Crystal silver ionic conductors

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). 

AgRSe2 compounds of the AgErSci-type. This structure was established, from a single crystal study, by Julien-Pouzol et al. (1977) (fig. 31.15). The flat orthorhombic cell, P2i2]2i, contains four formulas. The erbium atoms are at the center of nearly regular octahedra of selenium. The silver atoms are inside flat tetrahedra, with four nearly equal Ag-S distances. These tetrahedra form open chains along the Oz direction and constitute channels in which it might be possible for the silver ions to move. These compounds are ionic conductors. They are only known in the selenides series, from Dy to Lu, and for Y and Sc (Julien-Pouzol et al., 1969, 1973). 

When we study a solid that does not have the characteristic lustrous appearance of a metal, we find that the conductivity is extremely low. This includes the solids we have called ionic solids sodium chloride, sodium nitrate, silver nitrate, and silver chloride. It includes, as well, the molecular crystals, such as ice. This solid, shown in Figure 5-3, is made up of molecules (such as exist in the gas phase) regularly packed in an orderly array. These poor conductors differ widely from the metals in almost every property. Thus electrical conductivity furnishes the key to one of the most fundamental classification schemes for substances. 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

Metallic bonds are generally weaker than either covalent or ionic bonds, which explains why metallically bonded minerals (true metals), like silver, gold, and copper, can be worked— beaten into flat sheets, or drawn into thin wires. In metallic bonds, electrons move about the crystal constantly flowing between adjacent atoms, redistributing their charge. Because of this flow of electrons, true metals are also good electrical conductors. 

Disilver fluoride is a bronze-colored compound with a greenish cast when observed in bulk. It is an excellent electrical conductor. Crystal-structure determination3 shows the complete absence of elemental silver and silver(I) fluoride in the pure material and reveals the presence of successive layers of silver, silver, and fluorine in the lattice. The silver-silver distance is 2.86 A. (nearly twice the metallic radius of 1.53 A.), and the silver-fluorine distance is 2.46 A. [as in ionic silver(I) fluoride]. The compound is regarded as being intermediate in structure between a metal and a salt.4... 

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