The Chemically-Specific Structure of an Amorphous Molybdenum Germanium Alloy by Anomalous X-ray Scattering

Since its inception in the late 1970s, anomalous x-ray scattering (AXS) has been employed for chemically-specific structure determination in a wide variety of noncrystalline materials. These studies have successfully produced differential distribution functions (DDFs) which provide information about the compositionally-averaged environment of a specific atomic species in the sample. Despite the wide success in obtaining DDFs, there are very few examples of successful extraction of the fully-chemically-specific partial pair distribution functions (PPDFs), the most detailed description of an amorphous sample possible by x-ray scattering. Extracting the PPDFs is notoriously difficult since the matrix equation involved is ill-conditioned and thus extremely sensitive to errors present in the experimental quantities that enter the equation. Instead of addressing this sensitivity by modifying the data through mathematical methods, sources of error have been removed experimentally: A focusing analyzer crystal was combined with a position-sensitive linear detector to experimentally eliminate unwanted inelastic scattering intensity over most of the reciprocal space range probed. This instrumentation has been used in data collection for the extraction of PPDFs from amorphous (a)-MoGe{sub 3}. This composition arises as a phase separation endpoint in the Ge-rich region of the vapor-deposited Mo-Ge amorphous alloy system but is not present at equilibrium. Sincemore » the first Ge-rich compound in the Mo-Ge equilibrium system is MoGe{sub 2}, previous workers have speculated that perhaps a unique MoGe{sub 3} compound exists in the amorphous system. Rather than indicating a distinct MoGe{sub 3} compound with definitive local structure, however, the coordination results are more consistent with a densely-packed alloy having a wide range of solid solubility. Significant improvement in the quality and reliability of experimental PPDFs from a-MoGe{sub 3} by AXS has been achieved solely through the experimental modifications to eliminate inelastic scattering. The coordination uncertainties are estimated at 5% for the Mo-Ge and Ge-Ge coordinations and 15% for the Mo-Mo coordination. These PPDFs from data collected at a second generation synchrotron source demonstrate the promise of the technique for routine PPDF extraction from binary alloys when applied in the future on dedicated beamlines at third generation synchrotron.« less