Towards a comprehensive model of hydrogen-bonding interactions: past results and future research projects

Abstract

The Ag+ and Cu+ halides and chalcogenides are the prototype cation-disordered fast-ion conductors. The disuibution of mobile ions in their structures long remained enigmatic despite the fact that the anion arrangements are based on simple bee, hcp, or fcc packing. Disruptive phase transfonnations hindered the preparation of singlecrystal specimens and powder diffraction pattems, as a consequence of disorder and very large thermal vibration amplitudes, provide too few intensities to pe1mit specification of a satisfactory structural model. Methods for the preparation of a single crystal, coupled with neutron scatteling measurements (performed at the HFBR at Brookhaven National Laboratory) has enabled us to obtain precise descriptions of a number of these structures:cx-Agi, Ag2S. ~-Ag2Se, ~-Cu2S. cx-Cu2S, cx-Cu2Se, ~ AgCuS, cx-Ag 3 SI, and ~-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 su•uctures display cations that are delocalized by positional disorder and highly-anharmonic thern;al vibrations. To distinguish dynamic anharmonic vibration from positional disorder over closely-spaced equiliblium sites (i.e., "split atoms"), all of our su•ucture analyses were perfonned as a function of temperature to insure that the "thermal" parameters exu•apolate to zero at OK. Anharmonic vibration, "split atoms" and, sometimes, both were found in the structures. The distribution of cation scatte1ing 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.