Effect of alkali species on the crystallization behavior of alkali niobate silicate glass

Alkali niobate silicate glass-ceramics are of interest because they enable the precipitation of optically functional niobate-related phases while maintaining a nanoscale micro-structure. However, the role of alkali species in controlling phase selection and crystallization behavior has not been systematically clarified across the full Li-to-Cs series. In this work, glasses of 25M 2 O-17Nb 2 O 5 -58SiO 2 (M = Li, Na, K, Rb, and Cs) were prepared by melt-quenching and subsequently heat-treated isothermally to obtain the corresponding glass ceramics. Alkali substitution was found to affect not only structure, thermal, mechanical and electrical properties of the precursor glass, but also their crystallization behavior. With decreasing alkali field strength, crystallization evolves from the surface-dominated formation of binary alkali niobate phases in Li- and Na-containing glasses toward silicate-containing niobate phases and increasingly constrained volume crystallization in the Rb- and Cs-containing glasses, with K representing an intermediate case. Meanwhile, crystal growth changes from pronounced, temperature-dependent coarsening in Li/Na glasses to sub-10 nm, weakly temperature-dependent nanocrystallization in Rb/Cs glasses. These trends are accompanied by systematic changes in the local environments and medium-range ordering of [NbO 6 ]-based units in the precursor glasses. They suggest that alkali substitution could be an effective way for chemically controlling crystallization pathways in niobate-silicate glass-ceramics.