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Rare-earth (RE) containing alkali aluminoborosilicate glasses findincreasingly broad technological applications, with their further developmentonly impeded by yet-poor understanding of coordination environment andstructural role of RE ions in glasses. In this work we combine free inductiondecay (FID)-detected electron paramagnetic resonance (EPR), electron spin echoenvelope modulation (ESEEM), and MAS NMR spectroscopies, to examine thecoordination environment and the clustering tendencies of RE3+ in a series ofperalkaline aluminoborosilicate glasses co-doped with Nd2O3 (0.001-0.1 mol%)and 5 mol% La2O3. Quantitative EPR spectral analysis reveals three differentNd3+ forms coexisting in the glasses: isolated Nd3+ centers, dipole-coupled Ndclusters (Nd-O-X-O-Nd, where X = Si/B/Al), and spin-exchange-coupled Ndclusters, (Nd-O-Nd) and (Nd-O-La-O-Nd). Extensive RE clustering is observed athigh RE2O3 concentrations, with more than 90% REs converting to dipole- andexchange-coupled Nd clusters already at [RE2O3] = 0.01 mol%. ESEEM analysis ofthe EPR-detectable Nd centers indicates a Na/Si-rich environment (four Na+ perNd3+) for the isolated Nd3+ centers and the Na/Si/B-rich environment (2-3 Na+and 1-2 boron per each Nd3+) for the dipole-coupled Nd clusters, while theEPR-undetectable exchanged-coupled RE clusters are predicted to exist in aNa/B-rich environment. The RE clustering induces nano-scale glass phaseseparation, while the Na/B-rich environment of the RE clusters implies adepletion of the same elements from the remaining host glass. Based on ourresults, we develop a mechanistic model that explains the high tendency of RE3+to form clusters in alkali aluminoborosilicate glasses.

Insight into the partitioning and clustering mechanism of rare-earth cations in alkali aluminoborosilicate glasses