Trends in heavy-metal solid state ionic conductors: A comparison of Cu⁺, Ag⁺, In⁺, and Tl⁺ transport

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 C_2v 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.