Structure and properties of a-Agl

Silver iodide, Agl, exists in several polymorphic forms. In the a-Agl crystal, the 1 ions adopt the bcp structure, and the Ag+ cations are distributed statistically among the 6(b), 12(d), and 24(h) sites of space group — Im3m, as listed in Table 10.3.1, and also partially populate the passageways between these positions. The cubic unit cell, with a — 504 pm, provides 42 possible positions for two Ag+ cations, and the Ag+ I- distances are listed below  

12(d) positions having 4 I-neighbors at 282 pm, and 24(h) positions having 3 I-neighbors at 267 pm. 

The structure of Agl varies at different temperatures and pressures. The stable form of Agl below 409 K, y-Agl, has the zinc blende (cubic ZnS) structure. On the other hand, /3-AgI, with the wurtzite (hexagonal ZnS) structure, is the stable form between 409 and 419 K. Above 419 K, ft-Agl undergoes a phase change to cubic a-Agl. Under high pressure, Agl adopts the NaCl structure. Below room temperature, y-Agl obtained from precipitation from an aqueous solution exhibits prominent covalent bond character, with a low electrical conductivity of about 3.4 x 10-4 ohm 1cm 1. When the temperature is raised, it undergoes a phase change to a-Agl, and the electrical conductivity increases ten-thousandfold to 1.3 ohm-1 cm-1. Compound a-Agl is the prototype of an important class of ionic conductors with Ag+ functioning as the carrier. 

The transformation of /3-AgI to a-Agl is accompanied by a dramatic increase in the ionic electrical conductivity of the solid, which leaps by a factor of nearly 4000from3.4 x 10 4to 1.3 ohm- em-1. This arises because in /1-AgItheAg 

The reaction of Agl with Rbl forms RbAg, which has been found to have the highest specific electrolytic conductivity at room temperature (0.27 ohm 1cm 1) among known solids, and this property is retained at low temperature. 

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