MS44-P02 Crystal Structures of Solid Fluorine and Nitrogen TrifluorideThe crystal structures of the chemical elements belong to the fundamental knowledge of chemistry as atom distances, bond lengths and angles can be determined precisely and the data serve as benchmarks for quantum chemistry. Surprisingly, the crystal structures of α-fluorine and β-fluorine have so far only once been investigated [1,2]. Probably this is due to the fact that solid F2 is still extremely reactive and in the case of α-F2 the authors reported several explosions due to the highly exothermic phase change and the subsequent reaction of F2 with the sample holder . It was concluded that α‑F2 crystallizes in the monoclinic crystal system, probably in space group type C2/m, but space group type C2/c could not be ruled out. Later the original diffraction data were reinterpreted by others and space group C2/c was found to be more likely correct . The precise space group of α-F2 remained, however, uncertain, which naturally has an influence on the determined atom positions, bond lengths and displacement parameters. The crystal structure of β-F2 was first reported in 1964 and determined by means of single crystal X-ray diffraction , the problem to distinguish between two cubic space group types, P-43n and Pm-3n, was faced. The higher symmetric space group type was chosen since a disordered structure was expected. Neither the bond length of the F2 molecules, and not the anisotropic displacement parameters of the fluorine atoms could be refined.
Therefore, we constructed a gas line attached to a sample holder inside the SPODI neutron diffractometer at the FRM-II neutron facility of the Heinz Maier-Leibnitz Zentrum in Garching, Germany. The crystal structures of α- and β-fluorine were determined using powder neutron diffraction and Rietveld refinement. We will report on the obtained models providing much more precise lattice parameters, atomic coordinates as well as bond lengths for the two polymorphs of fluorine. Using the same gas line we attempted an elucidation of the previously unknown crystal structure of solid nitrogen fluoride at various temperatures.References:
 Meyer, L. et al. (1968) J. Chem. Phys. 49, 1902-1907..
 Jordan, T. H. et al. (1964) J. Chem. Phys., 41, 760-764.
 Pauling, L. et al. (1970) J. Solid State Chem., 2, 225-227.Keywords: fluorine, nitrogen trifluoride, neutron diffraction