X-ray determination of site occupation parameters in ordered ternaries Cu(AuxM(1-x))M = Ni,Pd.

摘要

X-ray scattering data obtained for multiple wavelengths with synchrotron radiation were analyzed by the Rietveld method to determine Ni and Pd distributions on the Cu(000) and Au(1/2 1/2 1/2) sites in the CuAuI tetragonal P4/mmm structure. Alloys of CuAu(sub x)M(sub 1-x) containing 6 at. % Ni or 10 and 25 at. % Pd were processed to obtain maximum ordering. Nickel is predominantly found on the Cu size and most all the Pd is found on the Au site. The uncertainty in site occupation parameters is discussed for various contributions which affect powder intensity measurements. For highly absorbing materials, an observed powder roughness effect decreases the low angle (2(theta)) intensities relative to the high 2(theta) intensities. This effect reduces the reliability of the thermal parameters and obscure a proper description of the thermal motion of the two sublattices. Corrections to the X-ray intensity data for surface roughness/porosity effects reduce uncertainties to about (plus minus)1 at. % on the refined value of the site occupations. This use of variable wavelength X-rays with simultaneous refinement of the corresponding data is capable of distinguishing site occupations even between two elements of almost equal scattering factor as, for example, Cu and Ni atoms in this investigation. Chemical phase stability is related to the site occupation parameters. 9 refs., 3 figs., 2 tabs.