Crystal Structure of α-Fluorine

The crystal structure of α‐fluorine, stable below 45.6°K, has been investigated from x‐ray powder‐diffraction patterns. Alpha fluorine is monoclinic, probably space group $\text{C2\hspace{0.17em}/\hspace{0.17em}m}$ , but space group $\text{C2\hspace{0.17em}/\hspace{0.17em}c}$ cannot be ruled out completely. There are four molecules per unit cell. At 23°K, the cell dimensions are: $$\text{a\hspace{0.17em}=\hspace{0.17em}5.50\hspace{0.17em}±\hspace{0.17em}0.01 Å}$$ , $$\text{b\hspace{0.17em}=\hspace{0.17em}3.28\hspace{0.17em}±\hspace{0.17em}0.01 Å}$$ , $$\text{c\hspace{0.17em}=\hspace{0.17em}10.01\hspace{0.17em}±\hspace{0.17em}0.01 Å}$$ , $$\text{β\hspace{0.17em}=\hspace{0.17em}134.66°\hspace{0.17em}±\hspace{0.17em}0.02°}$$ . The unit‐cell volume at 23°K is 128.4 ų, the calculated density is 1.97 g/cm³, and the calculated molar volume is 19.3 cm³/mole. The structure is very similar to $\mathrm{\alpha ‐}{\mathrm{O}}_2$ , the main difference being that the molecules are tilted, probably only by about 11° to alternate sides of the normal to the basal plane in alternate layers, leading to a doubling of the unit cell in the $c$ direction as compared to $\mathrm{\alpha ‐}{\mathrm{O}}_2$ . The fact that the unit cell contains four molecules admits the possibility of space group $\text{C2\hspace{0.17em}/\hspace{0.17em}c}$ with the molecules tilted out of the $\mathrm{ac}$ plane.