Structure and Phase Transition of Rock-Salt-Type Li@C₆₀ Crystal

Abstract

Fullerene with encapsulated lithium cation, Li⁺@C₆₀, can be regarded as an alkaline cation owing to the spherical shape and positively charged Li⁺ cation. We present the crystal structure and phase transition of a rock-salt-type Li⁺@C₆₀ crystal that consists of the freely rotating Li⁺@C₆₀ cations and orientationally disordered PF₆⁻ anions at 400 K. The orientations of the C₆₀ cages and PF₆⁻ anions are perfectly ordered below T_C＝370 K, whereas the Li⁺ cations are thermally hopping within the cages even at 155 K. A gradual localization of the Li⁺ cations at polar two positions in each C₆₀ below 100 K and a negative thermal expansion of the static C₆₀ are demonstrated.