Allotropes of plutonium

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

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

It has machining characteristics similar to cast iron but changes to the beta (β) phase at slightly higher temperatures.

[2] Plutonium in the delta (δ) phase[8] normally exists in the 310 °C to 452 °C range but is stable at room temperature when alloyed with a small percentage of gallium, aluminium, or cerium, enhancing workability and allowing it to be welded in weapons applications.

Silicon, indium, zinc and zirconium allow formation of a metastable δ state when rapidly cooled.