The thermodynamic properties and molecular dynamics of [Li+@C60](PF6−) associated with structural phase transitions†
Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for [Li+@C60](PF6−) at temperatures between 1.8 and 395 K. [Li+@C60](PF6−) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li+@C60 and PF6−. The transition occurred in a step-wise manner. The total transition entropy (ΔtrsS) of 40.1 ± 0.4 J K−1 mol−1 was smaller than that of the orientational order–disorder transition in a pristine C60 crystal (ΔtrsS = 45.4 ± 0.5 J K−1 mol−1). Thus, the orientational disorder of Li+@C60 in the high-temperature phase of [Li+@C60](PF6−) was much less excited than that of the pristine C60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of [Li+@C60](PF6−). At T < 100 K, upon cooling, Li+ ions were trapped in two pockets on the inner surface of C60, and no phase transition was observed. Finally, the Li+ ions achieved a complete order at 24 K through antiferroelectric transition. The ΔtrsS value of 4.6 ± 0.4 J K−1 mol−1 was slightly smaller than R ln 2 = 5.76 J K−1 mol−1 expected for the two-site order–disorder transition. The extent of the Li+ motion in the C60 cage was related to the selection rule in the THz-FIR and IR spectroscopy of the C60 internal vibrations, because a C60 cage should be polarized by the Li+ ion. It is shown that the local symmetry of the caged molecule can be modified by the rotational or hopping motion of the encaged ions.