parameters are a = 6.1835 A, b = 4.8244A, c = 10.973 A, and b = 101.81°. Figure 7.11a illustrates the (020) plane of the a-plutonium monoclinic lattice, which resembles an HCP plane. In Figure 7.11b, two stacked (020) planes of the crystal contain four unit cells. The bond lengths, however, can be categorized into two groups (long bonds ~3.19-3.71 A, and short bonds ~2.57-2.78 A); each of the eight numbered sites is crystallographically unique. Lawson et al. [7] showed that there are 16 atoms per unit cell with 8 distinct lattice positions.

From ~122 °C to ~206 °C, plutonium assumes a body-centered monoclinic lat­tice structure (34 atoms per unit cell) and is known as b-Pu. At 190 °C, the density of b-Pu is 17.70gcm~3 and the lattice parameters are a = 9.284A, b = 10.463 A, c = 7.859A and b = 92.13°.

From ~206 °C to ~319 °C, plutonium assumes a face-centered orthorhombic (8 atoms per unit cell) crystal structure. This phase is also known as gamma — plutonium (y-Pu). At 235 °C, the density of y-Pu is 17.14 gcm~3 and the lattice parameters are a = 3.1587 A, b = 5.7682 A, and c = 10.162 A

From ~319 °C to ~451 °C, plutonium takes up a face-centered cubic (FCC) crys­tal structure with usual 4 atoms per unit cell, and is known as delta-plutonium (6-Pu). At 320 °C, the density of 6-Pu is 15.92 gcm~3 and the lattice constant is a = 4.6871 A

From ~451 ° C to ~476 °C, plutonium assumes a body-centered tetragonal lattice structure with usual 2 atoms per unit cell. This phase is known as delta-prime (6′-Pu). At 465 °C, the density of 6′-Pu is 16.00 gcm~3 and the lattice constants are a = 3.327 A and c = 4.482 A.

From ~476 °C to just before the melting point (639.5 °C), plutonium maintains a body-centered cubic structure (2 atoms per unit cell) and is called “epsilon” phase.

The lattice constant and the physical density of this phase are 3.6361 A and 16.51 g cm~3, respectively.

The allotropic (or polymorphic) transformation kinetics is very sensitive to prior processing history and presence of impurities. In case of pure metal, significant hysteresis occurs on cooling. Plutonium has many anomalous characteristics. For instance, the transformation from b-Pu to a-Pu is very slow and generally other high temperature phases are found to be retained even below the transformation temperature except under application of high pressures. So, generally high strain is retained at room temperature in a-Pu when cooled at atmospheric pressure. So, martensitic type of transformation modes can also occur in plutonium, such as transformations from the delta phase to the gamma phase and from the gamma phase to the beta phase. Linear thermal expansion coefficient of plutonium as a function of temperature follows the trend as shown in Figure 7.12 from dilatometry

experiment. The temperature is generated due to its alpha-emitting characteristics with about 1.923 W g-1. A specimen of plutonium bit larger than a foil or a wire can be used as an adiabatic self-heater in the thermal expansion measurement so that no external heat needs to be provided for experimentation. The thermal expansion coefficient increases in the alpha-, beta-, and gamma-phase regimes. However, the delta phase contracts as the temperature rises. It also has positive temperature coefficient of resistivity. The delta-prime phase shows anomalous thermal expan­sion behavior. The electrical resistivity of plutonium is one of the highest in all met­als with much like semiconductors. Thermal conductivity and specific heat of plutonium increase with increasing temperature. Interestingly, plutonium has the largest low-temperature specific heat of any pure element [8].