The effect of glass structure and local rare earth site symmetry on the optical properties of rare earth doped alkaline earth aluminosilicate glasses

Understanding the connection of molecular structure and optical properties of rare earth doped luminescent materials is essential for fabrication of state-of-the-art active laser media. On the other hand, rare earth ions can be used as a probe ion for the molecular structure of the host material if the structure-property correlations are known. Therefore, this work combines molecular dynamics simulations, Judd-Ofelt theory and UV–vis-NIR absorption spectroscopy including the behavior of the structure-sensitive hypersensitive absorption transitions of Er ³⁺ to expand the knowledge on the local molecular structure in the immediate vicinity of the doped rare earth ions in dependence of glass composition. For this purpose, glasses of the compositions (35-x) BaO · x MgO · 10 Al ₂O ₃ · 55 SiO ₂ (mol%) ( x = 0, 7.5, 15, 25, 35) and (20-x) BaO · x MgO · 20 Al ₂O ₃ · 60 SiO ₂ (mol%) ( x = 0, 10, 20), doped with 2 × 10 ²⁰ ions/cm ³ Er ³⁺ were prepared and analyzed. Clear differences in the absorption spectra between glasses of different BaO/MgO ratios, i.e. different network modifier field strengths, and different network modifier oxide to Al ₂O ₃ ratios are found and discussed in detail. Glasses with high BaO concentrations and high network modifier oxide to Al ₂O ₃ ratios provide lower rare earth coordination numbers with oxygen in general but higher coordination probabilities with non-bridging oxygen, which results in notably increased splitting of the optical transitions of the doped rare earth ions and higher hypersensitivity / lower local site symmetry for the doped rare earth ions in the investigated compositions. Based on our results and results from other publications the local rare earth site symmetry in glasses can in general be correlated with the rare earth coordination number.