Superconductivity and cation-vacancy ordering in the rare-earth fulleride Yb2.75C60

Realization of Organocerium-Based Fullerene Molecular Materials Showing Mott Insulator-Type Behavior

Electron-rich organocerium complexes (C5Me4H)3Ce and [(C5Me5)2Ce(ortho-oxa)], with redox potentials E1/2 = -0.82 V and E1/2 = -0.86 V versus Fc/Fc+, respectively, were reacted with fullerene (C60) in different stoichiometries to obtain molecular materials. Structurally characterized cocrystals: [(C5Me4H)3Ce]2·C60 (1) and [(C5Me5)2Ce(ortho-oxa)]3·C60 (2) of C60 with cerium-based, molecular rare earth precursors are reported for the first time. The extent of charge transfer in 1 and 2 was evaluated using a series of physical measurements: FT-IR, Raman, solid-state UV-vis-NIR spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, and magnetic susceptibility measurements. The physical measurements indicate that 1 and 2 comprise the cerium(III) oxidation state, with formally neutral C60 as a cocrystal in both cases. Pressure-dependent periodic density functional theory calculations were performed to study the electronic structure of 1. Inclusion of a Hubbard-U parameter removes Ce f states from the Fermi level, opens up a band gap, and stabilizes FM/AFM magnetic solutions that are isoenergetic because of the large distances between the Ce(III) cations. The electronic structure of this strongly correlated Mott insulator-type system is reminiscent of the well-studied Ce2O3.

Probing the presence and absence of metal-fullerene electron transfer reactions in helium nanodroplets by deflection measurements

Metal-fullerene compounds are characterized by significant electron transfer to the fullerene cage, giving rise to an electric dipole moment. We use the method of electrostatic beam deflection to verify whether such reactions take place within superfluid helium nanodroplets between an embedded C₆₀ molecule and either alkali (heliophobic) or rare-earth (heliophilic) atoms. The two cases lead to distinctly different outcomes: C₆₀Na_n (n = 1-4) display no discernable dipole moment, while C₆₀Yb is strongly polar. This suggests that the fullerene and small alkali clusters fail to form a charge-transfer bond in the helium matrix despite their strong van der Waals attraction. The C₆₀Yb dipole moment, on the other hand, is in agreement with the value expected for an ionic complex.

Effect of Lanthanide Metal Complexation on the Properties and Electronic Structure of Single-Walled Carbon Nanotube Films

We spectroscopically analyze the effect of e-beam deposition of lanthanide metals on the electronic structure and conductivities of films of semiconducting (SC) single-walled carbon nanotubes (SWNTs) in high vacuum. We employ near-infrared and Raman spectroscopy to interpret the changes in the electronic structure of SWNTs on exposure to small amounts of the lanthanides (Ln = Sm, Eu, Gd, Dy, Ho, Yb), based on the behavior of the reference metals (M = Li, Cr) which are taken to exemplify ionic and covalent bonding, respectively. The analysis shows that while the lanthanides are more electropositive than the transition metals, in most cases they exhibit similar conductivity behavior which we interpret in terms of the formation of covalent bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = La, Nd, Gd, Dy, Ho]. However, only M = Eu, Sm, Yb show the continually increasing conductivity characteristic of Li, and this supports our contention that these metals provide the first examples of mixed covalent-ionic bis-hexahapto bonds [(η(6)-SWNT)M(η(6)-SWNT), where M = Sm, Eu, Yb].

Frontiers of organic conductors and superconductors

We review the development of conductive organic molecular assemblies including organic metals, superconductors, single component conductors, conductive films, conductors with a switching function, and new spin state (quantum spin liquid state). We emphasize the importance of the ionicity phase diagram for a variety of charge transfer systems to provide a strategy for the development of functional organic solids (Mott insulator, semiconductor, superconductor, metal, complex isomer, neutral-ionic system, alignment of chemical potentials, etc.). For organic (super)conductors, the electronic dimensionality of the solids is a key parameter and can be designed based on the self-aggregation ability of a molecule. We present characteristic structural and physical properties of organic superconductors.

Electronic structure of Eu-C(70) fullerides

The electronic structure of Eu-intercalated C(70) has been studied by a synchrotron radiation photoemission spectroscopy technique. At low intercalation levels (below the stoichiometry of Eu(3)C(70)), the photoemission data clearly exhibit charge transfer from Eu 6s states to the lowest-unoccupied-molecular-orbital (LUMO) and the LUMO + 1 of C(70). The amount of charge transfer reaches its maximum far before intercalation saturation. Detailed analysis reveals that most of the 5d6s electrons of Eu occupy the so-called interstitial states in the saturation phase (Eu(9)C(70)). The interstitial states are induced by a Eu sub-lattice formed at heavy intercalation levels, and comprise substantial 6s-π hybridized states. The π states participating in the hybridization are mainly the HOMO - n (n = 6-10) orbitals. The PES data also reveal the semiconducting property for both Eu(3)C(70) and Eu(9)C(70). The 6s-(HOMO - n) hybridization and the semiconducting property should play important roles in understanding the ferromagnetic mechanism for Eu(9)C(70).

Mixed valency in rare-earth fullerides

Mixed-valence phenomena associated with the highly correlated narrow-band behaviour of the 4f electrons in rare earths are well documented for a variety of rare-earth chalcogenides, borides and intermetallics (Kondo insulators and heavy fermions). The family of rare-earth fullerides with stoichiometry RE2.75C60 (RE=Sm, Yb, Eu) also displays an analogous phenomenology and a remarkable sensitivity of the rare-earth valency to external stimuli (temperature and pressure) making them the first known molecular-based members of this fascinating class of materials. Using powerful crystallographic and spectroscopic techniques which provide direct indications of what is happening in these materials at the microscopic level, we find a rich variety of temperature- and pressure-driven abrupt or continuous valence transitions-the electronically active fulleride sublattice acts as an electron reservoir that can accept electrons from or donate electrons to the rare-earth 4f/5d bands, thereby sensitively modulating the valence of the rare-earth sublattice.

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-T(c)) copper oxide superconductors. Metal fulleride salts-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 + epsilon) + 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 behaviour. We attribute the large negative thermal expansion, unprecedented in fullerene or other molecular systems, to a quasi-continuous valence transition from Sm(2+) towards the smaller Sm((2+epsilon)+), analogous to the valence or configuration transitions encountered in intermediate-valence Kondo insulators like SmS (ref. 3).

Synthesis, structure, and magnetic properties of the fullerene-based ferromagnets Eu3C70 and Eu9C70

Intercalation of C(70) with europium affords two kinds of magnetic compounds, a canted antiferromagnet Eu(x)C(70) (x approximately 3) and a ferromagnet Eu(x)C(70) (x approximately 9) with transition temperatures (T(C)) of 5 and 38 K, respectively. The Curie constants in the paramagnetic phase and the saturation moment in the ferromagnetic phase are both understood by the full moment of Eu(2+) for both systems. The structure of Eu(3)(-)(delta)C(70) (delta approximately 0.27) is pseudo-monoclinic, derived by a simple deformation of the parent face-centered cubic (fcc) structure. Eu(9)(-)(delta)C(70) (delta approximately 0.2) forms an fcc structure, in which cuboctahedral clustering of Eu(2+) ions is observed in the enhanced size octahedral holes. The observed T(C) of the Eu(9)(-)(delta)C(70) ferromagnet is comparable to or larger than those of simple binary Eu-based ferromagnets, such as Eu chalcogenides or carbides, despite the low atomic ratio of Eu in the chemical formulas. This can be understood by the short Eu(2+)-Eu(2+) distances and high coordination numbers permitted by the multiple occupation by Eu(2+) ions of the expanded octahedral interstitial sites in higher fullerene-based solids.