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 $C2 / m$⁠, but space group $C2 / c$ cannot be ruled out completely. There are four molecules per unit cell. At 23°K, the cell dimensions are: , , , . 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 $α‐O2$⁠, 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 $α‐O2$⁠. The fact that the unit cell contains four molecules admits the possibility of space group $C2 / c$ with the molecules tilted out of the $ac$ plane.