Title

L-edge x-ray absorption study of $\beta$-phase Ni$\rm\sb{x}Al\sb{100-x}$ and of the effects of substitutional Fe atoms on its local electronic structure

Date of Completion

January 1997

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

We present and discuss x-ray absorption near edge spectra (XANES) to determine the nature of the local electronic structure of Fe dopants in the ternary alloy system based on $\beta$-NiAl. Extended X-ray absorption fine structure (EXAFS) experiments using transition metal K shell excitation have previously been utilized by others to determine the local environment of dilute ternary Fe dopants in transition metal (TM) based aluminides, studies which were motivated by the discovery of a ductilization effect resulting from the Fe dopants in these systems. From the EXAFS results, the site occupancy of the Fe dopants in NiAl are well known; a major part of the present study is aimed toward elucidating the unoccupied 3d electronic levels for the Fe dopants in both the transition metal and aluminum sublattices of NiAl. Consequently, the L absorption edges of first row transition metals were of major interest, since unlike K absorption edges, L-edge studies probe unoccupied 3d states by dipole allowed transitions. The L-edge data were obtained at the EXXON U-1A soft x-ray beamline at the NSLS at Brookhaven and sample characterization K-edge data were obtained at the X-11A hard x-ray beamline. The L-edge data obtained here was then fitted as a superposition of two Lorentzians on top of two arc-tangents. From the values of the parameters for our best fit, we obtained the ratio of the areas under the L$\sb3$ and L$\sb2$ peaks, and the total area under the L$\sb3$ and L$\sb2$ peaks. As a measure of quantifying this information, we correlate these quantities to the number of d-holes for Fe-atoms in different types of sites in our Fe doped NiAl alloy system. We discuss a novel way of plotting data related to the anomalous L$\sb3$/L$\sb2$ ratio effect and its association with local magnetism. Finally, we also studied the effect of varying the substrate temperature on the structure of flash evaporated TM aluminides and refurbished the soft x-ray laboratory facility. This last effort involved the installation and testing of a new control system, and rebuilding and testing the demountable x-ray tube in a new location. ^