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Temperature-induced valence transition and associated lattice collapse in samarium fulleride


The different degrees of freedom of a given system are usually independent of each other but can in some materials be strongly coupled, giving rise to phase equilibria sensitively susceptible to external perturbations. Such systems often exhibit unusual physical properties that are difficult to treat theoretically, as exemplified by strongly correlated electron systems such as intermediate-valence rare-earth heavy fermions and Kondo insulators, colossal magnetoresistive manganites and high-transition temperature (high-Tc) copper oxide superconductors. Metal fulleride salts1—metal intercalation compounds of C60—and materials based on rare-earth metals also exhibit strong electronic correlations. Rare-earth fullerides thus constitute a particularly intriguing system—they contain highly correlated cation (rare-earth) and anion (C60) sublattices. Here we show, using high-resolution synchrotron X-ray diffraction and magnetic susceptibility measurements, that cooling the rare-earth fulleride Sm2.75C60 induces an isosymmetric phase transition near 32 K, accompanied by a dramatic isotropic volume increase and a samarium valence transition from (2 + ε) + to nearly 2 + . The negative thermal expansion—heating from 4.2 to 32 K leads to contraction rather than expansion—occurs at a rate about 40 times larger than in ternary metal oxides typically exhibiting such behaviour2. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm2+ towards the smaller Sm(2+ε)+, analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

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Figure 1: Selected region of the diffraction profile of Sm2.75C60 showing the temperature evolution of the (444) Bragg reflection (λ = 0.79980 Å).
Figure 2: Temperature evolution of structural parameters of Sm2.75C60.
Figure 3: Building block of the orthorhombic superstructure of Sm2.75C60, which can be obtained by doubling along all three lattice directions.
Figure 4: Temperature dependence of magnetic susceptibilities.


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We thank the ESRF for synchrotron X-ray beamtime, the Marie Curie Fellowship programme of the European Union “Improving the Human Research Potential” for support (J.A., K.P) and the Royal Society for a Dorothy Hodgkin Research Fellowship (S.M.).

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Correspondence to Konstantinos Papagelis or Serena Margadonna.

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Supplementary Information: Structural information derived from the Rietveld analysis of the synchrotron X-ray diffraction data of Sm2.75C60 at 5 K. List of final refined positional parameters and selected metal-carbon distances. Figures of final Rietveld analysis of the diffraction data. (PDF 235 kb)

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Arvanitidis, J., Papagelis, K., Margadonna, S. et al. Temperature-induced valence transition and associated lattice collapse in samarium fulleride. Nature 425, 599–602 (2003).

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