Abstract
The structure of crystalline [60]fullerene with a lithium cation inside (Li+@C60) was determined by synchrotron radiation X-ray diffraction measurements to understand the electrostatic and thermal properties of the encapsulated Li+ cation. Although the C60 cages show severe orientation disorder in [Li+@C60](TFPB−)·C4H10O and [Li+@C60](TFSI−)·CH2Cl2, the Li+ cations are rather ordered at specific positions by electrostatic interactions with coordinated anions outside the C60 cage. The Li+@C60 molecules in [Li+@C60](ClO4−) with a rock-salt-type cubic structure are fully disordered with almost uniform spherical shell charge densities even at 100 K by octahedral coordination of ClO4− tetrahedra and show no orientation ordering, unlike [Li+@C60](PF6−) and pristine C60. Single-bonded (Li+@C60−)2 dimers in [Li+@C60−](NiOEP)⋅CH2Cl2 are thermally stable even at 400 K and form Li+–C bonds which are shorter than Li+–C bonds in [Li+@C60](PF6−) and suppress the rotational motion of the Li+ cations.
Highlights
Hollow spherical carbon molecules called fullerenes or buckyballs with a molecular formula of C2n (n ≥ 30) can contain various atoms and molecules [1,2]
To understand the electrostatic and thermal properties of the mobile Li+ cation inside C60, we investigated the effects of the coordination structure and temperature on the position and motion of the encapsulated Li+ cations in Li+@C60 crystals by synchrotron radiation (SR) X-ray structure analysis in this study
The TFPB−, TFSI− and ClO4− salts were obtained by anion exchange of [Li+@C60](PF6−) [21]
Summary
Hollow spherical carbon molecules called fullerenes or buckyballs with a molecular formula of C2n (n ≥ 30) can contain various atoms and molecules [1,2]. The tunnelling motion is hindered below TC = 24 K with an abrupt decrease in the dielectric permittivity by antiferroelectric interactions among local electric dipole moments formed between the Li+ cations inside and the PF6− anions outside the C60 cages. The Li+ cation in the SbCl6− salt is localized at two adjoining off-centre positions at 370 K by asymmetric coordination of SbCl6− anions around an Li+@C60 molecule [11] Such anion exchange effects should be investigated in more depth to understand the electrostatic responses of the encapsulated Li+ cations. To understand the electrostatic and thermal properties of the mobile Li+ cation inside C60, we investigated the effects of the coordination structure and temperature on the position and motion of the encapsulated Li+ cations in Li+@C60 crystals by synchrotron radiation (SR) X-ray structure analysis in this study
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