Studies of Ion Transport in AgCl and AgBr

Abstract

Silver chloride and silver bromide, like sodium chloride and sodium bromide, are ionic crystals with the rock-salt structure, but despite the similarity in the crystal radii of Ag+ (1.29 Å) and Na+ (1.16 Å) the properties of these silver halides are very different from those of the corresponding sodium salts. Much of this difference is due to the filled 4d shell in the electron configuration of Ag+, which is responsible for the higher polarizability of Ag+ (2.37 − 2.44 Å3) than Na+ (0.41 Å3 and for the tendency of Ag+ to undergo quadrupolar deformation.Associated with the high polarizability of Ag+ are the large van der Waals cation-cation and cation-anion interactions in AgCl and AgBr and the consequent high cohesive energies of these silver salts (9.26 eV and 9.12 eV, respectively). Bear in mind, however, that the large van der Waals coefficients that result from empirical fitting of two-body model crystal potentials to crystal properties may be partly due to the model attempting to simulate features of the real crystal which the model does not really include, such as many-body interactions. That such many-body interactions are important in AgCl and AgBr is evident from the large violations of the Cauchy relation between the elastic constants, C12 = C44, which ought to hold when each ion is at a center of cubic symmetry and the inter-ionic forces are derivable from two-body potentials.