Journal
PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Volume 21, Issue 29, Pages 16147-16153Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c9cp02849h
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Funding
- Program for Key Interdisciplinary Research (Tohoku University)
- Japan Society for the Promotion of Science (JSPS) [15K05404, 17K19102]
- Grants-in-Aid for Scientific Research [15K05404, 17K19102] Funding Source: KAKEN
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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.
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