EXAFS studies of lattice dynamics and thermal expansion

Abstract

1 Introduction The acronym EXAFS (Extended X-ray-Absorption Fine-Structure) refers to the oscillations observed on the high-energy side of x-ray absorption edges in molecules and condensed systems [1]. EXAFS can be interpreted, within the framework of the time-dependent perturbation theory, as a final state effect due to the interaction of the photo-electron with the neighbouring atoms, the frequencies of oscillations being directly connected to inter-atomic distances. Selectivity of atomic species and insensitivity to long-range order make EXAFS a powerful probe of the local structure in disordered systems (glasses, random alloys, catalysts, proteins, etc.) Accurate temperature-dependent measurements can also give information on lattice dynamics of crystals, owing to EXAFS sensitivity to the correlation of atomic vibrations. The difference between the Debye-Waller factors of EXAFS and diffraction, quite early recognized [2], is due to the correlation parallel to the bond direction. The difference between the interatomic distances measured by EXAFS and diffraction, more recently evidenced [3], is due to the perpendicular relative vibrations. The sensitivity of EXAFS to the real expansion of selected atomic bonds and to the perpendicular correlation can be exploited to study the local behaviour of some crystals affected by macroscopic Negative Thermal Expansion (NTE). In this paper, after a short account of basic concepts (Section 2), the results obtained for copper and germanium will be critically compared (Section 3). Section 4 will be dedicated to NTE and to some preliminary results on cuprite crystals. Conclusions will be drawn in Section 5.