Bonding nature and structural phase transition in fullerene based nanomaterials

Rare-earth-metal-doped fullerides with a nominal composition of R ₃C ₇₀ (R -Sm, Eu, Yb) adopt a pseudomonoclinic structure in which C ₇₀ dimers glued with rare-earth ions are involved. High-temperature powder X-ray diffraction experiments revealed that the dimers undergo reversible first-order structural phase transitions, associated with reduction of the unit cell volume, similar to the results observed in previous high-pressure experiments. Structural analyses showed that C ₇₀ molecules are realigned to form closely packed structures, causing a reduction of volume at high temperature. The derived charge density map indicates that the transitions can be regarded as reversible structural changes from fullerene dimers to monomers. These features are ascribed to the unique bonding nature of rare-earth C ₇₀ compounds.