Crystal Structure and Dielectric Property of Endohedral Water Fullerene

Polar water molecules in normal ice have a frozen random orientation even at 0 K due to the three-dimensional network of hydrogen bonds. Encapsulation of the water molecule inside C₆₀ was recently achieved by the molecular surgical approach. The single water molecule trapped inside C₆₀ is completely free from the hydrogen bonds. The easy separation of the H₂O@C₆₀ molecules from the empty C₆₀ by high-performance liquid chromatography implies that H₂O@C₆₀ has a dipole moment. The dipolar interaction between the caged water molecules without hydrogen bonds in the H₂O@C₆₀ crystal could result in a net polarization and ferroelectricity. The crystal structure and dielectric property of pure H₂O@C₆₀ crystal is revealed in this study by the synchrotron radiation X-ray structure analysis and dielectric measurement.  The crystal has a cubic structure similar to that of the empty C₆₀ crystal except that water molecules are trapped inside the carbon cages. The water molecule rotates around its oxygen atom at the cage center even at 20 K. The rotating water molecules respond to an external electric field due to its permanent dipole moment.