  Rare-earth (RE) containing alkali aluminoborosilicate glasses find
increasingly broad technological applications, with their further development
only impeded by yet-poor understanding of coordination environment and
structural role of RE ions in glasses. In this work we combine free induction
decay (FID)-detected electron paramagnetic resonance (EPR), electron spin echo
envelope modulation (ESEEM), and MAS NMR spectroscopies, to examine the
coordination environment and the clustering tendencies of RE3+ in a series of
peralkaline aluminoborosilicate glasses co-doped with Nd2O3 (0.001-0.1 mol%)
and 5 mol% La2O3. Quantitative EPR spectral analysis reveals three different
Nd3+ forms coexisting in the glasses: isolated Nd3+ centers, dipole-coupled Nd
clusters (Nd-O-X-O-Nd, where X = Si/B/Al), and spin-exchange-coupled Nd
clusters, (Nd-O-Nd) and (Nd-O-La-O-Nd). Extensive RE clustering is observed at
high RE2O3 concentrations, with more than 90% REs converting to dipole- and
exchange-coupled Nd clusters already at [RE2O3] = 0.01 mol%. ESEEM analysis of
the EPR-detectable Nd centers indicates a Na/Si-rich environment (four Na+ per
Nd3+) 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 the
EPR-undetectable exchanged-coupled RE clusters are predicted to exist in a
Na/B-rich environment. The RE clustering induces nano-scale glass phase
separation, while the Na/B-rich environment of the RE clusters implies a
depletion of the same elements from the remaining host glass. Based on our
results, we develop a mechanistic model that explains the high tendency of RE3+
to form clusters in alkali aluminoborosilicate glasses.
