International audienceX-ray diffraction measurements on MnGa2Se4, a II-III2-VI4 magnetic semiconductor compound, are made as a function of pressure up to 25 GPa. It is found that in the range of 0P12 GPa the structure is a defect tetragonal; in the range between 12 and 14 GPa a two phase field +NaCl type exists. It is found that at about 14 GPa a transition to the cubic NaCl-type structure occurs and it appeared that this transition is irreversible. From the analysis of the x-ray diffraction lines, lattice parameter values are determined as a function of pressure. These results allow the evaluation of the compressibility coefficients of the corresponding parameters. The observed behavior of , a, and c with pressure are explained by using the relations proposed by Abrahams and Bernstein J. Chem. Phys. 52, 5607 1970. It is found that at about 3 GPa, a symmetry change, from an ordered tetragonal to a disordered tetragonal structure, occur

No abstract

We present here a novel and simple physical method for obtaining ZnO nanocrystals, using the pressure cycle method and the membrane diamond anvil cell (MDAC) up to 13 GPa at 500 K. The recovered nanocrystals in the wurtzitic ZnO phase (w-ZnO) were characterized by the high-resolution Raman scattering and X-ray diffraction. A single crystal of w-ZnO was used in this work. The high-pressure cell was a MDAC specifically designed for high-temperature studies. The Raman spectra of the high-quality ZnO single crystal and nanocrystals were compared, and we observed both a softening and an asymmetric broadening of the Raman peaks, which are in good agreement with the effect of relaxation of the q-vector selection rule due to quantum size confinement effects. The experimental results confirm the existence of ZnO nanocrystals in the w-ZnO phase with an average diameter of 17 nm.

In this Letter the electronic properties of double‐walled carbon nanotubes (DWCNTs) were studied by transmission measurements as a function of hydrostatic pressure up to 10 GPa. The energies of the optical transitions between the Van Hove singularities decrease with increasing pressure, which can be attributed to pressure‐induced hybridization and symmetry‐breaking effects. We observed a linear behaviour in the pressure‐induced shift of the optical transitions. This is in good agreement with previous studies on single‐walled carbon nanotubes (SWCNTs). High‐resolution TEM image of a bundle of DWCNTs (A), histogram of the number of walls (B), and diameter distribution for CNT samples plotted from 100 HRTEM images (C). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The electronic structure, band gap, density of states of the (8,8), (14,0) and (12,3) single-walled carbon nanotubes by the SIESTA (Spanish Initiative for Electronic Simulations with Thousands of Atoms) method in the framework density-functional theory (DFT) with the generalized gradients approximation (GGA) were studied. Also, we studied the vibrational properties of the (8,8) and (14,0) nanotubes. Only the calculated relaxed geometry for (12,3) nanotube show significant deviations from the ideal rolled graphene sheet configuration. The electronic transition energies of van Hove singularities were studied and compared with previous results. The calculated band structures, density of states and dispersion curves for all tubes were in good agreement with theoretical and experimental results.