151 research outputs found
Structural and superconducting transition in selenium under high pressures
First-principles calculations are performed for electronic structures of two
high pressure phases of solid selenium, -Po and bcc.
Our calculation reproduces well the pressure-induced phase transition from
-Po to bcc observed in selenium.
The calculated transition pressure is 30 GPa lower than the observed one, but
the calculated pressure dependence of the lattice parameters agrees fairly well
with the observations in a wide range of pressure.
We estimate the superconducting transition temperature of both
the -Po and the bcc phases by calculating the phonon dispersion and the
electron-phonon interaction on the basis of density-functional perturbation
theory.
The calculated shows a characteristic pressure dependence, i.e.
it is rather pressure independent in the -Po phase, shows a
discontinuous jump at the transition from -Po to bcc, and then decreases
rapidly with increasing pressure in the bcc phase.Comment: 8 pages, 11 figure
Spiral Chain O4 Form of Dense Oxygen
Oxygen is in many ways a unique element: the only known diatomic molecular
magnet and the capability of stabilization of the hitherto unexpected O8
cluster structure in its solid form at high pressure. Molecular dissociations
upon compression as one of the fundamental problems were reported for other
diatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solid
oxygen, making oxygen an intractable system. We here report the theoretical
prediction on the dissociation of molecular oxygen into a polymeric spiral
chain O4 structure (\theta-O4) by using first-principles calypso method on
crystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has been
observed as the third high pressure phase of sulfur (S-III). We find that the
molecular O8 phase remains extremely stable in a large pressure range of 0.008
- 2 TPa, whose breakdown is driven by the pressure-induced instability of a
transverse acoustic phonon mode at zone boundary, leading to the ultimate
formation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygen
from a superconductor into an insulator with a wide band gap (approximately 5.9
eV) originating from the sp3-like hybridized orbitals of oxygen and the
localization of valence electrons. (This is a pre-print version of the
following article: Li Zhu et al, Spiral chain O4 form of dense oxygen, Proc.
Natl. Acad. Sci. U.S.A. (2011), doi: 10.1073/pnas.1119375109, which has been
published online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)Comment: 13 apages, 3 figure
Phonon softening and superconductivity in tellurium under pressure
The phonon dispersion and the electron-phonon interaction for the -Po
and the bcc high pressure phases of tellurium are computed with
density-functional perturbation theory. Our calculations reproduce and explain
the experimentally observed pressure dependence of the superconducting critical
temperature (T) and confirm the connection between the jump in
T and the structural phase transition. The phonon contribution to the
free energy is shown to be responsible for the difference in the structural
transition pressure observed in low and room temperature experiments.Comment: Revtex, 4 Postscript figures, to appear in Phys. Rev. Let
Low-Energy Linear Structures in Dense Oxygen: Implications for the -phase
Using density functional theory implemented within the generalized gradient
approximation, a new non-magnetic insulating ground state of solid oxygen is
proposed and found to be energetically favored at pressures corresponding to
the -phase. The newly-predicted ground state is composed of linear
herringbone-type chains of O molecules and has {\it Cmcm} symmetry (with an
alternative monoclinic cell). Importantly, this phase supports IR-active
zone-center phonons, and their computed frequencies are found to be in broad
agreement with recent infrared absorption experiments.Comment: 4 pages, 4 figure
Pressure-induced alpha-to-omega transition in titanium metal: A systematic study of the effects of uniaxial stress
We investigated the effects of uniaxial stress on the pressure-induced
alpha-to-omega transition in pure titanium (Ti) by means of angle dispersive
x-ray diffraction in a diamond-anvil cell. Experiments under four different
pressure environments reveal that: (1) the onset of the transition depends on
the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa
(argon pressure medium); (2) the a and w phases coexist over a rather large
pressure range, which depends on the pressure medium employed; (3) the
hysteresis and quenchability of the w phase is affected by differences in the
sample pressure environment; and (4) a short term laser-heating of Ti lowers
the alpha-to-omega transition pressure. Possible transition mechanisms are
discussed in the light of the present results, which clearly demonstrated the
influence of uniaxial stress in the alpha-to-omega transition.Comment: 16 pages, 6 figures, 1 tabl
Quasi-molecular and atomic phases of dense solid hydrogen
The high-pressure phases of solid hydrogen are of fundamental interest and
relevant to the interior of giant planets; however, knowledge of these phases
is far from complete. Particle swarm optimization (PSO) techniques were applied
to a structural search, yielding hitherto unexpected high-pressure phases of
solid hydrogen at pressures up to 5 TPa. An exotic quasi-molecular mC24
structure (space group C2/c, stable at 0.47-0.59 TPa) with two types of
intramolecular bonds was predicted, providing a deeper understanding of
molecular dissociation in solid hydrogen, which has been a mystery for decades.
We further predicted the existence of two atomic phases: (i) the oC12 structure
(space group Cmcm, stable at > 2.1 TPa), consisting of planar H3 clusters, and
(ii) the cI16 structure, previously observed in lithium and sodium, stable
above 3.5 TPa upon consideration of the zero-point energy. This work clearly
revised the known zero-temperature and high-pressure (>0.47 TPa) phase diagram
for solid hydrogen and has implications for the constituent structures of giant
planets.Comment: accepted in The Journal of Physical Chemistr
Crystal structure of solid Oxygen at high pressure and low temperature
Results of X-ray diffraction experiments on solid oxygen at low temperature
and at pressures up to 10 GPa are presented.A careful sample preparation and
annealing around 240 K allowed to obtain very good diffraction patterns in the
orthorhombic delta-phase. This phase is stable at low temperature, in contrast
to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and
transforms with decreasing pressure into a monoclinic phase, which is
identified as the low pressure alpha-phase. The discontinuous change of the
lattice parameters, and the observed metastability of the alpha-phase
increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure
- …