ELECTRONIC-STRUCTURE OF AG2+ IMPURITIES IN HALIDE LATTICES

The composition of electronic levels as well as optical transitions as sociated with AgCl6(4-) and AgF6(4-) complexes have been studied throu gh MS-Xalpha and SCCEH calculations performed as a function of equator ial (R(eq)) and axial (R(ax)) metal-ligand distances. The scheme and c omposition of levels for AgCl6(4-) is rather different from that for A gF6(4-) and other more ionic systems. The first transition for KCl:Ag2 + (observed at 12 500 cm-1) is assigned to a jump involving the 5a1g o rbital which is mainly built (about 70%) from 3p orbitals of axial chl orine atoms. Aside from explaining reasonably the five optical bands e xperimentally observed for KCl:Ag2+, the present work indicates that t he first allowed charge-transfer transition of AgF6(4-) would lie in t he ultraviolet region and confirms that the unpaired electron in AgCl6 (4-) spends a little more time on equatorial ligands than on the centr al ion. All these results are consistent with a high value (chi = 2.8) for the optical electronegativity of Ag2+. The dependence of electron ic transitions (and also of unpaired spin densities f(sigma) and f(s)) on R(eq) and R(ax) is found to be rather similar for both AgF6(4-) an d AgCl6(4-) complexes. The relation between such a dependence and the band widths of optical transitions is outlined.