Solid-state chemistry of rare-earth compounds has been a central issue to investigate strongly correlated behavior in their optical, electronic, and structural properties. Of particular interest are compounds incorporating mixed-valence elements such as Sm, Eu, and Yb where divalent and trivalent states co-exist in a delicate balance. Triggered by physical perturbations (pressure, temperature, magnetic field) and chemical substitution, their average-valence state can be easily modified to induce various property changes (e.g. archetypal Kondo-insulator behavior as in samarium sulfide, which exhibits a pressure-driven valence transition from Sm2+ to Sm3+ with volume collapse accompanied by black-golden color change). Rare-earth C60 fullerides with stoichiometry RE2.75C60 (RE = Sm, Yb) constitute an intriguing class of strongly-correlated systems in which electronic instabilities of the RE cation sublattice couple to the electronic and lattice degrees-of-freedom of the fulleride C60 anion sublattice leading to a giant 2a×2a×2a super-lattice formation (a: corresponds to a typical fullerene compounds lattice size of ~14 Å). To date our synchrotron X-ray absorption work has revealed that a 1st-order phase transition takes place in these systems in the pressure range up to around 6 GPa, accompanied by a change in rare-earth oxidation state from close to 2+ towards the 3+ state. Although past synchrotron X-ray diffraction and Raman spectroscopic studies in Sm2.75C60 found signatures of strong coupling between pressure-induced mixed valence transitions and drastic volume changes Δ~6 % in combination with insulator-metal transitions, the origin of these phenomena has been difficult to understand due to the complicated crystal structures at high pressure. Here we present our results of successfully determining the crystal structure in the family of (Sm1-xCax)2.75C60 (0 ≤ x ≤ 1) systems following the structural transition at high pressure. The 1st-order isosymmetric structural transformation is associated with local geometry changes of the metal ions - off-centered Sm/Ca ions move towards the centers of both Td and Oh interstitial sites of the C60 sublattice. Excellent agreement between the pressure dependence of the unit cell volume and the rare-earth valence is found providing a complete understanding of the structural and electronic properties of these materials.

Pressure-induced isostructural volume collapse accompanied by local geometry changes in rare-earth C60 fullerides

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