Crystallographic studies of one-dimensional polyiodide conductors 3. Structure of non-stoichiometric coumarin-iodine complexes by X-ray analysis and exafs

Abstract

Coumarin forms highly conducting complexes with alkali iodides and iodine. The diffraction pattern of the isomorphous K, Rb and Cs complexes shows the iodine substructure to be incommensurate with the coumarin lattice along the triclinic c-axis direction. The iodine lattice has a translation repeat of 3.06 Å, which corresponds to the average I-I distance. The hk0 projection shows the cation columns to be surrounded by a polar mantle of oxygen atoms of the coumarin molecules. The crystals further contain infinite chains of iodine atoms, plus a set of three closely spaced iodine columns with occupancies of about 2 3 , 1 3 and 2 3 relative to the main columns. All columns are parallel to the c-axis. The structure refinement, in which all coumarin molecules are constrained to have identical, but variable, geometry, gives evidence for the non-stoichiometry of the complexes, with the cation and main iodine columns being deficient relative to a full occupancy of the coumarin sites. The deficiency varies from compound to compound and is believed to be an important factor explaining the high ionic conductivity of the complexes. In particular, the cation columns are deficient by as much as 20% relative to a full site occupancy. The structures are compared with those of previously determined benzophenone-iodide complexes of potassium and lithium. The higher conductivity of the coumarin complexes is thought to be related to the nature of the cation columns, which contain vacancies and are not interrupted by solvent molecules as in the benzophenone-iodide phases.