Laser-induced transformation of Li4C60 and Na4C60 polymers into metallic monomeric fulleride phases

High pressure study of Li-doped fullerides, Li(x)C60 (x = 4,12), by x-ray diffraction and Raman spectroscopy

In this article we study the alkali metal-intercalated 2D polymeric Li4C60 and the monomeric Li12C60 under pressure up to 40 GPa at room temperature, using x-ray diffraction and Raman spectroscopy. Li4C60 undergoes several transitions in the studied pressure range. At pressures lower than 8 GPa, we observed changes in both diffraction patterns and Raman scattering spectra, probably due to the displacement of Li atoms. At 8 GPa another structural and electronic transition occurs. We observe an enhancement of background and a broadening of diffraction peaks. Raman modes weaken and broaden considerably. An important structural transition occurs at around 16 GPa, in which new Raman bands exhibit features similar to those of a reported 3D C60 polymeric structure. The XRD data shows a collapse in volume with the simultaneous formation of amorphous material. The cell parameters deviate from their early pressure evolution and become less compressible. The high pressure study of highly doped monomeric Li12C60 shows that its structural integrity is retained up to 13 GPa, with increasing pressure-induced structural distortion and disorder. Above 13 GPa, Li12C60 transforms to a highly disordered state.

Raman study of the electron-phonon interaction in light alkali metal intercalated metallic fullerides

By laser-irradiating polymeric Li(4)C(60) and Na(4)C(60), we have obtained pure monomeric metallic phases stable at ambient conditions. Based on a systemic Raman analysis, we have determined the electron-phonon coupling constant for both metallic phases. The e-p coupling constants of Li- and Na-intercalated metallic fullerides are smaller than those of superconductive K(3)C(60) and Rb(3)C(60) and comparable to or slightly higher than that of ambient-pressure non-superconductive Cs(3)C(60). We predict that Na-doped fulleride could exhibit superconductivity with T(c) ∼ 10 K. Much lower T(c) or even no superconductivity can be expected for the Li-doped fulleride which exhibits a strong Li(+)-C interaction. These results contribute to the understanding of superconductivity in light alkali metal intercalated fullerides.