It is well reported that at low concentration of Dy2O3 acts as network modifier in place of network former in bismuth borate
glass system. The increase in molar volume may indicate that the volume of NBO sites produced by the modifier
Dy2O3. With the composition 30Bi2O3:70B2O3, NBOs are already present and on addition of Dy2O3 (as modifier),
concentration of NBOs increases. With the further addition of Dy2O3 we get the 6-membered rings with only one
BO4 tetrahedron.