No abstract

X-ray-absorption fine-structure (XAFS) measurements of lead from' 10 K through and above the melting transition at 600 K have been carried out. A cumulant expansion analysis provides data on the first four moments of the radial distribution of the first shell, which are used to construct a consistent temperature-dependent radial distribution function for the solid. The distribution is one given by a simple one-dimensional anharmonic oscillator. The results also demonstrate that XAFS, a local probe, can clearly distinguish between the liquid and solid state. In the liquid, the apparent coordination number is decreased by the fraction of the time the atom is diffusing. An improved method for determining the background at low k is discussed that gives a reliable determination of the background much closer to the threshold than allowed by standard methods.

The grain boundary regions of nanophase Cu metal are investigated using the x-ray absorption fine structure (XAFS) technique. Typical samples made by standard techniques need to be greatly thinned if measured in transmission in order to eliminate experimental artifacts which erroneously lower the apparent coordination number. To avoid this problem the samples were measured by the total electron yield technique. The results indicate a grain boundary structure which, on the average, is similar to that in conventional polycrystalline Cu, contrary to previous XAFS measurements made in transmission which indicated a lower coordination number.

A correlation between precise x-ray powder diffraction patterns and atomic size mismatch in disordered mixed crystals (alloys and ionic crystals) is observed. The anisotropy of the elastic moduli has been taken into account for evaluation of the strain energy density of the mixed crystals revealed in x-ray powder diffraction measurements. The precursor of the phase transformation for a quenched disordered Au-Cu alloy is identified. § Present address:

X-ray absorption fine structure (XAFS) measurements of mixed salts Rb^Ki-ccBr and RbBr^Cli-y at their congruent melting points compositions reveal that their actual structure buckles about the NaCl average structure with an rms angular deviation from collinearity of 7°-9°. XAFS measures the buckling directly through three-body correlations, and verifies the homogeneity and randomness of the mixtures. The characteristic ionic sizes are found to be dependent on concentration, causing changes to 0.1 A in interatom bond distances. A computer simulation based on the information from XAFS and diffraction displays the actual structure of the salts.