Comparative study of the rare earth environment in rare earth metaborate glass (REB 3 O 6 , RE = La, Nd) and in sodium borate glasses
Abstract The RE environment in the RE-metaborate glass (RE = La 3 + or Nd 3 + ) has been characterized by Nd L III -edge EXAFS and Nd 3 + and Eu 3 + 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% Na 2 O) bearing almost no NBOs (non-bridging oxygens), and a Na-rich borate glass (35 mol% Na 2 O) 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  3 + 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 RE 3 + , like in the crystalline RE-metaborate phase, but they point out stronger RE 3 + –NBO bonds. The structural and bonding differences involving the RE 3 + ions, between the glassy and crystalline forms of RE-metaborate, may be the basis of the high nucleation barrier observed in this congruent composition.