The Crystal Structures of α- and β-F₂ Revisited

The crystal structures of α-F₂ and β-F₂ have been reinvestigated using neutron powder diffraction. For the low-temperature phase α-F₂ , which is stable below circa 45.6 K, the monoclinic space group C2/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) ų , mS8, 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-F₂ , stable between circa 45.6 K and the melting point of 53.53 K, crystallizes in the cubic primitive space group Pm $$ n with the lattice parameter a=6.5314(15) Å, V=278.62(11) ų , cP16, Z=8, at 48 K. β-F₂ is isotypic to γ-O₂ and δ-N₂ . The centres of gravity of the F₂ molecules are arranged like the atoms in the Cr₃ Si structure type.