On the structural role of indium in aluminoborosilicate glasses: A multi-spectroscopic study

This study investigates the structural role of indium in aluminoborosilicate glasses designed for advanced transparent optical glass-ceramics. Using a comprehensive multi-spectroscopic approach, including nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS), as well as infrared and Raman spectroscopy, combined with transmission electron microscopy (TEM) and differential scanning calorimetry (DSC), we examine the impact of indium oxide on the glass structure and its crystallization behavior, with comparisons to an analogous gallium-containing series. The results reveal the ambivalent structural role of indium in these complex glasses. Glasses with 2 mol% In₂O₃ enable controlled crystallization, while those with ≥3 mol% exhibit phase separation and spontaneous devitrification, reflecting on the solubility limit of In₂O₃. In stark contrast, the gallium series shows no such behavior. High-resolution XPS analysis of In 3d binding energies confirms that indium in the glass network has a coordination number lower than six and exhibits high covalent character, supporting findings from vibrational and NMR spectroscopy, which show that the addition of both indium and gallium reduces boron tetrahedral units. TEM mapping reveals preferential depletion of indium and aluminum from silicon-rich regions, consistent with nanoscale phase separation. Glass transition and packing density measurements further support the complex structural role and bonding characteristics of indium in these glasses. We propose that a high aluminum content (>10 mol%) stabilizes partially 4-coordinated indium and promotes local agglomeration of indium oxide, reducing the activation energy required for the crystallization even at indium oxide concentrations as low as 2 mol%.