next up previous
Next: DAFS on non-crystalline materials Up: Crystallographic DAFS studies Previous: Crystallographic DAFS studies

Co-refinement of the EDAFS information

One of the interesting questions about the structure of alloys and of substitutionally doped materials is the nature of the disorder produced by the alloying or by the substitutional doping. Previously, the long-range order sensitivity of diffraction and the short-range order sensitivity of EXAFS measured separately have been used to provide complementary views of the true structure as a long-range ordered ``virtual crystal'' modulated by short-range distortions. For example, separate diffraction and XAFS results have been combined to probe the local distortions in the structure of strained pseudomorphic GeSi layers [27] and of binary salts [42]. These studies showed that the actual structures are distorted versions of the original lattices. EDAFS measurements enhance this capability, since measurements can be made on the same samples, at the same time, under the same conditions. Thus, the long- and short-range order information is guaranteed to be consistent.

It will be very interesting to do ``EDAFS crystallography'' by co-refining the atomic positions using all of the Bragg peak intensities, energy, but measured over a continuous range of energies, at the same time. This will force the atomic positions to simultaneously fit EDAFS's long-range diffraction information and short-range fine structure information. This combination of long-range and short-range information is reminiscent of the use of the short-range sequence information in protein crystallography. It will also be very interesting to apply EDAFS co-refinement techniques to structural phase transitions which involve order-disorder contributions to the transition.

Note that EDAFS crystallography also incorporates simultaneously both the diffraction Debye-Waller factors (reflecting the anisotropic mean-square atomic displacements from the equilibrium atomic positions) and the XAFS-like Debye-Waller factors (reflecting the mean-square multi-leg path length variations from the equilibrium path lengths). Thus specific correlated motions can be determined from the EDAFS co-refinement.