The thermodynamic properties and molecular dynamics of [Li@C](PF) associated with structural phase transitions

Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for [Li + @C 60 ](PF 6 − ) at temperatures between 1.8 and 395 K. [Li + @C 60 ](PF 6 − ) underwent a structural phase transition at around 360 K accompanied by the orientational order-disorder transition of Li + @C 60 and PF 6 − . The transition occurred in a step-wise manner. The total transition entropy (Δ trs S ) 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 (Δ trs S = 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 ](PF 6 − ) 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 ](PF 6 − ). 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 Δ trs S 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. Li + ions encapsulated in C 60 in the [Li + @C 60 ](PF 6 − ) crystal exhibit order-disorder transition at 24 K.