Mercury disilver tetraiodide (Ag₂HgI₄)

In its low temperature form, Ag₂HgI₄ adopts an ordered structure (a) in which the Ag⁺ and Hg²⁺ ions occupy some of the tetrahedral sites in the structure. At high temperatures, structure (b) is adopted, there are more sites in the structure than there are ions (Ag⁺ and Hg²⁺) present. As a result, the conductivity increases as the ions are more mobile and the I^– ions are easily polarizable.