Nano-heterogeneous structure of (1-x)KNbO3–xSiO2 glasses in the low glass-forming oxide content range 0.05 ≤ x ≤ 0.3

Glasses with a high content of niobium oxide are of significant interest for electro-optics and nonlinear optics. In the present paper we report the results of the investigation of the submicroscopic structure and nonlinear optical properties of (1-x)KNbO3–xSiO2 (KNS) glasses (x = 0.05–0.30) by XRD, SANS, electron microscopy and second harmonic generation (SHG) technique. Vitreous samples were fabricated by rapid melt cooling, via pressing the melt by steel plates, quenching between rotating metal rolls or splat cooling in air or nitrogen flow. Glasses with x < 0.15 are shown to possess a micro-inhomogeneous structure with regions enriched by SiO2. On the contrary, as-quenched glasses with x > 0.15 are found by SANS to be homogeneous, but form nanostructures enriched by SiO2 after heat-treatment. At temperatures below ~(Tg + 50 °C), SiO2-enriched regions grow slightly, whereas their chemical composition shifts considerably closer to SiO2. The data on the nano-inhomogeneous structure enables clarifying the complicated Tg(x) dependence of KNS glasses. SHG-active KNbO3 phase precipitates at later stages of crystallization when the glass starts to lose its transparency, and crystallization of perovskite-like KNbO3 is accompanied by the enhancement of SHG efficiency by several orders of magnitude.