Electron distribution in silver halides

Solid fast ion conductors such as silver halides and silver chalcogenides have been recently drawing attention, because of their unusually high ionic conductivity in the high-temperature phase at about 200°C. This peculiar property is of particular importance for both research field of codensed matter physics and practical application as solid state sensors. The information of the bonding nature relevant to the electron distribution is desirable, in order to obtain insights into their characteristic property of solid fast ion conductors. An attempt will be made in this paper to review the current information of the bonding nature of silver halides in terms of the electron density distribution. Thus, this paper is primarily concerned with some physical properties of silver halides including the characteristic features of the crystal structure and the phase diagram. The data of alkali halides are also described for comparison. The experimental results of the magnetic susceptibilities of silver halides are summarized from the phenomenological point of view. The magnetic susceptibilities of silver halides were found to be sensitive to the change of temperature and crystal structure. It implies that the valence electrons of silver halides are loosely bound by the ion core. The electron density distribution in Agl and AgBr is also discussed using the results by single crystal X-ray diffractometry and the following points are suggested; in AgBr, the deformation of the charge distribution in bromine ion is much larger than that of silver ion and the radial distribution of valence electrons in bromine ion is enlarged in comparison with that of free ion state. In Agl, charge accumulation is feasible only on the c-directed bond and the difference Fourier map on the (1 1 0) plane indicates the deficiency of electrons around the silver site.