Topic: Characterization of Crystallinity of Gold using X-Rays or Neutrons
Speaker: Prof. Heinrich Nakotte, New Mexico State University
Time: 15:00, June 8, 2017
Venue: Room 303, Liwu Building of Technology and Science, Wushan Campus

Abstract:
Gold is probably the most important precious metal: it is used in various electronics and optics applications, it provides a safe monetary equivalent for the investment market, and it is greatly appreciated in jewelry. Bulk gold crystallizes in a face-centered-cubic structure, but it is currently impossible to grow large single crystals of gold cannot in the laboratory because of difficult and slow growth conditions. In general, large natural gold nuggets are also polycrystalline, with very few exceptions of crystals found in certain parts of the world with favorable geological conditions. These crystals can have dimensions of several centimeters, and they are more rare than similar-sized diamonds. Subsequently, they are highly desirable specimens for collectors, but the market has been plagued by some ‘fake’ gold crystals. Full determination of the crystallinity of such large gold crystals requires a highly penetrating probe, such as neutrons. I will discuss our neutron diffraction studies on some of the large gold crystals that came up for auction, including the largest known crystal in the world. On the other end, micron-sized gold crystals grown under non-equilibrium conditions, tend to crystallize in highly geometrical shapes, such as five-fold decahedral ones, rather than forming the extended bulk face-centered-cubic (fcc) structure found in bulk gold. The five-fold symmetry of decahedral particles is inconsistent with a continuous crystal lattice and it is generally explained by multiple twinning of a tetrahedral subunit about a symmetry axis, with or without structural modification to the fcc motif. The small size of decahedral gold particles severely limits the application of traditional techniques, such as Bragg diffraction. I will discuss the sensitivity and the application of Pair Distribution Function (PDF) analysis for modeling of total (neutron and X-ray) scattering data of decahedral gold nanoparticles.