Morphology-tuned wurtzite-type ZnS nanobelts.

Nanometre-sized inorganic dots, wires and belts have a wide range of electrical and optical properties, and variable mechanical stability and phase-transition mechanisms that show a sensitive dependency on size, shape and structure. The optical properties of the semiconductor ZnS in wurtzite structures are considerably enhanced, but the lack of structural stability limits technological applications. Here, we demonstrate that morphology-tuned wurtzite ZnS nanobelts show a particular low-energy surface structure dominated by the +/-[210] surface facets. Experiments and calculations show that the morphology of ZnS nanobelts leads to a very high mechanical stability to approximately 6.8 GPa, and also results in an explosive mechanism for the wurtzite-to-sphalerite phase transformation together with in situ fracture of the nanobelts. ZnS wurtzite nanobelts provide a model that is useful not only for understanding the morphology-tuned stability and transformation mechanism, but also for improving synthesis of metastable nanobelts with quantum effects for electronic and optical devices.

Sound velocity and elasticity of tetragonal lysozyme crystals by Brillouin spectroscopy.

Quasilongitudinal sound velocities and the second-order elastic moduli of tetragonal hen egg-white lysozyme crystals were determined as a function of relative humidity (RH) by Brillouin scattering. In hydrated crystals the measured sound velocities in the [110] plane vary between 2.12 +/- 0.03 km/s along the [001] direction and 2.31 +/- 0.08 km/s along the [110] direction. Dehydration from 98% to 67% RH increases the sound velocities and decreases the velocity anisotropy in (110) from 8.2% to 2.0%. A discontinuity in velocity and an inversion of the anisotropy is observed with increasing dehydration providing support for the existence of a structural transition below 88% RH. Brillouin linewidths can be described by a mechanical model in which the phonon is coupled to a relaxation mode of hydration water with a single relaxation time of 55 +/- 5 ps. At equilibrium hydration (98% RH) the longitudinal moduli C(11) + C(12) + 2C(66) = 12.81 +/- 0.08 GPa, C(11) = 5.49 +/- 0.03 GPa, and C(33) = 5.48 +/- 0.05 GPa were directly determined. Inversion of the measured sound velocities in the [110] plane constrains the combination C(44) + (1/2)C(13) to 2.99 +/- 0.05 GPa. Further constraints on the elastic tensor are obtained by combining the Brillouin quasilongitudinal results with axial compressibilities determined from high-pressure x-ray diffraction. We constrain the adiabatic bulk modulus to the range 2.7-5.3 GPa.

Elasticity of MgO and a primary pressure scale to 55 GPa.

We have studied the elasticity and pressure-density equation of state of MgO in diamond cells to 55 GPa and have doubled the previous pressure limit of accurate elasticity determinations for crystals. Integrating single-crystal velocity data from Brillouin scattering measurements and density data from polycrystalline x-ray diffraction, we obtained the three principal elastic tensor elements (C(11), C(12), and C(44)) and various secondary elasticity parameters, including single-crystal elastic anisotropy, Cauchy relation, aggregate sound velocities, and Poisson's ratio, as functions of pressure. The present study also provides a direct determination of pressure without recourse to any prior pressure standard, thus creating a primary pressure scale. The commonly used ruby fluorescence pressure scale has thus been improved to 1% accuracy by the new MgO scale.

Sound velocities in dense hydrogen and the interior of jupiter.

Sound velocities in fluid and crystalline hydrogen were measured under pressure to 24 gigapascals by Brillouin spectroscopy in the diamond anvil cell. The results provide constraints on the intermolecular interactions of dense hydrogen and are used to construct an intermolecular potential consistent with all available data. Fluid perturbation theory calculations with the potential indicate that sound velocities in hydrogen at conditions of the molecular layer of the Jovian planets are lower than previously believed. Jovian models consistent with the present results remain discrepant with recent free oscillation spectra of the planet by 15 percent. The effect of changing interior temperatures, the metallic phase transition depth, and the fraction of high atomic number material on Jovian oscillation frequencies is also investigated with the Brillouin equation of state. The present data place strong constraints on sound velocities in the Jovian molecular layer and provide an improved basis for interpreting possible Jovian oscillations.

Synchrotron X-ray Diffraction Measurements of Single-Crystal Hydrogen to 26.5 Gigapascals.

The crystal structure and equation of state of solid hydrogen have been determined directly to 26.5 gigapascals at room temperature by new synchrotron x-ray diffraction techniques. Solid hydrogen remains in the hexagonal close-packed structure under these pressure-temperature conditions and exhibits increasing structural anisotropy with pressure. The pressure-volume curve determined from the x-ray data represents the most accurate experimental measurement of the equation of state to date in this pressure range. The results remove the discrepancy between earlier indirect determinations and provide a new experimental constraint on the molecular-to-atomic transition predicted at higher pressures.