Pressure and Temperature Transformations of the Molecular Conformation and Crystal Structure of Ferrocene Fe2+(η5-C5H5)2–

Abstract

X-ray diffraction, optical spectroscopy, and numerical calculations were carried out on pristine ferrocene Fe2+(η5-C5H5)2– single crystals under various pressure/temperature conditions. Structural data were obtained at room temperature and pressures up to 6.5 GPa as well as at ambient pressure and low temperatures up to 100 K. The iron–carbon bond length (l = 2.043 Å under ambient conditions) shows linear temperature dependence and quadratic pressure dependence with the coefficients ∂l/∂T = −2.84 × 10–5 Å K–1, ∂l/∂P = −1.36 × 10–3 Å GPa–1, and ∂2l/∂P2 = −3.04 × 10–4 Å GPa–2, respectively. Jumping of the Fc molecular configuration between D5h and D5d conformations decreases at high pressure/low temperature, leading to complete ordering in low-temperature (LT, <161.3 K) and high-pressure (HP, >3 GPa) phases. The initially large bandwidth of lattice phonon modes increases with pressure to a maximum at 2 GPa and decreases to the common value after molecular ordering at pressures P > 3 GPa. The electronic structure calculations in the HP phase at high pressure show a linear increase in the fundamental gap Eg with the pressure coefficient of 28 meV/GPa. Optical transmission measurements at high pressure demonstrate an almost 2 times smaller pressure coefficient of about 12 meV/GPa and an abrupt decrease in the gap by ∼14.5 meV at P = 2 GPa.