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

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 (20 mol% Na2O) bearing almost no NBOs (non-bridging oxygens), and a Na-rich borate glass (35 mol% 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.