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]
