In solid C-60, Complex impedance spectroscopy measurements revealed a relax
ational process due to glass transition. However, the spectral features ass
ociated with this relaxation became indiscernible, when CO molecules are pr
esent at octahedral sites in molar ratio exceeding 0.67. We explain this ph
enomenon by the existence of a phase with almost purely pentagon oriented C
-60, and this phase is favored by a negative chemical pressure on C-60 latt
ice exerted by CO, as well as the electrostatic interaction between the two
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