It can be shown that volume changes in silicate, borate and germanate glasses can be simply described by the ionic volumes of the corresponding cation addition, if these changes are related to the total number of oxygen atoms. Changes in the volume per mole oxygen are also caused by a varying coordination number as in the case of borate and germanate glasses. Molar volumes defined this way obey simple superposition rules for binary and ternary glasses. In addition, the volume per oxygen atom (or mole), a measure of the average distance of oxygen atoms, provides a quantity which is closely related to the diffusion of gas atoms and alkali ions. Thus a decreasing (increasing) volume per mole oxygen gives rise to a decrease (increase) of the diffusion coeffieiciet of cations or gas atoms.
