J. Mustre de Leon, Centro de Investigaci'on y de Estudios Avanzados del IPN, Merida, Yucatan 97310, Mexico

The harmonic approxiamtion to the atomic motion leads to the usual XAFS formalism of a Gaussian Debye-Waller factor. It has been known for several years that this approximation is not adequate to extract reliable structural information from XAFS in several situations. Typical situations are materials at high temperatures (T $\geq$ T$_{\rm Debye}$) and materials near a structural phase transition.

Some approaches to the treatment of nonharmonic systems are cumulant expansions of the Debye-Waller factor or the use of model interatomic potentials to derive a radial distribution function for the nearest neighbor shell.[1,2] Specifically, the radial distribution function for the nearest neighbor shell, $g(z)$, is given in terms of single particle wave functions $\{\psi_i(z)\}$ and single particle energy levels $\{E_i\}$; $g(z)={\sum_i\left\vert{\psi_i(z)}\right\vert^2e^{-\beta E_i}\over \sum_ie^{-\beta E_i}} \quad .$ The wavefunctions are determined by solving the Schr\"odinger equation using the reduced mass for the absorbing atom-nearest neighbor pair and the model potential.

The effect of anharmonicity in higher shell contributions, specifically in multiple scattering paths appears as one of the important problems to solve to achieve reliable structural determinations up to 5 or 6 neighbor shells. Another aspect to be addressed when comparing XAFS results with results from other techniques is the presence of inelastic events which produce excitations of the phononic spectrum.[3] Work supported by CONACyT, Mexico, and U.S. Department of Energy.

References:
[1] J. Mustre de Leon et al., Phys. Rev. Lett. 65, 1675 (1990).
[2] W.E. Jacson et al., Science 262, 229 (1993).
[3] M. I. Salkola et al., Phys. Rev. B. 51 8878 (1995).

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