与结构相变相关的[Li@C](PF)的热力学性质和分子动力学。

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

作者信息

Suzuki Hal, Ishida Misaki, Otani Chiko, Kawachi Kazuhiko, Kasama Yasuhiko, Kwon Eunsang, Miyazaki Yuji, Nakano Motohiro

机构信息

Department of Chemistry, Kindai University, 3-4-1 Kowakae, Higashiosaka, Osaka 577-8502, Japan.

Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aoba, Sendai, Miyagi 980-8578, Japan and Terahertz Sensing and Imaging Research Team, RIKEN Center for Advanced Photonics, RIKEN, 519-1399 Aramaki-Aoba, Sendai, Miyagi 980-0845, Japan.

出版信息

Phys Chem Chem Phys. 2019 Jul 24;21(29):16147-16153. doi: 10.1039/c9cp02849h.

DOI:10.1039/c9cp02849h

PMID:31292592

Abstract

Calorimetric and terahertz-far-infrared (THz-FIR) spectroscopic and infrared (IR) spectroscopic measurements were conducted for Li+@C60 at temperatures between 1.8 and 395 K. Li+@C60 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 was much less excited than that of the pristine C60 owing to the Coulombic interactions, which stabilized the ionic crystal lattice of Li+@C60. 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.

摘要

在1.8至395 K的温度范围内，对Li⁺@C₆₀进行了量热法、太赫兹 - 远红外（THz - FIR）光谱法和红外（IR）光谱法测量。Li⁺@C₆₀在约360 K时经历了结构相变，同时伴随着Li⁺@C₆₀和PF₆⁻的取向有序 - 无序转变。该转变以逐步方式发生。总转变熵（ΔtrsS）为40.1±0.4 J K⁻¹ mol⁻¹，小于原始C₆₀晶体中取向有序 - 无序转变的熵（ΔtrsS = 45.4±0.5 J K⁻¹ mol⁻¹）。因此，由于库仑相互作用，Li⁺@C₆₀高温相中的Li⁺@C₆₀的取向无序比原始C₆₀的取向无序激发程度小得多，这稳定了Li⁺@C₆₀的离子晶格。在T < 100 K时，冷却时Li⁺离子被困在C₆₀内表面的两个口袋中，未观察到相变。最后，Li⁺离子在24 K时通过反铁电转变实现了完全有序。4.6±0.4 J K⁻¹ mol⁻¹的ΔtrsS值略小于两点有序 - 无序转变预期的R ln 2 = 5.76 J K⁻¹ mol⁻¹。C₆₀笼中Li⁺的运动程度与C₆₀内部振动的THz - FIR和IR光谱中的选择规则有关，因为C₆₀笼会被Li⁺离子极化。结果表明，笼中分子的局部对称性可通过笼中离子的旋转或跳跃运动来改变。