Neodymium(III) in alumino-borosilicate glasses

Optical spectroscopy measurements were performed on Al2O3-B2O3-SiO2 glasses containing 7-25 mol% Nd2O3 (soluble). The local chemical environment of Nd (III) in glass was systematically studied as a function of the Nd2O3 concen tration using Judd-Ofelt (J-O) optical oscillator parameters, Omega (2) and Omega (6), that are related to the ligand field symmetry (LFS) and the deg ree of bond covalency (BC), respectively. Two transition points (TPs), in t erms of Omega (lambda) versus Nd2O3 concentration, were found for Omega (2) that is sensitive to LFS and Omega (6) that is sensitive to the degree of BC. The first TP is defined by Nd:3(B + Al) and the second by Nd:3(B + Al Si). Below the first TP, Omega (2) and Omega (6) were nearly independent o f Nd concentration. Between the first and the second TP, Omega (2) and Omeg a (6) increased and then decreased above the second TP. After reviewing lit erature data on rare earths (REs) in Na2O-B2O3, Na2O-SiO2, Na2O-B2O3-SiO2, and Al2O3-B2O3 systems, we proposed that up to the first TP, Nd preferentia lly dissolves in a berate-rich environment composed of two trigonal boron, B-III, and one tetrahedral boron, B-IV, where Al-IV substitutes for B-IV. F urther increases in Nd2O3 concentrations result in excess Nd cations, [Nd-3 (B + Al)], partitioning to a silicate-rich environment, possibly in a form of Nd-Q(3), and above the second TP, Nd-Q(2) could form at the expense of N d-Q(3) (Q(n) is the number of bridging oxygens per Si tetrahedron), whereas the concentration of Nd cations in the berate sites is expected to be unch anged. Crystallization of Nd silicate above its solubility, 25-30 mol% Nd2O 3, indirectly supports the proposed Nd dissolution mechanisms. The dissolut ion of rare earth elements in alumino-borosilicate glasses may be best desc ribed in terms of their partitioning to these structure groups. (C) 2000 El sevier Science B.V. All rights reserved.