Régis Debord

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.

Thermoelectric La-doped SrTiO3 epitaxial layers with single-crystal quality: from nano to micrometers

Abstract High-quality thermoelectric La0.2Sr0.8TiO3 (LSTO) films, with thicknesses ranging from 20 nm to 0.7 μm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 × 10−4 Ω cm at room temperature), one order of magnitude lower than standard commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectric measurements where Seebeck coefficients of approximately –60 μV/K have been recorded for all films. These LSTO films can be integrated on Si for non-volatile memory structures or opto-microelectronic devices, functioning as transparent conductors or thermoelectric elements.