Isosymmetric Lattice Collapse in Mixed-Valence Rare-Earth Fullerides at High Pressure─Coupling of Lattice and Electronic Degrees of Freedom

The orthorhombic-structured (Sm1/3Ca2/3)2.75C60 is a member of the family of rare-earth metal intercalated C60 fullerides with stoichiometry, RE2.75C60 (RE = rare-earth metal). At ambient conditions, it crystallizes with the formation of a 2a × 2a × 2a (a ∼ 14 Å) supercell of the face-centered cubic (fcc) A3C60 (A = alkali metal) unit cell. The superstructure is stabilized by the long-range ordering of tetrahedral-site Sm/Ca metal defects (O phase). Synchrotron X-ray powder diffraction measurements at high pressure and ambient temperature reveal that (Sm1/3Ca2/3)2.75C60 is a compressible solid with a bulk modulus, K0 = 24(1) GPa, that transforms to a more densely packed isostructural high-pressure O′ phase above ∼4 GPa. The first-order phase transition retains the formation of the superstructure and is accompanied by a discontinuous lattice size decrease (ΔV/V ∼ 2%). The O′ phase is stabilized by the release of the steric crowding that develops upon compression; the Sm/Ca metal ions, which reside in the tetrahedral and octahedral holes of the fulleride sublattice, shift from their off-centered positions at low pressure (O phase) to nest at the centers of the interstices (O′ phase). The nearly exact coincidence of the pressure response of the reversible hysteretic O ↔ O′ phase transformation with that of the samarium valence transition from +2.33(2) to +2.71(3), as established before by synchrotron X-ray absorption spectroscopy, unambiguously establishes an intimate link between the crystal and electronic structures of the (Sm1/3Ca2/3)2.75C60 fulleride.