Pressure-induced valence transition in the mixed-valence (Sm1/3Ca2/3)2.75C60 fulleride

Abstract

(Sm_1/3Ca_2/3)_2.75C₆₀ is a member of the family of non-stoichiometric strongly-correlated rare-earth fullerides, (Sm_1−xCa_x)_2.75C₆₀ (0 ≤ x ≤ 1), in which an orthorhombic 2 × 2 × 2 supercell of the face-centred cubic (fcc) unit cell of stoichiometric A₃C₆₀ (A = alkali metal) fullerides is stabilized by the long-range ordering of partially-occupied metal sites. At ambient temperature and pressure, it is a mixed valence compound with an average Sm valence of +2.33(2) implying a formal charge of −5.78 for the C₆₀ anions. Here we study its electronic response to the application of pressure in the range 0–9 GPa. Synchrotron X-ray absorption measurements in the high-resolution partial fluorescence yield mode (PFY-XAS) at ambient temperature show the onset of an abrupt strongly hysteretic (width ∼2.5 GPa) first-order reversible phase transition at ∼4 GPa, accompanied by a drastic increase in the bulk Sm valence by ∼20% to +2.71(3). This is coincident with the huge lattice contraction and concomitant insulator-to-metal transition encountered before for Sm_2.75C₆₀ in the same pressure range and provides a possible explanation of the physical properties in terms of the strong coupling between the lattice and electronic degrees of freedom. The behaviour is reminiscent of the electronic and lattice response to pressure of highly correlated Kondo insulators like SmS and its ternary derivatives, Sm_1−xR_xS (R = Ca, Y, etc.). However, a distinguishing feature of the fulleride systems is that the C₆₀ anionic sublattice can act as an electron reservoir due to the availability of a close-lying band derived by the t_1g orbitals and can accept excess charge as the 4f-electron occupation number decreases. The observed electronic response as a function of pressure is thus opening new possibilities for accessing metallic fullerides at elevated pressures.