Tunneling motion and antiferroelectric ordering of lithium cations trapped inside carbon cages

Dielectric and X-ray diffraction measurements of [Li@C₆₀](PF₆) single crystals reveal the motion of the Li⁺ cations inside the C₆₀ cages at low temperature. An increase in the dielectric permittivity below 100 K is consistent with a combined tunneling and hopping motion of the Li⁺ cation between two positions inside the C₆₀ cage. A phase transition accompanied by a decrease in the dielectric permittivity at T_C = 24 K is explained by an antiferroelectric ordering of the Li₊ cations. The Li₊ ordering is caused by interactions among electric dipole moments formed between the Li⁺ cations inside and the PF₆ ^- anions outside the C₆₀ cages. The electric dipole moments that are switched by the Li⁺ tunneling and interact with each other are potential qubits in a quantum computer using electric dipole moments.