Ionic Conductivity - Silver Iodide

Silver Iodide

In 1921, Tubandt et al. found that silver iodide (AgI) has extraordinary high ionic conductivity. While measuring conductivity, they found that above 147 °C, AgI changes into a phase that has an ionic conductivity of ~ 1 –1 cm−1, similar to that of its liquid phase. As a result, the low temperature phase of AgI was the first superionic conductor ever discovered. The highly conductive phase of AgI is now known as -AgI. It was shown that a sublattice cationic disorder takes place in -AgI. The liquid-like state of Ag+ ions, as proposed by Strock (1934, 1936) and later reinforced by others (Geller, 1977; Funke, 1976), consists of a cubic unit cell of iodide ions (I-), in which a total of 42 sites (6 octahedral, 12 tetragonal and 24 trigonal bipyramidal) are available for 2 Ag+ ions, as shown in the Figure 1. O' Keeffe and Hyde (1976) have argued that this phase transition in AgI is dramatic and powerful, nothing less than the melting and have also shown that the entropy change at the superionic transition is comparable to its value at the melting. Thus, in the -phase, I− ions form a body-centered cubic lattice and the Ag+ ions are distributed in such a way that 42 crystallographic equivalent interstices are available for the two Ag+ ions.

Alpha phase crystals of various materials like Ag₂S, Ag₂Se, Ag₂Te, etc. were soon discovered (Tubandt, 1932). By the early 1930s, it was demonstrated that these fast ionically conducting solids could be treated entirely the same as aqueous electrolytes from the viewpoint of chemical reactions and thermodynamics, hence these materials were labeled solid electrolytes.