Structure-Property Relations in Lanthanide Borate Glasses

Glass formation in the system Ln₂O₃-B₂O₃ (Ln = Nd, Sm) was studied. Glasses could be formed in the range from 0 to -28 mol% rare-earth oxide (Ln₂O₃), but liquid immiscibility in these systems limits the range of homogeneous glasses to 0 to 1.5 and 25 to 28 mol% Ln₂O₃. The infrared spectra indicate that the rare-earth-rich glasses are structurally similar to rare-earth metaborates (LnB₃O₆) which contain (B₃O₆)- chains. The variation in density, transformation temperature, thermal expansion coefficient, and transformation-range viscosity of these glasses with the size of the rare-earth ion is discussed. Glasses near the metaborate composition have a transformation temperature of =700°C, which is high for binary borate glasses. Glasses could not be formed in the systems EU₂O₃-, Gd₂O₃-, HO₂O₃-, and Er₂O₃-B₂O₃, even by quenching at =1300°C/s. The sudden lack of glass formation in the system Ln₂O₃-B₂O₃ with Ln³⁺ ions smaller than Sm³⁺ is explained on the basis of the size effect of the Ln³⁺ ion on the stability of (B₃O₆)- chains in these metaborates.