CF4: Crystal Structure and Solid Phase Diagram with Ar

The crystal structure of solid CF₄ and the temperature–composition solid phase diagram of CF₄ with Ar have been studied by x‐ray powder diffraction. The diffraction pattern of α‐CF₄, stable from 0–76.2°K, has been indexed with a monoclinic unit cell where $\text{a\hspace{0.17em}=\hspace{0.17em}8.71 Å, b\hspace{0.17em}=\hspace{0.17em}4.16 Å, c\hspace{0.17em}=\hspace{0.17em}15.27 Å}$ , and $\text{β\hspace{0.17em}=\hspace{0.17em}150.7°}$ , at 40°K. The α‐CF₄ unit cell is C‐centered with four molecules per unit cell and is probably in space group $\mathrm{Cc}$ with molecules centered at (0, 0, 0) and $(\frac{1}{2},\frac{1}{2}\text{, 0)}$ . The reflections of β‐CF₄, which is stable from 76.2°K to the melting point at 89.5°K, could not be indexed, but it was established that the structure is not cubic and is probably also monoclinic. The CF₄–Ar solid phase diagram shows a eutectic from about 12% Ar to essentially 100% Ar. The eutectic temperature is 70 ± 1°K, coinciding with the solidus line in the region of the eutectic.