The mixed alkali effect as a consequence of network density and site energy distribution

By reviewing experimental findings on ion diffusion and electrical conducta nce in binary and ternary glasses it will be shown that three effects deter mine ion mobility. These are (i) the direct Coulomb interaction between alk ali cations and non-bridging oxygen anions, (ii) a broad distribution of ca tion site energies arising from structural and/or Coulomb disorder and (iii ) the network packing density as expressed by the molar volume per mole of oxygen. In ternary glasses with two network modifying oxides the site energ y distribution leads to a competition for the low energy sites. By assuming that smaller cations succeed because they come closest to anions, their ac tivation energies for diffusion have to be larger on the average. Thus subs tituting cations by bigger cations reduces the mobility of the smaller ones . The diffusivity of bigger cations is reduced as well, because they have t o move through a network with a mesh size being reduced by the presence of the smaller cations. Describing these counteracting effects with two reason able parameters allows one to explain the mixed alkali effect quantitativel y for a variety of glasses. From a simple estimation of elastic energies on e of the parameters can be calculated from the bulk modulus and? therefore, only one parameter which is the width of an energy distribution is left as a free fitting parameter to explain a large body of experimental results. (C) 2000 Elsevier Science B.V. All rights reserved.