The Crystal Structures of α‐ and β‐F 2 Revisited

The crystal structures of α‐F 2 and β‐F 2 have been reinvestigated using neutron powder diffraction. For the low‐temperature phase α‐F 2 , which is stable below circa 45.6 K, the monoclinic space group C 2/ c with lattice parameters a= 5.4780(12), b= 3.2701(7), c= 7.2651(17) Å, β =102.088(18)°, V= 127.26(5) Å 3 , mS 8, Z= 4 at 10 K can now be confirmed. The structure model was significantly improved, allowed for the anisotropic refinement of the F atom, and an F−F bond length of 1.404(12) Å was obtained, which is in excellent agreement with spectroscopic data and high‐level quantum chemical predictions. The high‐temperature phase β‐F 2 , stable between circa 45.6 K and the melting point of 53.53 K, crystallizes in the cubic primitive space group Pm 3 ‾ n with the lattice parameter a= 6.5314(15) Å, V= 278.62(11) Å 3 , cP 16, Z= 8, at 48 K. β‐F 2 is isotypic to γ‐O 2 and δ‐N 2 . The centres of gravity of the F 2 molecules are arranged like the atoms in the Cr 3 Si structure type.