I. Loa

High Pressure Studies of the Raman‐Active Phonons in Carbon Nanotubes

We report high pressure Raman studies on as-prepared, purified, and solubilized single-walled and aligned multi-walled carbon nanotubes. The pressure dependence of radial (R) and tangential (T) vibrational bands in these samples is measured and compared with the results from other studies. In single-walled nanotubes, an abrupt drop in the intensity of these bands is seen near 2 GPa, indicative of a phase transition. Experiments on single (unbundled) tubes reveal a ≈10 cm—1 upshift of the R band relative to its frequency in bundled tubes. This is opposite to the predictions of calculations that include the intertube van der Waals interaction only and is explained by the changes in the electronic band dispersion driven by tube–tube interactions. Surprisingly, the pressure dependence of the R and T bands in unbundled tubes is very similar to that seen in bundled tubes, which indicates that the changes in the electronic band structure might significantly influence the pressure dependence.

Calculated elastic and electronic properties of MgB2 at high pressures

Abstract The effect of high pressure on structural and electronic properties of the novel superconductor MgB 2 has been calculated using the full-potential linearized augmented-plane-wave method. Despite the layered crystal structure of MgB 2 nearly isotropic compression (bulk modulus B 0 =140.1(6) GPa) is found with only a 1.2% decrease of the c/a ratio at 10 GPa. The effect of pressure on the critical temperature has been estimated on the basis of BCS theory and good agreement with experimental data is found. Our results suggest that it is a combination of increasing phonon frequency and decreasing electronic density of states at the Fermi level which leads to the observed decrease of the critical temperature under pressure.

Radial superlattices and single nanoreactors

We investigate the wall structure and thermal stability of individual freestanding rolled-up nanotubes (RUNTs) using micro-Raman spectroscopy, transmission electron microscopy, and selected area electron diffraction. Our studies reveal that the walls of the InAs/GaAs RUNTs consist of a radial superlattice comprising alternating crystalline and noncrystalline layers. Furthermore, we locally heated individual RUNTs with a laser beam, and Raman spectroscopy was used in situ to monitor any structural changes. At about 300 °C the heated part of a RUNT starts to oxidize and eventually transforms into crystalline β-Ga2O3. This result shows that RUNTs can serve as nanoreactors that locally synthesize material at intentional places on a substrate surface.

MgB2 under pressure : phonon calculations, Raman spectroscopy, and optical reflectance

The effect of pressure on optical phonon frequencies of MgB2 has been calculated using the frozen-phonon approach based on a pseudopotential method. Gruneisen parameters of the harmonic mode frequencies are reported for the high-frequency zone-centre E2g and B1g and the zone-boundary E2u and B2u modes at A. Anharmonic effects of phonon frequencies and the implications of the calculated phonon frequency shifts for the pressure dependence of the superconducting transition temperature of MgB2 are discussed. Also reported are Raman and optical reflectance spectra of MgB2 measured at high pressures. The experimental observations in combination with calculated results indicate that the broad spectral features that we observed in the Raman spectra at frequencies between 500 and 900 cm-1 cannot be attributed to first-order scattering by zone-centre modes, but originate in part from a chemical species other than MgB2 at the sample surface and in part from a maximum in the MgB2 phonon density of states. Low-temperature Raman spectra taken at ambient pressure showed increased scattering intensity in the region below 300 cm-1.

Structure of sodium above 100 GPa by single-crystal x-ray diffraction

At pressures above a megabar (100 GPa), sodium crystallizes in a number of complex crystal structures with unusually low melting temperatures, reaching as low as 300 K at 118 GPa. We have utilized this unique behavior at extreme pressures to grow a single crystal of sodium at 108 GPa, and have investigated the complex crystal structure at this pressure using high-intensity x-rays from the new Diamond synchrotron source, in combination with a pressure cell with wide angular apertures. We confirm that, at 108 GPa, sodium is isostructural with the cI16 phase of lithium, and we have refined the full crystal structure of this phase. The results demonstrate the extension of single-crystal structure refinement beyond 100 GPa and raise the prospect of successfully determining the structures of yet more complex phases reported in sodium and other elements at extreme pressures.

Effect of pressure on the Raman modes of antimony

The effect of pressure on the zone-center optical phonon modes of antimony in the A7 structure has been investigated by Raman spectroscopy. The A(g) and E-g frequencies exhibit a pronounced softening with increasing pressure, the effect being related to a gradual suppression of the Peierls-like distortion of the A7 phase relative to a cubic primitive lattice. Also, both Raman modes broaden significantly under pressure. Spectra taken at low temperature indicate that the broadening is at least partly caused by phonon-phonon interactions. We also report results of ab initio frozen-phonon calculations of the A(g) and E-g mode frequencies. The presence of strong anharmonicity is clearly apparent in calculated total energy versus atom displacement relations. Pronounced nonlinearities in the force versus displacement relations are observed. Structural instabilities of the Sb A7 phase are briefly addressed in the Appendix.

Crystal structure and lattice dynamics of AlB2 under pressure and implications for MgB2

The effect of high pressures, up to 40 GPa, on the crystal structure and lattice dynamics of

Vanadium oxides V2O5 and NaV2O5 under high pressures: Structural, vibrational, and electronic properties

{\mathrm{AlB}}_{2}

Vibrational properties of ZnTe at high pressures

was studied by synchrotron x-ray powder diffraction, Raman spectroscopy, and first-principles calculations. There are no indications for a pressure-induced structural phase transition. The Raman spectra of the metallic sample exhibit a well-defined peak near

Extraordinarily complex crystal structure with mesoscopic patterning in barium at high pressure

980{\mathrm{cm}}^{\ensuremath{-}1}