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: a = 5.50 ± 0.01 Å, b = 3.28 ± 0.01 Å, c = 10.01 ± 0.01 Å, β = 134.66° ± 0.02°. The unit-cell volume at 23°K is 128.4 Å3, 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.

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