Silver iodide, crystal structure

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... 

A silver iodide crystal has a surface pattern that closely resembles that of an ice crystal it was this resemblance that led Langmuir to try silver iodide as a seeding material. The silver atoms and iodine atoms occupy the positions of alternate oxygen atoms in the ice structure (Figure 9-8), and the silver-iodine distance, 280 pm, is only 1.5 percent greater than the oxygen-oxygen distance in ice. 

S. V. Shevkunov, Structure of water in microscopic fractures of a silver iodide crystal, Russ. J. Phys. Chem. A, 88,313-319 (2014). 

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... 

The complexation of anionic species by tetra-bridged phosphorylated cavitands concerns mainly the work of Puddephatt et al. who described the selective complexation of halides by the tetra-copper and tetra-silver complexes of 2 (see Scheme 17). The complexes are size selective hosts for halide anions and it was demonstrated that in the copper complex, iodide is preferred over chloride. Iodide is large enough to bridge the four copper atoms but chloride is too small and can coordinate only to three of them to form the [2-Cu4(yU-Cl)4(yU3-Cl)] complex so that in a mixed iodide-chloride complex, iodide is preferentially encapsulated inside the cavity. In the [2-Ag4(//-Cl)4(yU4-Cl)] silver complex, the larger size of the Ag(I) atom allowed the inner chloride atom to bind with the four silver atoms. The X-ray crystal structure of the complexes revealed that one Y halide ion is encapsulated in the center of the cavity and bound to 3 copper atoms in [2-Cu4(//-Cl)4(//3-Cl)] (Y=C1) [45] or to 4 copper atoms in [2-Cu4(/U-Cl)4(/U4-I)] (Y=I) and to 4 silver atoms in [2-Ag4(/i-Cl)4(/i4-Cl)] [47]. NMR studies in solution of the inclusion process showed that multiple coordination types take place in the supramolecular complexes. 

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, 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 ... 

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). 

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. 

Figure 7. (a) Crystal structure of tetra-coppeifl) (61) with included iodide anion. (b) Crystal structure of tetra-silver(I) (61) with included chloride anion. [Reprinted with permission from W. Xu. J. J. Vittal, R. J. Puddephatt, J. Am. Chem. Soc., 117, 8362 (1995). Copyright 1995 American Chemical Society.]... 

Both in the wurtzite and zinc blende structures (coordination number = 4) of silver iodide, stable at atmospheric pressure, the Ag—I-distances are 278 pm and 280 pm respectively. At 4 kbar the NaCl-type structure (coordination number = 6) becomes stable, in which the Ag—I-distances are remarkably greater, namely 303 pm. Increase in pressure to 100 kbar contracts all of these distances to 283 pm with a decrease in molar volume from 33.8 to 27.6 cm /mol. Thus the transformation with increase in coordination number from the low-pressure modification into the high pressure modification involves an increase in Ag—I-distances by 8.4%, but further increase in pressure does not produce another geometrical rearrangement and hence all of the equidistant bonds within the crystal lattice are shortened 34). 

Silver iodide exists in one of three crystal structures depending on the temperature, a phenomenon frequently referred to as trimorphism. Below 137°C, silver iodide is in the cold cubic, or y-form at 137—145.8°C, it exists in the green-yellow colored hexagonal, or p-form above 145.8°C, the yellow cubic or a-form of silver iodide is the stable crystal structure. Silver iodide decomposes into its elements at 552°C. 

This table lists only hexagonal crystals and only the best known of these. Silver iodide in the form of smoke particles a few hundred angstroms in average diameter, for example, is commonly used to seed clouds and induce ice formation at temperatures —10 to — i5°C instead of the — 20°C more characteristic of natural nuclei. Many materials which are not hexagonal and some which bear no obvious relation to ice structure, such as certain organic materials, are also quite efficient nuclei. Extensive experimental... 

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