NEUTRON SCATTERING STUDIES OF THE DISTRIBUTION AND ANHARMONIC VIBRATION OF Ag⁺ AND Cu⁺ IN FAST-ION CONDUCTORS. Bernhardt J. Wuensch, Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, MA 02139

The Ag⁺ and Cu⁺ halides and chalcogenides are the prototype cation-disordered fast-ion conductors. The distribution of mobile ions in their structures long remained enigmatic despite the fact that the anion arrangements are based on simple bcc, hcp, or fcc packing. Disruptive phase transformations hindered the preparation of single-crystal specimens and powder diffraction patterns, as a consequence of disorder and very large thermal vibration amplitudes, provide too few intensities to permit specification of a satisfactory structural model. Methods for the preparation of a single crystal, coupled with neutron scattering measurements (performed at the HFBR at Brookhaven National Laboratory) has enabled us to obtain precise descriptions of a number of these structures:[[alpha]]-AgI, Ag₂S, [[beta]]-Ag₂Se, [[beta]]-Cu₂S, [[alpha]]-Cu₂S, [[alpha]]-Cu₂Se, [[beta]]-AgCuS, [[alpha]]-Ag₃SI, and [[beta]]-Ag₃SI. Fast-ion conduction is a consequence of the cations residing in shallow potential wells at sites that form a continuous pathway through the structure. All of the structures display cations that are delocalized by positional disorder and highly-anharmonic thermal vibrations. To distinguish dynamic anharmonic vibration from positional disorder over closely-spaced equilibrium sites (i.e., "split atoms"), all of our structure analyses were performed as a function of temperature to insure that the "thermal" parameters extrapolate to zero at 0K. Anharmonic vibration, "split atoms" and, sometimes, both were found in the structures. The distribution of cation scattering density is not solely a function of the geometry of the anion arrangement, but depends on the concentration and bonding characteristics of the mobile cations as well.