Unraveling structural models of graphite fluorides by density functional theory calculations

Using density functional theory (DFT) calculations, we have studied structural models of graphite fluorides for five fluorine compositions; C ₁F (CF₁), C₂F (CF_0.5), C ₃F (CF_0.33), C₄F (CF_0.25), and C₁₆F (CF_0.0625). For each composition, we considered several possible structural models and calculated heat of formation relative to the pristine graphite and F₂ molecule. We also simulated X-ray diffraction patterns for each structural model and compared those with experiments. We find, in agreement with earlier experiments, that the most stable structure of the C₁F (CF₁) has an infinite array of trans-linked cyclohexane chairs of covalent C-F bonds (1.38 A). We also find that the effects of the layer stacking sequence such as AB or AA′ is not significant. For the C₂F (CF_0.5) system, an earlier model in the literature indicated that all carbon atoms have only sp³ hybridization due to coexistence of C-C and C-F covalent bonds. However, in this work, we propose a new C₂F (CF_0.5) crystal structure in which half of the carbon atoms has sp³ hybridization due to C-F covalent bonds and the other half has sp² hybridization as found in pristine graphite. Besides, with the structural models of graphite fluorides considered in this work, their formation mechanism is also clarified. Initially, two fluorines are positioned at adjacent carbon atoms with the trans geometry, and then, one graphene layer is fully covered with fluorine while other layers are still pristine. After full coverage of the graphene layer, newly added fluorine will get located on this pristine graphene layer, and then, finally, all carbon layers are covered with fluorine leading to formation of C ₁F crystal. Through this mechanism, we can explain phase transition from the C₂F to the C₂F through further fluorination, which was demonstrated by an earlier experiment.