Formation of Metallic Polyferrocene Chains under Pressure

The effect of pressure on the structural reorganization of ferrocene, Fc = (C₅H₅)₂Fe, is studied using density functional theory (DFT) calculations assisted by evolutionary crystal structure prediction algorithms based on USPEX code. Pressure brings the individual molecules in close contact, and above 220 GPa, the 18-electron closed-shell molecular unit undergoes polymerization through the formation of quasi-one-dimensional (1D) chains, [(C₅H₅)₂Fe]_∞, termed as polyferrocene (p-Fc). Pressure induced polymerization (PIP) of Fc causes significant deviations from the 5-fold symmetry of the cyclopentadiene (Cp, C₅H₅ rings) and loss of planarity due to the onset of envelope-like distortions. This triggers distortions within the multidecker sandwich structures and σ­(C–C) bond formation between the otherwise weak noncovalently interacting Cp rings in Fc crystals. Pressure gradually reduces the band gap of Fc, and for p-Fc, metallic states are found due to increased electronic coupling between the covalently linked Cp rings. Polyferrocene is significantly more rigid than ferrocene as evident from the 5-fold increase in its bulk modulus. Pressure dependent Raman spectra show a clear onset of polymerization in Fc at P = 220 GPa. Higher mechanical strength coupled with its metallicity makes p-Fc an interesting candidate for high pressure synthesis.