Neodymium(III) in alumino-borosilicate glasses

Abstract

Optical spectroscopy measurements were performed on Al 2O 3–B 2O 3–SiO 2 glasses containing 7–25 mol% Nd 2O 3 (soluble). The local chemical environment of Nd(III) in glass was systematically studied as a function of the Nd 2O 3 concentration using Judd–Ofelt (J–O) optical oscillator parameters, Ω 2 and Ω 6 , that are related to the ligand field symmetry (LFS) and the degree of bond covalency (BC), respectively. Two transition points (TPs), in terms of Ω λ versus Nd 2O 3 concentration, were found for Ω 2 that is sensitive to LFS and Ω 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, Ω 2 and Ω 6 were nearly independent of Nd concentration. Between the first and the second TP, Ω 2 and Ω 6 increased and then decreased above the second TP. After reviewing literature data on rare earths (REs) in Na 2O–B 2O 3, Na 2O–SiO 2, Na 2O–B 2O 3–SiO 2, and Al 2O 3–B 2O 3 systems, we proposed that up to the first TP, Nd preferentially dissolves in a borate-rich environment composed of two trigonal boron, B III, and one tetrahedral boron, B IV, where Al IV substitutes for B IV. Further increases in Nd 2O 3 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 Nd–Q 3 (Q n is the number of bridging oxygens per Si tetrahedron), whereas the concentration of Nd cations in the borate sites is expected to be unchanged. Crystallization of Nd silicate above its solubility, 25–30 mol% Nd 2O 3, indirectly supports the proposed Nd dissolution mechanisms. The dissolution of rare earth elements in alumino-borosilicate glasses may be best described in terms of their partitioning to these structure groups.