Plutonium

In nature, plutonium is found in the Earth's crust, 10⁻¹⁸ ppm [4].

In pure solid form, plutonium exist in the following forms:
α-form: monoclinic crystal at < 122 ± 4 °C [4]
β-form: body-centered monoclinic crystal at 122 ± 4 to 207 ± 5 °C [4]
γ-form: orthorhombic crystal at 207 ± 5 to 315 ± 3 °C [4]
δ-form: face-centered cubic crystal at 315 ± 3 to 457 ± 2 °C [4]
δ'-form: body-centered tetragonal crystal at 457 ± 2 to 479 ± 4 °C [4]
ϵ-form: body-centered cubic crystal at > 479 ± 4 °C [4]