Polymorphism silver iodide

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

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 ... 

Alpha silver iodide (a-Agl), a fast ion conductor, is one of the different polymorphic structures of Agl showing a cubic structure [51], where I occupies anionic positions, that is, the Cl- sites in the CsCl-type structure (see Figure 2.19). On the other hand, the low temperature phase, that is, p-Agl, exhibits a hexagonal wurtzite-type structure. 

The majority of unipolar ionic conductors identified to date are polymorphic compounds with several phase transitions, where the phases have different ionic conductivities owing to modifications in the substructure of the mobile ions [28], One of the first studied cationic conductors was a-Agl [21]. Silver iodide exhibits different polymorphic structures. Agl has a low-temperature phase, that is, [3-Agl, which crystallizes in the hexagonal wurtzite structure type, and a high-temperature cubic phase, a-Agl, which shows a cubic CsCl structure type [20,22] (see Section 2.4.5). 

---