Heat capacity and phase transitions of the plastic crystal formylferrocene

Abstract

Heat capacities of the plastic crystal formylferrocene have been measured with adiabatic calorimetry in the 13–405 K temperature region. Three phase transitions hitherto reported were observed at 293.53 K (second-order phase transition), 317.03 K (first-order phase transition to plastic crystal phase), and 397.60 K (fusion). The enthalpy and entropy of transition were determined as ΔH = (0.887 ± 0.01) kJ mol⁻¹/ΔS = (3.39 ± 0.05) J K⁻¹ mol⁻¹, (13.29 ± 0.01) kJ mol⁻¹/(42.10 ± 0.01) J K⁻¹ mol⁻¹, and 2.76 kJ mol⁻¹/6.88 J K⁻¹ mol⁻¹, respectively. Mechanisms governing the phase transitions are discussed. Because formylferrocene has an almost spherical molecular structure, it can reorient quasi-isotropically in the solid state. This situation bears a close resemblance to that encountered in fullerene C₆₀.

Keywords:

• Phase transition
• Plastic crystal
• Formylferrocene
• Heat capacity
• Entropy of transition
• Orientationally disordered crystal

Acknowledgements

Dedicated to Professor Jerzy A. Janik of the Henryk Niewodniczański Institute of Nuclear Physics in Kraków, Polish Academy of Sciences, in honour of his 80th birthday.