X-ray Synchrotron Radiation Laboratory University of Trento - Department of Physics |
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Local origin of
negative thermal expansion in crystals |
Several
crystals with the diamond or zincblende structure, such as Si, Ge,
GaAs, CuCl, undergo isotropic negative thermal expansion (NTE) in a
limited low-temperature interval. Stronger NTE, extending over large
temperature intervals, is exhibited by framework structures, such as
the cuprites Cu2O, Ag2O or the more complex ZrW2O8. The lattice thermal expansion is the result of a competition between bond stretching (positive contribution) and tension effects (negative contribution). When tension effects prevail, the lattice undergoes NTE. Bragg diffraction experiments can measure the lattice thermal expansion and the thermal factors of single atoms. It has recently been shown that EXAFS accurately measures the bond thermal expansion and one can evaluate the intensity of relative atomic vibrations both parallel and perpendicular to the a given bond. This potential is being exploited to disentangle the effects of bond stretching from the effect of relative vibrations perpendicular to the bond, in order to get original information on the local mechanisms of NTE.
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Isotopic effects in
germanium |
The isotopic
composition has subtle but non-negligible influence on
some basic properties of crystals, such as density, phonon widths, and
electronic energy gaps. Let us consider the dynamical properties of
crystals. The force constants depend on atomic species and crystal
structure. The zero-point amplitude of atomic vibrations is however
influenced also by the nuclear masses, the lighter isotopes undergoing
larger oscillations than the heavier ones. As a consequence of
anharmonicity, the difference in zero-point amplitude of motion
reflects on a difference of interatomic equilibrium distances and lattice
parameters]. These effects, of genuine quantum origin,
progressively disappear when temperature increases.
EXAFS has been measured on the two isotopes 70Ge and 76Ge as a function of temperature. The effect of isotopic mass difference on the amplitude of relative atomic vibrations is neatly evidenced by the temperature dependence of the difference of Debye-Waller factors. The isotopic effect is also detected on the difference of nearest-neighbor average interatomic distances, thanks to a resolution better than 10 fm.
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Ionic conductivity in
AgI-doped glasses |
The mechanism of
ionic conduction in glasses is still an open question. EXAFS can
give original information on the local structure and dynamics around
selected atomic species. Strengths and limitations of the technique
have been thoroughly studied. Significant results have been obtained on
AgI-doped glasses.
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Interpretation of
EXAFS |
An accurate
interpretation of EXAFS results requires a thorough understanding of
the effects of thermal disorder and the implementation of effective
procedures of data analysis. A long-term research program is under way,
based on measurements on model systems accompanied by a theoretical
support. Because of the different sensitivity to correlation and
anharmonicity, EXAFS measures different thermal expansions and
Debye-Waller factors with respect to Bragg diffraction.The comparison
of the two techniques leads to e better understanding of the relations
between local and average dynamical properties of solids.
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