Crystal Structures and Ionic Conductivities of Ternary Derivatives of the Silver and Copper Monohalides. I. Superionic Phases of Stoichiometry MA 4I 5:RbAg 4I 5, KAg 4I 5, and KCu 4I 5
Abstract
The superionic properties of the compounds RbAg4I5, KAg4I5 and KCu4I5 have been investigated by powder neutron diffraction and complex impedance spectroscopy. RbAg4I5 and KAg4I5 have room-temperature ionic conductivities of σ=0.21(6) and 0.08(5) Ω-1 cm-1, respectively, which increase gradually on increasing temperature. KCu4I5 is only stable in the temperature range between 515(5) K and its melting point of 605 K, and its ionic conductivity is σ=0.61(8) Ω-1 cm-1, at T=540 K. At lower temperatures, KCu4I5 disproportionates into KI+4CuI and the ionic conductivity falls by over three orders of magnitude. Least-squares refinements of the powder neutron diffraction data for RbAg4I5 at ambient temperature confirm the reported structure (space group P4132, Z=4, a=11.23934(3) Å), though with some differences in the preferred locations of the mobile Ag+. KAg4I5 and KCu4I5 are found to adopt the same basic structure as RbAg4I5, with the I- forming a β-Mn-type sublattice, with the K+ located in a distorted octahedral environment and the Ag+(Cu+) predominantly distributed over two sites which are tetrahedrally co-ordinated to I-. The implications for the conduction mechanism within these compounds are discussed, using a novel maximum entropy difference Fourier technique to map the distribution of the Ag+(Cu+) within the unit cell.
- Publication:
-
Journal of Solid State Chemistry France
- Pub Date:
- May 2002
- DOI:
- 10.1006/jssc.2002.9552
- Bibcode:
- 2002JSSCh.165..363H
- Keywords:
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- superionic conduction;
- Ag<SUP>+</SUP> diffusion;
- neutron diffraction;
- maximum entropy techniques.