The thermodynamic properties and molecular dynamics of [Li+@C60](PF6−) associated with structural phase transitions

Abstract

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 (Δ_trsS) 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 (Δ_trsS = 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 Δ_trsS 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.