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

Rare-earth-metal-doped fullerides with nominal composition of $R_3{\mathrm{C}}_{70}$ $(R=\mathrm{Sm},$ Eu) adopt a pseudomonoclinic structure in which ${\mathrm{C}}_{70}$ 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, ${\mathrm{C}}_{70}$ molecules are realigned so that the fivefold (long) axes are perpendicular to the $\left(101\mathrm{}¯\right)$ or $(111\mathrm{}¯{)}_{\mathrm{fcc}}$ 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 ${\mathrm{C}}_{70}$ compounds.