Early-stage clustering in Tb-doped borosilicate glasses revealed by high-resolution scanning transmission electron microscopy and luminescence properties

Abstract

Glasses with the base composition 60 SiO2·37 B2O3·3 Na2O (in mol%) were doped with different concentrations of terbium oxide. These glasses show phase separation in borate-rich droplets surrounded by a silicate matrix. By means of scanning transmission electron microscopic analyses including energy dispersive X-ray spectroscopy, structural changes during phase separation are investigated, focusing mainly on the local sites of the luminescent ions. Moreover, phase separation structures of as cast and heat-treated glasses are characterized regarding droplet shape and size, as well as size distribution. The luminescence properties of the phase separated glasses are correlated with their nano- and microstructure. Already in the glass with the lowest Tb3+ concentration, the local environments in which the Tb3+ ions reside in could be identified. This early clustering of the luminescent ions remains undetected in fluorescence lifetime measurements. Only in the time-resolved fluorescence measurement of the glass with the highest Tb3+ concentration, which contains very large Tb-rich agglomerates, an indication of clustering is found. Since clustering of rare earth ions can be detrimental to the performance of an optical material, early detection is beneficial. The studies show that high-resolution transmission electron microscopy can assist with this concern.