The mobility of Cu+, Ag+, In+, and Tl+ in halide compounds has been investigated to develop an understanding of the factors which are important in fast ion conduction. Many of the compounds in the comparison are of the type MM′X4, where M is one of the monopositive ions listed above, M′ is Zn2+, Cd2+, or Hg2+, and X is Br- or I-. These compounds are characterized by transitions at elevated temperatures to disordered phases in which the M ion is highly mobile. Trends in the transition temperatures, conductivities, and activation energies for ion transport demonstrate that In+ and Tl+ are less mobile than Cu+ and Ag+. The crystal structure of the low-temperature phase of Tl2ZnI4 has been determined at -160°C. The material crystallizes with two formula units in space group C22-P21 of the monoclinic system in a cell of dimensions a = 7.661 (9) Å, b = 7.971 (8) Å, c = 10.074 (13) Å, β = 118.39 (4)°. The Zn2+ ions are tetrahedrally coordinated by I- ions. The Tl+ ions are found to reside in seven-coordinate sites approximating C2v capped trigonal prisms. There is no clear-cut migration path for the Tl+ ions, but some plausible conduction mechanisms are discussed. The origin of the mobility difference between d10 ions (e.g., Cu+ and Ag+) and d10s2 ions (e.g., In+ and Tl+) is attributed to the symmetry of the polarized ion. Compounds synthesized for the first time in the present work are In2ZnBr4 and Tl2ZnBr4. Materials which were discovered to be fast ion conductors are Ag2ZnI4, Ag2CdBr4, and Tl2ZnBr4.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Physical and Theoretical Chemistry
- Inorganic Chemistry
- Materials Chemistry