1,721,051 research outputs found
Local dynamical properties of crystalline germanium and their effects in extended x-ray absorption fine structure
Full text linkThe local dynamics of crystalline germanium has been investigated by molecular-dynamics simulations. The
radial distribution functions of the first six coordination shells, as well as their parallel and perpendicular
mean-square relative displacements, have been determined as a function of temperature. The agreement with
the available extended x-ray absorption fine-structure data is very good. Original insights on the outer shells
have been achieved. In particular, differently from the first shell, the bond thermal expansion of the outer shells
is mainly due to the shift of the maximum of the distance distribution while the contribution of the distribution
asymmetry is smaller than 15%
On the Einstein model for EXAFS parallel and perpendicular Mean-Square Relative Displacements
Full text linkThe correlated Einstein model for EXAFS parallel and perpendicular meansquare
relative displacements (MSRDs) is discussed. By means of dynamical
simulations on different crystalline structures, the error owing to the Einstein-fit
model on the EXAFS MSRDs is estimated as a function of the standard
deviation of the density of vibrational states. This error should be taken into
account to improve the accuracy of the MSRDs
On the switching between negative and positive thermal expansion in framework materials
Full text linkThe control of thermal expansion represents a challenge in materials design. This work shows that
the length of the lattice parameter is a key element for controlling thermal expansion. By varying
the lattice parameter through external pressure, chemical or other methods, the single-well
potential energy of the polyhedra rotations (or of the atomic transverse vibrations) responsible
for negative thermal expansion (NTE), can be turned into a quartic anharmonic potential or into a
double-well potential, thus enhancing or suppressing the NTE, respectively. This result applied to
framework materials should be taken into account to overcome the challenge of controlling thermal
expansion
On the neglecting of higher-order cumulants in EXAFS data analysis
Full text linkThe cumulant expansion is one of the most powerful and useful methods for
EXAFS data analysis, in which the higher-order cumulants allow to consider
deviations from a simple Gaussian distribution. In this work, analytical
expressions have been derived to show the effects of neglecting higher-order
cumulants in EXAFS analysis by the ratio method. The errors in the best-fitting
procedure owing to the omission of the higher-order cumulants, as well as of the
coordination number, can be determined
Displacive phase-transition of cuprite Ag2O revealed by extended x-ray absorption fine structure
No full textThe low-temperature phase-transition of silver oxide (Ag2O) has been investigated by extended x-ray absorption fine structure (EXAFS) spectroscopy as a function of temperature.The thermal evolution of the local structure around Ag atoms has been determined. In particular, below the phase-transition temperatureat ∼35 K, a progressive splitting of the Ag–Ag next-nearest-neighbor distances is observed. This definitely supports the idea that the phase-transition of Ag2O is due to displacive disorder of the Ag atoms
Comment on "On the discrimination between magnetite and maghemite by XANES measurements in fluorescence mode"
No full textIn a recent article (Espinosa et al 2012 Meas. Sci. Technol. 23 015602), the authors have
presented a method for the quantification of magnetite (Fe3O4) and maghemite (γ -Fe2O3)
from x-ray absorption near edge spectroscopy at the Fe K-edge in the fluorescence mode. In
this comment, I point out that the study was erroneously conducted in hematite (α-Fe2O3)
instead of maghemite. Accordingly, the authors’ conclusions need to be revised
Tension effect in local dynamics of cuprite structures
No full textCuprite crystals, Cu2O and Ag2O, display a negative thermal expansion in a wide temperature range. This
work shows, through a phenomenological approach, how the next-nearest-neighbors dynamics
observed by EXAFS in both crystals [A. Sanson, F. Rocca, G. Dalba, P. Fornasini, R. Grisenti, M. Dapiaggi,
G. Artioli, Phys. Rev. B 73 (2006), 214305] is directly connected to a tension mechanism. As a result, the
negative thermal expansion in cuprite crystals could be described within the Rigid Unit Modes
formalism, but replacing the typical rigid polyhedral units by rigid rods between nearest-neighbors
atoms
Toward an Understanding of the Local Origin of Negative Thermal Expansion in ZrW2O8: Limits and Inconsistencies of the Tent and Rigid Unit Mode Models
No full textAlthough zirconium tungstate (ZrW2O8) is the most popular negative thermal expansion (NTE) material, the exact mechanism responsible for its NTE still remains controversial. Specifically, the “Tent” model [Cao et al. Phys. Rev. Lett. 2002, 89, 215902; Bridges et al. Phys. Rev. Lett. 2014, 112, 045505] and the “rigid unit mode”
(RUM) model [Tucker et al. Phys. Rev. Lett. 2005, 95, 255501] were subjects of debate during recent years. This work aims to shed light on this issue by means of molecular
dynamics simulations which allow us to separate, for each bond distance, the “true” thermal expansion from the “apparent” thermal expansion, as well as to study the effective bond strength and the anisotropy of relative thermal motion. In spite of the good agreement with the experimental data of Cao, Bridges, and co-workers, a decrease of the “true” W−Zr distances has been observed accompanied by large transverse vibrations of the O atoms in the middle of the W−O−Zr linkage, in sharp contrast to the “tent” model. Moreover, in contrast to the RUM model, it has been found that the WO4 and ZrO6 polyhedra are
strongly distorted by thermal motion, and, more importantly, that intra-polyhedra contributions to NTE are present. Accordingly, we can conclude that both the tent and RUM models are inadequate to explain NTE in ZrW2O8, and a more flexible model, simply based on rigid nearest W−O and Zr−O bonds and tension effect, should be adopted
A first-principles study of vibrational modes in Cu2O and Ag2O crystals
Full text linkA first-principles investigation of cuprite crystals (Cu2O and Ag2O) has been performed. For Cu2O,
the calculated frequencies at the Γ point of the Brillouin zone are in very good agreement with
the experimental frequencies. For Ag2O, the presence of Eu and F2u vibrational modes with negative
frequencies indicates a low temperature phase transition, in agreement with recent high resolution X-ray
and neutron diffraction measurements. The energy scanning along these two modes shows a double-well
potential, within which only the Ag atoms vibrate. As a result, the origin of the phase transition can be
attributed to displacive disorder of the Ag atoms
Isotopic effect on the local dynamics of crystalline germanium
No full textThe effect of isotopic mass difference on the local dynamics of crystalline germanium has been investigated by lattice dynamics calculation. The effect on the total, parallel and perpendicular mean-square
relative displacements, as well as on the average bond distance, correlation and anisotropy of the relative
atomic motion, has been calculated for the first three coordination shells. The isotopic differences are
more pronounced where the relative atomic motion is more correlated. The agreement with recent high-quality EXAFS measurements provides evidence of the great potential of EXAFS in the study of the local
dynamics of crystals
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