Packing fractions in borate and silicate glasses with an emphasis on lead and bismuth systems.

• Packing is a physical property that is directly related to atomic structure. • Lithium, zinc, lead, and bismuth borates were studied. • The fraction of four-coordinated boron is strongly correlated to packing in these borate glasses. • Lithium, lead, and bismuth silicate glasses were studied. • Changes in the numbers of non-bridging per silicon are related to packing trends in silicates. The densities of lead oxide and bismuth oxide modified glasses determined over a wide range of compositions were collected from the literature. Using these data as well as Shannon atomic radii and atomic masses, the glass systems' total packing fractions and oxygen packing fractions were computed. For borates, these fractions were correlated against the trend of the fraction of tetrahedral borate (N 4) units. This analysis was compared to that of lithium borate and silicate glasses and zinc borate glasses. The peak of the N 4 distribution versus R (= molar fraction of modifier/molar fraction B 2 O 3) corresponds well to the peak in oxygen packing in lithium and lead borate glasses but does not closely coincide with the peak in oxygen packing for zinc and bismuth glasses. In contrast to the borates, silicon in glassy silicate systems does not change coordination and the resulting packing trends change less than in the borates. Furthermore, the packing in the silicate systems is correlated to the change in Q n units. [ABSTRACT FROM AUTHOR]