27 research outputs found
Thermal Stabilization of the HCP Phase in Titanium
We have used a tight-binding model that is fit to first-principles
electronic-structure calculations for titanium to calculate quasi-harmonic
phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega
crystal structures. We show that the true zero-temperature ground-state is the
omega structure, although this has never been observed experimentally at normal
pressure, and that it is the entropy from the thermal population of phonon
states which stabilizes the hcp structure at room temperature. We present the
first completely theoretical prediction of the temperature- and
pressure-dependence of the hcp-omega phase transformation and show that it is
in good agreement with experiment. The quasi-harmonic approximation fails to
adequately treat the bcc phase because the zero-temperature phonons of this
structure are not all stable
Ab initio study of the beta$-tin->Imma->sh phase transitions in silicon and germanium
We have investigated the structural sequence of the high-pressure phases of
silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase
transitions. We have used the plane-wave pseudopotential approach to the
density-functional theory implemented within the Vienna ab-initio simulation
package (VASP). We have determined the equilibrium properties of each structure
and the values of the critical parameters including a hysteresis effect at the
phase transitions. The order of the phase transitions has been obtained
alternatively from the pressure dependence of the enthalpy and of the internal
structure parameters. The commonly used tangent construction is shown to be
very unreliable. Our calculations identify a first-order phase transition from
the cd to the beta-tin and from the Imma to the sh phase, and they indicate the
possibility of a second-order phase-transition from the beta-tin to the Imma
phase. Finally, we have derived the enthalpy barriers between the phases.Comment: 12 pages, 16 figure
Magnetism and structural distortions in uranium sulfide under pressure
Uranium sulde belongs to a class of uranium monochalcogenides that crystallize in the rock-salt structure and exhibit ferromagnetism at low temperature. The magnetism is believed to play a role in the low-temperature rhombohedral distortion, possibly due to its large magnetic anisotropy. We have performed electrical and structural characterization along with density-functional theory calculations as functions of pressure to help understand the interplay between structure and magnetism in US. Theoretical calculations suggest that ferromagnetic order is responsible for the small distortion at ambient pressure and low temperature. Under pressure, the Curie temperature is reduced
monotonically until it discontinuously disappears near a pressure-induced deformation of the crystal structure. This high-pressure distortion is identical to the one correlated with the onset of magnetic order, but with a larger change in the cell angle. Calculations imply a reduction in the electronic band energy as the driving force for the pressure-induced structure, but the loss of magnetic order associated with this distortion remains a mystery. The high-pressure electronic phase diagram may shed light on the magneto-structural free energy landscape of US.JRC.E.6-Actinide researc
Diffraction study of actinides under pressure
Uranium and thorium have sufficiently low radioactive dose rates to allow their study at synchrotrons and neutron facilities. Correspondingly, numerous compounds of these two actinides have been studied under pressure by synchrotron x-ray diffraction. The maximum pressures reached were on the order of 60-80 GPa, and 300 GPa in one case.The situation is much more difficult for all other actinides. Their high level of radioactivity has up to now prevented their study at synchrotrons, except in a few special cases. In contrast, all actinide metals available in sufficient quantities, and a large number of compounds of highly radioactive actinides, have been studied in highpressure laboratory facilities.Recent examples of in situ high pressure x-ray diffraction work will be described