Chemical tuning of samarium valence in mixed valence (Sm1-xCax)2.75C60 fullerides

Abstract

Rare-earth (RE) fullerides are an intriguing family of materials in which electronic instabilities on the RE sublattice couple to the electronic and lattice degrees-of-freedom of the strongly-correlated fulleride sublattice. In particular, insulating Sm2.75C60 adopts an orthorhombic superstructure, arising from long-range ordering of Sm partial vacancies, and can be driven to a lattice-collapsed metallic state due to a valence transition towards Sm3+ upon pressurization. Here we use synchrotron X-ray absorption spectroscopy (XAS) at the Sm L3 edge at ambient conditions to authenticate the mixed valence character of the material by identifying two features due to distinct Sm2+ and Sm3+ components – their relative intensity allows a direct measure of the average Sm valence in Sm2.75C60 at +2.07(3). We then attempt to mimic the physical pressure effect on the electronic properties by co-intercalation of the smaller-size valence-precise Ca2+ ion to form the series of ternary solid solutions, (Sm1-xCax)2.75C60 (0 ≤ x ≤ ⅔). XAS measurements in their high-resolution partial fluorescence yield (PFY) variance find that chemical pressure leads to an increase of the Sm3+ contribution to the average valence by >10% in the most contracted member, (Sm⅓Ca⅔)2.75C60 of the present study with the average valence reaching a value of +2.33(2). Assuming full charge transfer between the metal ions and C60, the charge on the C60 units remains invariant throughout at approximately −5.78. This opens the possibility that the system can be tuned further towards the higher average valences needed for the high-pressure insulator-to-metal transition in Sm2.75C60 to be shifted to ambient pressure with a single-valence Sm3+ state accessible at a Ca content of x ~0.9.