Comparative study of the rare earth environment in rare earth metaborate glass (REB3O6, RE = La, Nd) and in sodium borate glasses

Abstract

The RE environment in the RE-metaborate glass (RE=La3+ or Nd3+) has been characterized by Nd LIII-edge EXAFS and Nd3+ and Eu3+ optical spectroscopy. The spectroscopic data have been compared with that of crystalline RE-metaborate and of two sodium borate glasses containing RE, a Na-poor borate glass (20mol% Na2O) bearing almost no NBOs (non-bridging oxygens), and a Na-rich borate glass (35mol% Na2O) bearing a significant number of NBOs. Spectroscopic data indicate that the RE–O ionicity and mean distance increase in the order Na-rich borate glass<RE-metaborate glass<Na-poor borate glass<RE-metaborate crystal. The distribution of RE3+ sites in the RE-metaborate glass is wider than in the two sodium borate glasses, as revealed by a larger RE–O distance distribution and larger inhomogeneous widths of optical transitions. Data are consistent with a model in which both NBOs and BOs take part to the coordination sphere of RE3+, like in the crystalline RE-metaborate phase, but they point out stronger RE3+–NBO bonds. The structural and bonding differences involving the RE3+ ions, between the glassy and crystalline forms of RE-metaborate, may be the basis of the high nucleation barrier observed in this congruent composition.