Vittoria Pischedda

High pressure study of Li-doped fullerides, LixC60 (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.

Small angle scattering methods to study porous materials under high uniaxial strain

We developed a high pressure cell for the in situ study of the porosity of solids under high uniaxial strain using neutron small angle scattering. The cell comprises a hydraulically actioned piston and a main body equipped with two single-crystal sapphire windows allowing for the neutron scattering of the sample. The sample cavity is designed to allow for a large volume variation as expected when compressing highly porous materials. We also implemented a loading protocol to adapt an existing diamond anvil cell for the study of porous materials by X-ray small angle scattering under high pressure. The two techniques are complementary as the radiation beam and the applied pressure are in one case perpendicular to each other (neutron cell) and in the other case parallel (X-ray cell). We will illustrate the use of these two techniques in the study of lamellar porous systems up to a maximum pressure of 0.1 GPa and 0.3 GPa for the neutron and X-ray cells, respectively. These devices allow obtaining information on the evolution of porosity with pressure in the pore dimension subdomain defined by the wave-numbers explored in the scattering process. The evolution with the applied load of such parameters as the fractal dimension of the pore-matrix interface or the apparent specific surface in expanded graphite and in expanded vermiculite is used to illustrate the use of the high pressure cells.

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.

Pressure-induced polyamorphism in TiO{sub 2} nanoparticles

Two different nanometric (6 nm) TiO{sub 2} compounds, anatase polycrystals and amorphous particles, were investigated under high pressure using Raman spectroscopy. Nanoanatase undergoes a pressure-induced amorphization. The pressure-induced transformations of this mechanically prepared amorphous state are compared with those of a chemically prepared amorphous particles. In the mechanically prepared amorphous state, a reversible transformation from a low-density amorphous state to high-density amorphous state (HDA1) is observed in the range 13-16 GPa. In the chemically prepared sample, a transformation to a new high-density amorphous state (HDA2) is observed at around 21 GPa. Further compression leads to the transformation HDA2{yields}HDA1 at {approx}30 GPa. We demonstrate that depending on the starting amorphous material, the high-pressure polyamorphic transformations may differ. This observation indicates that pressure is a suited tool to discriminate between nanomaterials apparently similar at ambient conditions.