Analysis of the structural and electronic properties of (fluoranthene)₂PF₆ and characterization of its (011) surface by scanning tunneling microscopy

The phase transitions of the conducting salts (FA)₂X (FA = fluoranthene; X = PF₆, AsF₆) at 200 and 180 K were probed by analyzing the short intermolecular contacts of their crystal structures and calculating their electronic band structures. The scanning tunneling microscopy (STM) images of (FA)₂PF₆ were interpreted by calculating the partial electron density plots for its (011) surface. Our study indicates that the 200 K phase transition leads to a more compact crystal packing in which all F atoms of a PF₆^- anion make short F⋯H contacts with the surrounding FA molecules. The 180 K phase transition originates from the charge density wave (CDW) instability of each FA stack. It is most likely that the dimerization expected from the CDW instability involves rotations of FA dimers around the stacking axis, and that the occurrence of domain structures in each FA stack prevents the observation of a dimerized structure by single-crystal X-ray diffraction. The STM images of (FA)₂PF₆ indicate that a few FA molecules under the STM tip apex undergo a reversible, rotational rearrangement.