Orientational phase transition in methane-intercalated fullerite C60статьяИсследовательская статья
Статья опубликована в высокорейтинговом журнале
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Дата последнего поиска статьи во внешних источниках: 16 января 2019 г.
Аннотация:Above236 K, the intercalate adopts the face-centered cubic structure which is characterized by freely rotating C60 and disordered CH4 molecules. The ordered low-temperature crystal structure possesses simple cubic unit cell in the space group Pa-3 in which both C60 and CH4 molecules are fixed on three-fold inversion axes. In the structure of (CH4)0.72C60 at 100 K, a C60 molecule is fixed by its 12 C60 neighbors due to donor-acceptor van der Waals interactions between electron-rich 6:6 C-C bonds and electron-deficient cage pentagons. At 200 K, the second orientation of a C60 molecule is present (with 7.5 % occupancy) in which 6:6 C-C bonds are directed to cage hexagons. The methane molecules are situated in octahedral holes and statistically disordered over two orientations around inversion centers. The phase transition temperature of (CH4)C60 intercalate is lower than in C60 by ca. 24 K due to the increased C60C60 separation. This study provides, for the first time, a reliable structural information about the low-temperature structure of the (CH4)0.7(C60) intercalate thus demonstrating significant differences between orientational phase transitions in pure fullerite, C60, and C60 intercalated with methane. In addition, the molecular structure of C60 fullerene in the (CH4)0.7C60 intercalate is distinguished by a high precision which is superior to the former experimental data for C60 and its adducts reported in the literature.