Structural characterization of rare-earth doped soda magnesia alumina silica glasses for holographic storage: Brillouin, Raman and NMR spectroscopy studies

Scope and method of study. This work was directed to perform (Brillouin and Raman) light scattering and (29Si, 27Al, 23Na) MAS NMR spectroscopies to study structural, vibrational, elastic, photoelastic and magnetic properties of Eu3+ -doped soda magnesia alumina silica (EDSMAS) glasses as a function of content of individual components: Eu₂O₃, Al ₂O₃ and Na₂O to provide new knowledge related to the mechanism underlying the formation of holographic gratings in these materials.; Findings and conclusions. Europium doping and alumina substitution for silica causes glass to become harder but at the same time more polarizable. In both glass series the enhanced hardness is due to reduction of the free space. In addition, in the former case the europiums have high coordination and enhanced field strength with oxygen atoms. In the latter one, the aluminums entering the glass recover Si-O-Si bridges by attracting charge balancing modifiers from depolymerized SiO₄ network. Growing polarizability in Eu- and Al-series is mostly due to depolymerization by Eu 3+ doping and the appearance of AlO₄ tetrahedral groups weakly bound to sodium ions, respectively. The Eu3+ ions form the broad distribution of Qn species with possible clustering of rare-earths at large Eu3+ ion concentration, while sodium ions entering the glass cause the formation of rather distinct Qn species. In Na-series the glass becomes more depolymerized although less compressible in the volumetric sense . In this series Na environment is sensitive to the first neighbor Na+ network-modifiers. The overall bonding in all three EDSMAS glass series becomes more polarizable compared to fused silica. The depolymerization (polymerization) of the EDSMAS glass is mainly characterized by the presence of NBOs (BOs) and three (four) member silicate and aluminosilicate rings. Si and Al atoms were found to be in tetrahedrally coordinated sites. No Al was observed in 6- or 5-fold coordinated sites. Lorentz-Lorentz effect was the dominant contribution to the photoelastic constant P ₁₂. The strength of the holographic storage signal is defined by many factors among which the number of the excited Eu3+ ions, the high-energy phonons associated with Si-NBO vibrations, weakly bound Na+ ions charge balancing [AlO₄]− complexes and the stability of the traps for the mobile light ions play very important roles.