Alkali-Niobate aluminosilicate glasses: Structure and properties

Abstract

K0.5Na0.5NbO3 (KNN) based ceramics are considered optimal candidates to replace the toxic Pb-based piezoelectric materials; nonetheless, they are quite difficult to process. Therefore, obtaining an efficient glass-ceramic (GC) will have several advantages in lead-free piezoelectric production. The first step to developing advantageous functional GCs is to determine an adequate glass matrix and the network connectivity of the different structural entities depending on the bulk chemistry. Here, we investigated the structural variations occurring in aluminosilicate systems having Na2O/Al2O3 molar ratios from 1 to 5.7 upon the addition of KNN (0.4 up to 30 mol%) to understand how the bulk composition influences KNN incorporation and the properties of the glasses.Our study shows that more depolymerized aluminosilicate systems incorporate a larger amount of KNN because the Nb structural environment's evolution strongly depends on glass polymerization and the availability of charge compensator ions. No strong crosslinking between [SiO4], [AlO4], and [NbO6] structural entities was found, and there is an evolution from a heterogeneous environment with isolated [NbO6] units having different degrees of distortion to Nb–O–Nb linkages, up to the formation of clusters composed of corner-shared slightly distorted [NbO6]. All glass series tend to move toward these 3D clusters whenever the starting structure includes different entities or a more homogeneous environment. The evolution of the different niobate entities marks the changes in the properties that show non-linear variations with KNN content. Furthermore, the glass chemistry influences the crystallization kinetics, with activation energies for crystallization that vary from ∼300 ± 10 kJ/mol to ∼220 ± 10 kJ/mol depending on the alkali/Al2O3 molar ratios.The developed niobo-aluminosilicate glasses show high refractive indices, high transparency in the visible– NIR region and intense UV absorption. Therefore, these glasses are excellent candidates for photonic applications and optoelectronic devices. The stabilization of a perovskite-like phase upon crystallization confirms the potentiality for use as electro-optical glass-ceramics.