Si and Ge K-edge x-ray absorption fine structure ͑XAFS͒ spectra of strained and relaxed Si x Ge 1Ϫx crystalline alloys grown by molecular beam epitaxy on Si͑001͒ substrates are reported. For alloys with less than 30% Si, fluorescence yield detection is shown to be essential to avoid distortions of the Si K-edge XAFS signal caused by the underlying Ge L-edge XAFS signal from the majority Ge species. The average first shell structure has been deduced using simultaneous fitting of all data for relaxed alloys, while imposing physically reasonable constraints. The Ge-Ge, Ge-Si, and Si-Si first-shell distances are found to vary with composition. The results are compared with other experimental results and theoretical predictions in the literature. Our results are generally consistent with other experimental studies but they differ from recent theoretical predictions based on macroscopic elastic properties in that we observe a different compositional dependence ͓i.e.,

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CYb, Eu)Fe204 crystallizes in space group R]m with hexagonal parameters a=b=3.486 and c= 24.92 A. The crystal structure has been determined by the heavy-atom method and Fourier synthesis.Least-squares refinement with anisotropic temperature factors yielded a final R value of 0.059. The rare-earth metals are octahedrally coordinated by six oxygen atoms and the iron atoms by five oxygen atoms at the apices of a trigonal bipyramid.