NEUTRON-DIFFRACTION STUDY OF HIGH IONIC CONDUCTOR RB4CU16I7+XCL13-X AT 50-300-K - CORRELATION WITH IONIC-CONDUCTIVITY

The high ionic conductor Rb4Cu16I7+xCl13-x was characterized by electrical conductivity, X-ray diffraction, and neutron diffraction measurements. The compositions Rb4Cu16I7Cl13 and Rb4Cu16I7.2Cl12.8 were synthesized using a solid-state reaction at 403 K. Arrhenius conductivity plots showed smooth slope changes at 150-300 K, corresponding to a so-called diffuse phase transition. Rb4Cu16I7.2Cl12.8 showed slightly higher conductivities above 200 K, whereas no significant difference was found below 200 K. The structural parameters of Rb4Cu16I7.2Cl12.8 were refined by Rietveld analysis of TOF neutron diffraction data at 50-300 K. In three sets of Cu sites, a gradual migration of copper ions from the Cu(3) site into the Cu(2) site was observed in the temperature range 110 to 300 K, which corresponds to the migration of nonconducting copper ions into the conduction pathways. All the structural changes proceed gradually over a wide range of temperature, 110-300 K; this corresponds exactly to the temperatures where the conductivity curves showed smooth slope changes. © 1993 Academic Press, Inc.