Silver iodide ionic conductivity

Every ionic crystal can formally be regarded as a mutually interconnected composite of two distinct structures cationic sublattice and anionic sublattice, which may or may not have identical symmetry. Silver iodide exhibits two structures thermodynamically stable below 146°C sphalerite (below 137°C) and wurtzite (137-146°C), with a plane-centred I- sublattice. This changes into a body-centred one at 146°C, and it persists up to the melting point of Agl (555°C). On the other hand, the Ag+ sub-lattice is much less stable it collapses at the phase transition temperature (146°C) into a highly disordered, liquid-like system, in which the Ag+ ions are easily mobile over all the 42 theoretically available interstitial sites in the I-sub-lattice. This system shows an Ag+ conductivity of 1.31 S/cm at 146°C (the regular wurtzite modification of Agl has an ionic conductivity of about 10-3 S/cm at this temperature). 

Some ionic solids have been discovered that have a much higher conductivity than is typical for such compounds and these are known as fast-ion conductors. One of the earliest to be noticed, in 1913 by Tubandt and Lorenz, was a high temperature phase of silver iodide. 

In purely ionic compounds, the conductivity from these mechanisms is intrinsic and relates only to the entropy-driven Boltzmann distribution the conductivity will thus increase with increase in temperature. Because the number of defects is quite limited, the conductivities are low, of the order of I0-6 ft 1 cm-1. In addition, extrinsic vacancies will be induced by ions of different charge (see page 264). There exist, however, a few ionic compounds that as solids have conductivities several orders of magnitude higher. One of the first to be studied and the one with the highest room-temperature conductivity, 0.27 fl-1 cm-1, is rubidium silver iodide, RbAg j.1 1... 

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],... 

Translation of ions within crystals is less frequently observed than is rotation. Perhaps one of the most interesting cases is that of silver iodide which may actually be said to melt in halves. When this solid is heated to 145.8° C, the crystal structure then changes and the ionic conductivity increases tremendously the iodide ions are hexagonally closest-packed below the transition temperature but at this temperature they rearrange to form a more open structure, and the silver atoms are allowed to move within the lattice. At 555° C, the network of iodide ions collapses, and the compound becomes a liquid. The solids Cul and Ag2Se show similar behavior. 

The use of anion-centred polyhedra can be particularly useful in describing diffusion in fast ion conductors. These materials, which are solids that have an ionic conductivity approaching that of liquids, find use in batteries and sensors. An example is the high temperature form of silver iodide, a-Agl. In this material, the iodide anions form a body-centred cubic array, (Figure 7.20a). 

Many ionic crystals have an appreciable electrical conductivity in the solid state, due to the motion of anions or cations or both. We have already given an extreme example of this effect in the case of the high-temperature form of silver iodide ( 8.05), where the silver atoms are... 

Silver iodide undergoes a first order structural phase transition at 420 K from the / -phase (hexagonal Wurtzite structure), which is metastable with respect to the / -phase (cubic sphalerite structure), to the a-phase where the I - ions occupy a bcc lattice within which the Ag+ ions jump rapidly between a number of possible sites. The ionic conductivity is very high upon melting it actually decreases. Agl is probably the most widely studied fast ion conductor, with much of the work concentrating on determination of the exact distribution of Ag+ sites and conduction pathways. 

Silver iodide, which undergoes a solid state phase transition at 147 from the low-temperature p-phase (Wurtzite structure) to the high-temperature, ionically conducting a-phase (body-centred cubic iodide containing a disordered silver ion sub-lattice) was investigated by means of modulated temperature calorimetry, using a novel apparatus that could expose the sample to... 

The super ionic conductivity can occur after a phase transformation at a characteristic transformation temperature. A representative example is Agl. The material has considerable conductivity already at room temperature. The large iodide ions form a lattice in which the much smaller Ag" ions can move via interstitials. At this temperature the stable form of silver iodide is the sphalerite lattice (see Chapter 2). 

P"-aluminas, structures derived from quartz or cristobalite, e.g. LiAlSi04 and structures based on silver iodide. Before discussion of the specific behaviour of protonic conductors, the role of defects in the ionic and electronic conductivity of solids will be reviewed. 

Electrolytes are distinguished from pure electronic conductors by the fact that the passage of an electric current is only insured by displacement of charged species called ions and hence accompanied by a transfer of matter. Therefore, electrolytes are entirely ionic electrical conductors without exhibiting any electronic conductivity (i.e., no free electrons). They can be found in the solid state (e.g., fluorite, beta-aluminas, yttria-stabilized zirconia, and silver iodide), liquid state (e.g., aqueous solutions, organic solvents, molten salts and ionic liquids), and gaseous state (e.g., ionized gases and plasmas). The ions (i.e., anions or cations)... 

Silver iodide is an example of a crystal with large ionic conductivity, which reaches the value 2.5 ohm" cm" at 555 C, 3° below the melting point. At the melting point the conductivity of the crystal is greater than that of the liquid. 

The very large ionic conductivity of silver iodide crystal is explained by its structure. The crystal is cubic, with the four iodide ions in the unit cell in the close-packed positions 00 0, (Figure 2-7). The... 

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