Phase Transformations in Bulk Nanostructured Potassium Niobiosilicate Glasses

In potassium niobiosilicate (KNS) glasses, nanostructuring can be driven and controlled by thermal treatments at the glass transition temp. and/or by modulation of the chem. compn. The tight relationship between nanostructure and nonlinear optical properties suggests these bulk nanomaterials as an appealing route to nanophotonics. The focus of this paper is placed on assessing the phase transformations which occur in these materials upon annealing at the glass transition temp. and subsequent heating. High-temp. resolved X-ray diffraction (HTXRD) and high-resoln. TEM (HRTEM) expts. are integrated with previously published results for in-depth insight. It will be shown that nanostructuring evolves from nucleation of niobium-rich nanocrystals, which are up to 20 nm large, uniformly distributed in the matrix bulk, and metastable. Formation kinetics as well as phase transformation of the nanocrystals are detd. by the glass compn. Depending on it, nanocrystal nucleation can be preceded or not by phase sepn., and the nanocrystals' phase transition can be of first or second order.