Silver iodide structure

The crystal structure of ice is hexagonal, with lattice constants of a = 0.452 nm and c = 0.736 nm. The inorganic compound silver iodide also has a hexagonal structure, with lattice constants (a = 0.458 nm, c = 0.749 nm) that are almost identical to those of ice. So if you put a crystal of silver iodide into supercooled water, it is almost as good as putting in a crystal of ice more ice can grow on it easily, at a low undercooling (Fig. 9.2). 

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). 

KEY TERMS supercooled liquid crystal structure silver iodide... 

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. 

The crystal structure of the complex between silver iodide and piperidine has been determined.57 The colourless crystals were prepared by warming silver iodide with sufficient piperidine to allow the silver iodide to dissolve and then allowing the resulting solution to cool. The structure consisted of tetrahedral clusters of iodide ions with the silver atoms embedded into the faces of the tetrahedron. The (Agl)4 clusters were separated by the piperidine molecules which were bound to the silver via the N atom. The Ag—N bond lengths were 232.9 pm, while the Ag—I distances were 285.3, 293.6 and 294.2 pm. 

Silver iodide derivatives of trialkyl-phosphines and -arsines were prepared in 1937 for comparison with their copper(I) iodide analogues.201 The preparations involved shaking the ligands with silver iodide dissolved in concentrated aqueous KI. The products were found to be tetramers and of similar structure to the Cu1 complexes. The Pr As silver complex was isomorphous with [Cul-AsEt3]4. Molecular weight determinations in a range of organic solvents showed that partial dissociation occurred in solution. 

It is convenient here to consider the compound disilver monofluoride (or silver subfluoride). This was reported to have an anti-cadmium iodide structure in early work (5), and this has been confirmed more recently (6). From X-ray single-crystal results the unit cell is hexagonal, a = 2.996, c = 5.691 A, space-group C3m, and the Ag—F separation 2.451 A, almost identical with that in silver monofluoride (2.468 A). The structure consists of layers, with a plane of fluorine atoms sandwiched between two planes of silver atoms, giving the fluorine atom a coordination of six silver atoms, the same as in silver monofluoride. The silver atoms have separations of 2.996 and 2.814 A, compared with 2.889 A in the metal itself, and in line with the metallic conductivity of the compound. 

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). 

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. 

Structural transformation in the superionic conductor silver iodide has been investigated by employing the modified... 

The adsorption of alkali Ions (and of earth alkali ions, not shown) differs from that of the anions SO and HPO In that the latter adsorb specifically on uncharged silver Iodide, with the concomitant change In p.z.c. (sec. 3.8. fig. 3.23 -25), whereas the former do not shift the p.z.c. For alkali ions, specificity starts only when there Is already 1 on the surface. This Is an example of specific adsorption of the second kind, as defined in sec. 3.6d. Apparently, the alkali Ions only adsorb on 1 sites, so that there will be some analogy with water structure-originating alkali lon-iodlde Ion interaction In solution. We will come back to this in sec. 3.10g. 

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. 

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