X-ray Synchrotron Radiation Laboratory
University of Trento - Department of Physics

Home
Home

Highlights

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.


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.



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.


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.