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+@C-60](PF6-) at temperatures between 1.8 and 395 K. [Li+@C-60](PF6-) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li+@C-60 and PF6-. The transition occurred in a step-wise manner. The total transition entropy (Delta S-trs) of 40.1 +/- 0.4 J K-1 mol(-1) was smaller than that of the orientational order-disorder transition in a pristine C-60 crystal (Delta S-trs = 45.4 +/- 0.5 J K-1 mol(-1)). Thus, the orientational disorder of Li+@C-60 in the high-temperature phase of [Li+@C-60](PF6-) was much less excited than that of the pristine C-60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of [Li+@C-60](PF6-). At T < 100 K, upon cooling, Li+ ions were trapped in two pockets on the inner surface of C-60, and no phase transition was observed. Finally, the Li+ ions achieved a complete order at 24 K through antiferroelectric transition. The Delta S-trs 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 C-60 cage was related to the selection rule in the THz-FIR and IR spectroscopy of the C-60 internal vibrations, because a C-60 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.