Silver iodide conductivity

Starting in the 1960s, many compounds with such properties were discovered (i.e., with high conductivities and low-temperature coefficients of conductivity). Some of them are double salts with silver iodide (uAgFmMX) or other silver halides where MX has either the cation or the anion in common with the silver halide. The best-known example is RbAgJj (= 4AgFRbI), where this sort of conduction arises at - 155°C and is preserved up to temperatures above 200°C. At 25°C this compound has a conductivity of 26 S/m (i.e., the same value as found for a 7% KOH solution). Another example is Ag3SI, which above 235°C forms an a-phase with a conductivity of 100 S/m. 

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). 

Silver(I) halide complexes of oA could not be prepared. The phosphine ap, however, reacts with silver iodide to give a colourless, unstable, non-conducting compound of empirical formula Agl(ap). This compound reacts with excess ap to give the stable 2 1 adduct Agl(ap)2- Silver bromide and silver chloride react directly with the ligand to give similar 2 1 adducts. These complexes are essentially monomeric, contain three-coordinate silver (I) and uncoordinated olefinic groups. The structure of the 1 1 adduct is unknown. 

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. 

Tubandt was a pioneer of - solid state electrochemistry. He introduced a methodology to determine the - transport numbers of ions in -> solid electrolytes [i], which is now referred to as -> Tubandt method. Together with his co-workers he performed seminal studies of conductivities and transport numbers of solid electrolytes, e.g., of silver, lead, and copper halides, and silver sulfide. He showed for the first time that the entire dark current of silver bromide is transported by silver ions, and also that slightly below the melting point silver iodide has a higher conductivity than the melt. 

Other properties of aqueous solutions investigated are density,3 refractive index,4 molecular elevation of the boiling-point,6 vapour-pressure,6 specific heat,7 and electric conductivity.8 References are also appended to work on the compressibility,9 the solubility in organic solvents10 and sulphurous acid,11 the molecular weight in liquid sulphur dioxide,11 the electric conductivity in acetone12 and dilute alcohol,13 the non-existence of polyiodides,14 isomorphism with potassium iodide,15 and the formation of a double salt with silver iodide.16... 

I) For materials that can be made Into an electrode, or that can be deposited on an electrode, the differential capacitance can sometimes be measured directly. From this, the surface charge follows by Integration. A number of technical problems have to be surmounted, to be discussed In sec. 3.7c. One of these Is that Faradaic currents (currents across the Interface) have to be suppressed or accounted for. Another Intrinsic problem Is whether the surface properties of the electrode are Identical to those of the dispersed particles. For silver Iodide and some oxides the capacitance approach has worked well. It Is recalled that for polarizable, conducting Interfaces, with mercury as the prototype, this Is virtually the sole method. 

At low temperatures silver iodide forms a structure in which silver has a coordination number of four. On raising the temperature the lattice is deformed so that three iodine atoms are closer to the silver atom than the fourth. Above 146° C a further transformation occurs to give a structure in which the iodine ions form a body centred cubic lattice and the silver ions move freely in the interstices. Owing to the free mobility of the silver ions, the high temperature form conducts electricity. 

Silver iodide exhibits an unusual property. In addition to a y-(blende) form and a -(wurtzite) form it has an a-form stable between 146° and 552° (the m.p.). In this the iodide ions are arranged in a body-centred cubic lattice but the Ag+ ions form what may be called an interstitial fluid, being apparently free to move through the rigid network of 1 ions. The variation of conductance with temperature in silver iodide (Table 23) is particularly interesting. 

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