Fullerides: superconductivity at the limit

High-T C superconductivity in Cs3C60 compounds governed by local Cs-C60 Coulomb interactions

Unique among alkali-doped A ₃C₆₀ fullerene compounds, the A15 and fcc forms of Cs₃C₆₀ exhibit superconducting states varying under hydrostatic pressure with highest transition temperatures at [Formula: see text]  =  38.3 and 35.2 K, respectively. Herein it is argued that these two compounds under pressure represent the optimal materials of the A ₃C₆₀ family, and that the C₆₀-associated superconductivity is mediated through Coulombic interactions with charges on the alkalis. A derivation of the interlayer Coulombic pairing model of high-T _C superconductivity employing non-planar geometry is introduced, generalizing the picture of two interacting layers to an interaction between charge reservoirs located on the C₆₀ and alkali ions. The optimal transition temperature follows the algebraic expression, T _C0  =  (12.474 nm² K)/ℓζ, where ℓ relates to the mean spacing between interacting surface charges on the C₆₀ and ζ is the average radial distance between the C₆₀ surface and the neighboring Cs ions. Values of T _C0 for the measured cation stoichiometries of Cs_3-x C₆₀ with x  ≈  0 are found to be 38.19 and 36.88 K for the A15 and fcc forms, respectively, with the dichotomy in transition temperature reflecting the larger ζ and structural disorder in the fcc form. In the A15 form, modeled interacting charges and Coulomb potential e²/ζ are shown to agree quantitatively with findings from nuclear-spin relaxation and mid-infrared optical conductivity. In the fcc form, suppression of [Formula: see text] below T _C0 is ascribed to native structural disorder. Phononic effects in conjunction with Coulombic pairing are discussed.

Mixing Li and Cs in the reduction of corannulene for the assembly of a cesium-capped sandwich with a hexanuclear heterometallic core

In the first Li/Cs organometallic self-assembly, the tetrareduced corannulene decks are angled up to keep the highly-charged (Li₃Cs₃)⁶⁺ unit in between, with two external Cs⁺ ions capping the sandwich from outside.

Jahn-Teller instability in cationic boron and carbon buckyballs B80+ and C60+: a comparative study

This paper investigates the Jahn-Teller effect in the icosahedral cation B(80)(+) and compares the descent in symmetry with that in C(60)(+). For both cations the icosahedral ground state is a (2)H(u) state, which exhibits a H ⊗ (g ⊕ 2h) Jahn-Teller instability. A detailed construction of the potential energy surface of B(80)(+) using different DFT methods including B3LYP/6-31G(d), VWN/6-31G(d), PBE/TZP and PBE/6-31G(d) shows that, contrary to C(60)(+), which prefers D(5d) symmetry, the ground state of B(80)(+) adopts S(6) point group symmetry. A D(3d) structure is identified as a saddle point among the S(6) minima of B(80)(+). The distortion of D(3d) to S(6) in B(80)(+) is attributed to a superposition of Jahn-Teller and pseudo-Jahn-Teller effects. Imaginary modes, transforming as the g(g) representation, which are present in neutral icosahedral B(80), form the dominant symmetry breaking active modes. The pronounced difference between the JT effects in the boron and carbon buckyball cations is due to the plasticity of the boron caps. The calculated Jahn-Teller stabilization of B(80)(+) is nearly 1549 cm(-1) (PBE/TZP), which exceeds the stabilization of 596 cm(-1) computed for C(60)(+) at the same level.