The density of two mixed alkali borate glass systems related to atomic arrangements

Densities of the two mixed alkali borate glass systems, R( 1 2 Na 2 O + 1 2 Li 2 O)· B 2 O 3 and R( 1 2 Rb 2 O + 1 2 Li 2 O)· B 2 O 3 have been measured over an extremely wide range of alkali content: from boron oxide through the orthoborate composition. The glasses were prepared with metal carbonate and oxide starting materials. As with the binary alkali borates, a semiempirical model has been applied to the densities in order to determine the volumes of the structural units present. These values, in conjunction with volumes found in the binary borate cases, and with volumes calculated from the ionic radii, were used in a general discussion of the filling of space by the structural units. The results indicate that the size increases of the structural groupings as one goes from small to large alkali are primarily due to the alkali being used.