Influence of lanthanides on the melt viscosity and glass density of sodium disilicate

High temperature viscosity data for 14 sodium disilicate (NS2; Na2Si2O5) melts with equimolar (5.288 mol%) additions of lanthanum oxides (Ln2O3; Ln = La; Ce; Pr; Nd; Sm; Eu; Gd; Tb, Dy; Ho; Er; Tm; Yb; Lu) have been determined in the temperature range of 1003-1490 degrees C. At the highest temperatures the viscosity of Ln2O3 modified NS2 generally decreases, whereas towards lower temperatures the heaviest lanthanides effect a slight increase of melt viscosity. In general, the lanthanides behave highly coherently with respect to their viscosity reducing effect, while a "tetrad effect" might be present. Two exceptions exist, Ce and Tm, both exhibiting anomalously stronger viscosity reducing effects. For Ce we attribute this behavior to its multivalent oxidation state in the melt, while for Tm its origin remains unclear. Glass densities also correlate with the atomic number, again with exceptions for Ce-and Tm-bearing samples, which exhibit the lowest and highest densities, respectively.