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 concentration using Judd–Ofelt (J–O) optical oscillator parameters, Ω2 and Ω6, that are related to the ligand field symmetry (LFS) and the degree of bond covalency (BC), respectively. Two transition points (TPs), in terms of Ωλ versus Nd2O3 concentration, were found for Ω2 that is sensitive to LFS and Ω6 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, Ω2 and Ω6 were nearly independent of Nd concentration. Between the first and the second TP, Ω2 and Ω6 increased and then decreased above the second TP. After reviewing literature 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 preferentially dissolves in a borate-rich environment composed of two trigonal boron, BIII, and one tetrahedral boron, BIV, where AlIV substitutes for BIV. Further 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-Q3, and above the second TP, Nd–Q2 could form at the expense of Nd–Q3 (Qn 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% Nd2O3, 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.