A general study of packing in oxide glass systems containing alkali

The packing fraction, defined as the ionic volume divided by the molar volume, appears to hold promise as a general measure of the structure of alkali oxide modified glasses. For modifying ions having volumes larger than oxygen (K, Rb, and Cs), the packing is dominated by the modifier as alkali oxide concentration increases and is largely independent of glass former; we define this as ionic packing. For alkali smaller than oxygen (Li and Na) the packing is controlled by the oxygen covalent network and is heavily dependent on glass former; we call this covalent packing. In this paper we compare the packing fractions of alkali borate, silicate, germanate, phosphate, and vanadate glass systems.