FROM 3-COORDINATION TO 4-COORDINATION IN COPPER(I) AND SILVER(I)

An orbital picture is presented of the factors influencing the choice of three- versus four-coordination for solids containing Cu(I) and Ag(I), of importance in appreciating the mobility of these ions in ionic conductors. The electronic preferences of both the nd orbital manifold and the (n + 1)(s + p) orbital manifold are for the regular tetrahedral arrangement, but mixing between the occupied nd and unoccupied (n + 1)s orbitals on distortion, via a second-order Jahn-Teller effect, as in Orgel's model for Hg(II), favors the lower coordination number. This balance is shown by calculation to be particularly sensitive at the end of the transition metal series.