Examples of Unipolar Cationic 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). 

Alpha silver iodide, the high-temperature phase of Agl, shows an extraordinarily high Ag+ ionic conductivity. On the other hand, the conductivity at room temperature, that is, the conductivity of P-AgI is considerably lower [12,20], In the a-Agl phase, there are only two Ag+ ions distributed over the octahedral and tetrahedral positions of the cubic lattice, producing many vacancies that are accessible for Ag+ ions to bypass through the structure [20], Then, in order to operate these materials it is necessary to stabilize the high-temperature a phase at low temperatures [12], Subsequently, a considerable amount of studies of ternary and quaternary compounds have been performed with the purpose of stabilizing fast ionic phases at low temperatures [29], 

One largely investigated system in the field of silver ion conductivity is the Rbl-AgI system, including the fast ionic conductor, RbAgJj [19,28,29], Structural investigations of these compounds started 40 years ago and the field has been active during the last four decades. The mechanism of conduction and the favored conduction pathway in ionic conductors is important for the development of solid-state batteries [28], 

optimized ion conductors, such as Ag5Te2Cl, demonstrate the characteristic performance of ion-conducting materials, specifically, polymorphism in combination with ion conduction in a high-temperature phase [28], 

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