Crystal structure, electron density and diffusion path of the fast-ion conductor copper iodide CuI

Abstract

The crystal structure of copper iodide (CuI) has been studied at 300 K and 760 K using synchrotron X-ray powder diffraction data. An accurate electron density distribution of CuI has been obtained analyzing the data with a combination of the Rietveld method, a maximum-entropy method (MEM) and MEM-based pattern fitting. The MEM electron-density map of the zinc-blende type γ-CuI at 300 K indicated that the copper ions are located at the regular 4c position. It was found that the fluorite-type α-CuI at 760 K has a large spatial distribution of copper ions along the <111> directions around the regular 8c position and diffusion pathway of mobile copper ions along the <100> direction. These features are responsible for the high ionic conductivity of α-CuI. The electron-density maps of CuI at 300 K and 760 K also show covalent bonding between the copper and iodine atoms.