α-form: body-centered cubic crystal at < ~710 °C [7]
β-form: cubic crystal at 710 to 1079 °C [7]
γ-form/electrolytic form: face-centered cubic crystal at 1079 to 1143 °C [7]
γ-form, stabilized at room temp: face-centered tetragonal crystal [7]
δ-form: body-centered cubic crystal at > 1143 °C [7]
