Correlation effects studied based on Debye–Waller factors. Application to fcc crystals.

Correlation effects described by the displacement–displacement correlation function C R (T) have been studied based on Debye–Waller factors presented by the mean square displacement (MSD) u 2 (T) and mean square relative displacement (MSRD) σ 2 (T) in X-ray Absorption Fine Structure. Analytical expressions have been derived for σ 2 (T) based on the anharmonic correlated Debye model (ACDM) and for u 2 (T) based on anharmonic Debye model (ADM) or uncorrelated Debye model. Many-body effects have been taken into account in the present one-dimensional model by a simple measure based on the anharmonic effective potentials that include interactions of absorber and backscatterer atoms with their nearest neighbors. Morse potential is used for describing single-pair atomic interaction. The reasons for the difference between MSRD and MSD have been discussed in detail. The theory is applied to fcc crystals and can be generalized to any crystal structure. Numerical results for Cu are found to be in good agreement with the experimental values and with those taken from the measured Morse parameters, as well as with the values of C R / u 2 calculated using the other theories. [ABSTRACT FROM AUTHOR]

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