Distorted and inequivalently chargedC60anions inYb2.75

Abstract

Near-edge x-ray absorption measurements from single-phase Yb 2.75 C 60 establish that all of the Yb cations are divalent, meaning that the average negative formal charge of the C 60 anions is 5.5. Extended x-ray absorption fine structure ͑EXAFS͒ and Raman measurements also reveal that the C 60 anions are distorted in shape, despite their well-ordered local structure around the Yb cations. These findings are combined with a model to show that the charge transferred from Yb to C 60 is both anisotropically distributed on the anions and inequivalently partitioned among them within the unit cell. In the model, the inhomogeneously distributed transferred charge ͑which arises from the comparatively greater electron affinity of the C 60 pentagonal faces͒ interacts strongly with the divalent cations and leads to significant anion distortions. The redistribution of transferred charge is traceable to the three crystallographically different types of C 60 anions that are surrounded by 8, 10, or 12 Yb cations, a situation similar to the inequivalently coordinated and charged O anions in the high-T c compound YBa 2 Cu 3 O 7 . Additional EXAFS and Raman data from Yb 2.75 C 60 heated above 670°C show the removal of local order around the Yb cations and the production of an amorphous carbidic phase containing Yb͑III͒. Our collective results, along with comparisons to analogous fulleride and organometallic compounds, provide insight into the distinctive structural features of Yb 2.75 C 60 and its unique distribution of anion charge. They also have general implications for a variety of other metal-doped fullerides.

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

“…6 Although the original work on the lanthanide complexes of benzene [Ln(η 6 -t Bu 3 C 6 H 3 ) 2 ] confirmed the formulation of these compounds as neutral, covalent, π-bonded molecules in which the metal was zerovalent, 3 the lanthanide compounds of C 60 began with the idea that they would form salts 10 in analogy with the ionic alkali metal fullerides. 11,12 Indeed, the observation that some of these compounds (M = Yb, Sm) are superconductors strongly supports the idea that there are free electrons in these compounds which reside primarily on C 60 , 10,13,14 as do the physical properties of the Eu x C 60 compounds. 15 All of these authors assign the oxidation state of the lanthanide as +2 but the importance of covalent Special Issue: Advances towards Electronic Applications in Organic Materials character in these compounds is discussed at length.…”

“…22,35,36 Furthermore, there is the promise of new ferromagnetic and superconducting materials based on the organometallic single layer graphene and graphene sandwich compounds and their relationship to the graphite and C 60 intercalation compounds. 4,5,10,[12][13][14][15]21,35,37 …”

“…15 All of these authors assign the oxidation state of the lanthanide as +2 but the importance of covalent Special Issue: Advances towards Electronic Applications in Organic Materials character in these compounds is discussed at length. 14,15 In this respect, the C 60 compounds are rather different from the lanthanide complexes of benzene and SWNTs discussed above in that C 60 is strongly curved and possesses five-membered rings (5-MRs); these features of the geometric and electronic structure contribute to the ability of the fullerenes to stabilize excess electron density. 16,17 Although the fullerenes are incapable of forming stable π-complexes with transition metals requiring a hapticity greater than two, given the size and orbital extent of the lanthanides, they may be effective in facilitating η 5 -coordination of the 5-MRs. 14,18 Nevertheless, it is apparent that similar issues are relevant in the C 60 and SWNT complexes of the lanthanides; that is, the question of covalent and ionic bonding, although the fullerides prepared to date only include M = Sm, Eu, and Yb, which are known to readily form the +2 oxidation state.…”

Electronic structure ofYb2.75C60

“…The reported valence photoemission spectra of Xia et al 3 were for the Yb/C 60 interface rather than Yb 2.75 C 60 . On the other hand, some reported works 4,5 have made it feasible now to study the valence-band structure of Yb 2.75 C 60 . Core-level x-ray photoemission ͑XPS͒ data 4 of a film sample showed that C 60 mixing with Yb produced single-phase fulleride ͑the single phase was incorrectly determined to be Yb 2 C 60 in Ref.…”

Stereochemical effect of covalent chemistry on the electronic structure and properties of the carbon allotropes and graphene surfaces

“…Nevertheless it has been argued that some of the lanthanide compounds of C 60 possess significant covalent character, perhaps because the d-orbitals of these large metals are able to effectively overlap with the rehybridized C 60 p-orbitals [51][52][53]. The overlap of transition metal d-orbitals with the carbon nanotube p-orbitals has been extensively discussed previously and clearly constitutes an intermediate case between the fullerenes and graphene [32,54,55].…”