85 research outputs found
Landau theory applied to phase transitions in calcium orthotungstate and isostructural compounds
The pressure-driven tetragonal-to-monoclinic phase transition in CaWO4 and
related scheelite-structured orthotungstates is analysed in terms of
spontaneous strains. Based upon our previous high-pressure x-ray diffraction
results and the Landau theory, it is suggested that the
scheelite-to-fergusonite transition is of second order in nature.Comment: 14 pages, 3 figure
Pressure screening in the interior of primary shells in double-wall carbon nanotubes
The pressure response of double-wall carbon nanotubes has been investigated
by means of Raman spectroscopy up to 10 GPa. The intensity of the radial
breathing modes of the outer tubes decreases rapidly but remain observable up
to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of
single-wall carbon nanotubes, which undergo a shape distortion at higher
pressures. In addition, the tangential band of the external tubes broadens and
decreases in amplitude. The corresponding Raman features of the internal tubes
appear to be considerably less sensitive to pressure. All findings lead to the
conclusion that the outer tubes act as a protection shield for the inner tubes
whereas the latter increase the structural stability of the outer tubes upon
pressure application.Comment: PDF with 15 pages, 3 figures, 1 table; submitted to Physical Review
Pressure dependence of the Boson peak in glassy As2S3 studied by Raman Scattering
A detailed pressure-dependence study of the low-energy excitations of glassy
As2S3 is reported over a wide pressure range, up to 10 GPa. The spectral
features of Boson peak are analysed as a function of pressure. Pressure effects
on the Boson peak are manifested as an appreciable shift of its frequency to
higher values, a suppression of its intensity, as well as a noticeable change
of its asymmetry leading to a more symmetric shape at high pressures. The
pressure-induced Boson peak frequency shift agrees very well with the
predictions of the soft potential model over the whole pressure range studied.
As regards the pressure dependence of the Boson peak intensity, the situation
is more complicated. It is proposed that in order to reach proper conclusions
the corresponding dependence of the Debye density of states must also be
considered. Employing a comparison of the low energy modes of the crystalline
counterpart of As2S3 as well as the experimental data concerning the pressure
dependencies of the Boson peak frequency and intensity, structural or
glass-to-glass transition seems to occur at the pressure ~4 GPa related to a
change of local structure. Finally, the pressure-induced shape changes of the
Boson peak can be traced back to the very details of the excess (over the Debye
contribution) vibrational density of states.Comment: To appear in J. Non-Cryst. Solids (Proceedings of the 5th IDMRCS,
Lille, July 2005
Effects of pressure on the local atomic structure of CaWO4 and YLiF4: Mechanism of the scheelite-to-wolframite and scheelite-to-fergusonite transitions
The pressure response of the scheelite phase of CaWO4 (YLiF4) and the
occurrence of the pressure induced scheelite-to-wolframite (M-fergusonite)
transition are reviewed and discussed. It is shown that the change of the axial
parameters under compression is related with the different pressure dependence
of the W-O (Li-F) and Ca-O (Y-F) interatomic bonds. Phase transition mechanisms
for both compounds are proposed. Furthermore, a systematic study of the phase
transition in 16 different scheelite ABX4 compounds indicates that the
transition pressure increases as the packing ratio of the anionic BX4 units
around the A cations increases.Comment: 38 pages, 10 figures (Figure 5 corrected), accepted for publication
in Journal of Solid State Chemistr
Effects of high pressure on the optical absorption spectrum of scintillating PbWO4 crystals
The pressure behavior of the absorption edge of PbWO4 was studied up to 15.3
GPa. It red-shifts at -71 meV/GPa below 6.1 GPa, but at 6.3 GPa the band-gap
collapses from 3.5 eV to 2.75 eV. From 6.3 GPa to 11.1 GPa, the absorption edge
moves with a pressure coefficient of -98 meV/GPa, undergoing additional changes
at 12.2 GPa. The results are discussed in terms of the electronic structure of
PbWO4 which attribute the behavior of the band-gap to changes in the local
atomic structure. The changes observed at 6.3 GPa and 12.2 GPa are attributed
to phase transitions.Comment: 14 pages, 3 figure
Surface profile gradient in amorphous Ta<inf>2</inf>O<inf>5</inf> semi conductive layers regulates nanoscale electric current stability
© 2016 The Author(s)A link between the morphological characteristics and the electric properties of amorphous layers is established by means of atomic, conductive, electrostatic force and thermal scanning microscopy. Using amorphous Ta2O5 (a-Ta2O5) semiconductive layer, it is found that surface profile gradients (morphological gradient), are highly correlated to both the electron energy gradient of trapped electrons in interactive Coulombic sites and the thermal gradient along conductive paths and thus thermal and electric properties are correlated with surface morphology at the nanoscale. Furthermore, morphological and electron energy gradients along opposite conductive paths of electrons intrinsically impose a current stability anisotropy. For either long conductive paths (L > 1 μm) or along symmetric nanodomains, current stability for both positive and negative currents i is demonstrated. On the contrary, for short conductive paths along non-symmetric nanodomains, the set of independent variables (L, i) is spanned by two current stability/intability loci. One locus specifies a stable state for negative currents, while the other locus also describes a stable state for positive currents
Surface profile gradient in amorphous Ta<inf>2</inf>O<inf>5</inf> semi conductive layers regulates nanoscale electric current stability
© 2016 The Author(s)A link between the morphological characteristics and the electric properties of amorphous layers is established by means of atomic, conductive, electrostatic force and thermal scanning microscopy. Using amorphous Ta2O5 (a-Ta2O5) semiconductive layer, it is found that surface profile gradients (morphological gradient), are highly correlated to both the electron energy gradient of trapped electrons in interactive Coulombic sites and the thermal gradient along conductive paths and thus thermal and electric properties are correlated with surface morphology at the nanoscale. Furthermore, morphological and electron energy gradients along opposite conductive paths of electrons intrinsically impose a current stability anisotropy. For either long conductive paths (L > 1 μm) or along symmetric nanodomains, current stability for both positive and negative currents i is demonstrated. On the contrary, for short conductive paths along non-symmetric nanodomains, the set of independent variables (L, i) is spanned by two current stability/intability loci. One locus specifies a stable state for negative currents, while the other locus also describes a stable state for positive currents
Surface profile gradient in amorphous Ta<inf>2</inf>O<inf>5</inf> semi conductive layers regulates nanoscale electric current stability
© 2016 The Author(s).A link between the morphological characteristics and the electric properties of amorphous layers is established by means of atomic, conductive, electrostatic force and thermal scanning microscopy. Using amorphous Ta2O5 (a-Ta2O5) semiconductive layer, it is found that surface profile gradients (morphological gradient), are highly correlated to both the electron energy gradient of trapped electrons in interactive Coulombic sites and the thermal gradient along conductive paths and thus thermal and electric properties are correlated with surface morphology at the nanoscale.Furthermore, morphological and electron energy gradients along opposite conductive paths of electrons intrinsically impose a current stability anisotropy. For either long conductive paths (L .>. 1. μm) or along symmetric nanodomains, current stability for both positive and negative currents . i is demonstrated. On the contrary, for short conductive paths along non-symmetric nanodomains, the set of independent variables (L, i) is spanned by two current stability/intability loci. One locus specifies a stable state for negative currents, while the other locus also describes a stable state for positive currents
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