Pressure-induced structural phase transition in fullerides doped with rare-earth metals

Rare-earth-metal-doped fullerides with nominal composition of R3C70 (R = Sm, Eu) adopt a pseudomonoclinic structure in which C-70 dimers glued with rare-earth ions are involved. High-pressure powder x-ray diffraction experiments revealed that these compounds undergo a reversible first-order structural phase transition at 1.5 GPa, associated with 2.7%-2.9% reduction of the unit cell volume. Structural analyses showed that the rare-earth ions, which are located close to the edge of tetrahedral sites at ambient pressure, move back to the center of the tetrahedral sites. Simultaneously, C-70 molecules are realigned so that the fivefold (long) axes are perpendicular to the (10 (1) over bar) or (11 (1) over bar)(fcc) plane at high pressure. The derived charge density map indicates that the transition is regarded as a structural change from dimers to three-dimensional polymers of fullerenes. These features are ascribed to the unique bonding nature in rare-earth C-70 compounds.