A-Agl

It had been discovered long ago that the character of conduction in Agl changes drastically at temperatures above 147°C, when (I- and I -Agl change into a-Agl. At the phase transition temperature the conductivity, o, increases discontinuously by almost four orders of magnitude (from 10 to 1 S/cm). At temperatures above 147°C, the activation energy is very low and the conductivity increases little with temperature, in contrast to its behavior at lower temperatures (see Fig. 8.2). 

Crystals with Frenkel or Schottky defects are reasonably ion-conducting only at rather high temperatures. On the other hand, there exist several crystals (sometimes called soft framework crystals ), which show surprisingly high ionic conductivities even at the room or slightly elevated temperatures. This effect was revealed by G. Bruni in 1913 two well known examples are Agl and Cul. For instance, the ar-modification of Agl (stable above 146°C, sometimes denoted also as y-modification ) exhibits at this temperature an Ag+ conductivity (t+ = 1) comparable to that of a 0.1m aqueous solution. (The solid-state Ag+ conductivity of a-Agl at the melting point is actually higher than that of the melt.) This unusual behaviour can hardly be explained by the above-discussed defect mechanism. It has been anticipated that the conductivity of ar-Agl and similar crystals is described... 

Figure 6.9 Structure of the high-temperature form of Agl (a-Agl) (a) the body-centered cubic arrangement of iodide (I-) ions the unit cell is outlined (b) two (of four) tetrahedral sites on a cube face, indicated by filled circles and (c) the four tetrahedral sites found on each cube face, indicted by filled circles. Ag+ ions continuously jump between all of the tetrahedral sites.

Table 10.5 Selected experimental determinations of the enthalpy of transition of Agl (a-Agl = j8-Agl).

Fig. 2.4 (a) Body centred lattice of I ions in a-Agl. (b) Sites available to Ag ions in the conduction pathway. 

The absorption band at 320-330 nm seen in Fig. 4.4.10B was reported in a Agl particle formed by pulse radiolysis technique. The unstable species thus formed was attributed to initial aggregates of Agl molecules whose lifetime was on the order of milliseconds. Therefore in the present experiment, these transient species were successfully stabilized with RSH and can survive for several days before further... 

Below 146°C, two phases of Agl exist y-Agl, which has the zinc blende structure, and (3-Agl with the wurtzite structure. Both are based on a close-packed array of iodide ions with half of the tetrahedral holes filled. However, above 146°C a new phase, a-AgI, is observed where the iodide ions now have a body-centred cubic lattice. If you look back to Figure 5.7, you can see that a dramatic increase in conductivity is observed for this phase the conductivity of a-Agl is very high, 131 S m , a factor of 10 higher than that of (3- or y-AgI, comparable with the conductivity of the best conducting liquid electrolytes. How can we explain this startling phenomenon ... 

FIGURE 5.9 Building up the structure a-Agl. (a) The hody-centered cubic array of r ions, (b) hcc array extended to next-nearest neighbours, (c)... 

FIGURE 5.10 Possible cation sites in the /icc structure of a-Agl. The thick solid and dashed lines mark possible diffusion paths. 

A table of ionic conductors that behave in a similar way to a-Agl is given in Table 5.4. Some of these structures are based on a close-packed array of anions and this is noted in the table the conducting mechanism in these compounds is similar to that in a-Agl. The chalcogenide structures, such as silver sulfide and selenide, tend to demonstrate electronic conductivity as well as ionic, although this can be quite useful in an electrode material as opposed to an electrolyte. 

The zinc blende structure of y-AgI, a low-temperature polymorph of Agl, is illustrated in Figure 5.43. Discuss the similarities and differences between this structure and that of a-Agl. Why do you think the conductivity of the Ag" ions is lower in y-AgI ... 

The existence of Schottky or Frenkel defects, or both, within an ionic solid provides a mechanism for significant electrical conductance through ion migration from site to empty site (leaving, of course, a fresh empty site behind).4 Solid /3-AgI provides a classic example of a nonmetallic solid with substantial electrical conductivity at elevated temperatures at 147 °C, it undergoes a transition to a-Agl in which the silver ion sublattice is disordered and consequently allows for relatively free movement of Ag+ and... 

There are two modifications of Agl at ordinary temperatures, (3-AgI has the wurtzite (2 2PT) structure and y-Agl has the zinc blende (3 2PT) structure. For both of these structures Ag+ and I ions have CN 4 (tetrahedral). Above 145.8°C, a-Agl is formed with a bcc (3 2PTOT) structure for I ions. For a bcc structure all P, T, and O layers are filled by I ions for a-Agl. There are secondary interstitial sites for a bcc structure—four distorted tetrahedral sites (T ) in each face of the cube and distorted octahedral sites (O ) in the centers of the edges (12) and in the centers of the faces (6) of the cube. The T sites are shown as squares in each face of the bcc cube in Figure 7.28. The bcc cell... 

Figure 7.28. The 24 secondary T sites available for Ag+ (solid circles) in a bcc cell for a-Agl. In the bcc cell the positions of I ions are large open circles.

Potential-determining ions are those whose equilibrium between two phases, frequently between an aqueous solution and an interface, determines the difference in electrical potential between the phases. Consider a Agl dispersion in water. There will exist some concentrations of Ag+ and I" such that the surface charge of the Agl particles is zero. This is called the point of zero charge (pzc). It is usually determined by a titration method (called a colloid titration). 

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 ... 

Figure 10.3.2 shows the crystal structure of a-Agl and the possible positions of the Ag+ ions, the mobility of which accounts for the prominent ionic transport properties of a-Agl as a solid electrolyte. 

The structure of Agl varies at different temperatures and pressures. The stable form of Agl below 409 K, y-Agl, has the zinc blende (cubic ZnS) structure. On the other hand, /3-AgI, with the wurtzite (hexagonal ZnS) structure, is the stable form between 409 and 419 K. Above 419 K, ft-Agl undergoes a phase change to cubic a-Agl. Under high pressure, Agl adopts the NaCl structure. Below room temperature, y-Agl obtained from precipitation from an aqueous solution exhibits prominent covalent bond character, with a low electrical conductivity of about 3.4 x 10-4 ohm 1cm 1. When the temperature is raised, it undergoes a phase change to a-Agl, and the electrical conductivity increases ten-thousandfold to 1.3 ohm-1 cm-1. Compound a-Agl is the prototype of an important class of ionic conductors with Ag+ functioning as the carrier. 

The transformation of /3-AgI to a-Agl is accompanied by a dramatic increase in the ionic electrical conductivity of the solid, which leaps by a factor of nearly 4000from3.4 x 10 4to 1.3 ohm- em-1. This arises because in /1-AgItheAg... 

---