Evolution of Nanocrystalline Ba2NaNb5O15 in 2BaO–0.5Na2O–2.5Nb2O5–4.5B2O3 Glass System and Its Refractive Index

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• 作者：Swarup Kundu ; K. B. R. Varma
• 刊名：Crystal Growth & Design
• 出版年：2014
• 出版时间：February 5, 2014
• 年：2014
• 卷：14
• 期：2
• 页码：585-592
• 全文大小：566K
• ISSN：1528-7505

文摘

Monophasic Ba₂NaNb₅O₁₅ was crystallized at nanometer scale (12鈥?6 nm) in 2BaO鈥?.5Na₂O鈥?.5Nb₂O₅鈥?.5B₂O₃ glass system. To begin with, optically transparent glasses, in this system, were fabricated via the conventional melt-quenching technique. The amorphous and glassy characteristics of the as-quenched samples were respectively confirmed by X-ray powder diffraction and differential thermal analyses. Nearly homogeneous distribution of Ba₂NaNb₅O₁₅ (BNN) nanocrystals associated with tungsten bronze structure akin to their bulk parent structure was accomplished by subjecting the as-fabricated glasses to appropriate heat-treatment temperatures. Indeed transmission electron microscopy (TEM) carried out on these samples corroborated the presence of Ba₂NaNb₅O₁₅ nanocrystals dispersed in a continuous glass matrix. The as-quenched glasses were 75% transparent in the visible range of the electromagnetic spectrum. The optical band gap and refractive index were found to have crystallite size (at nanoscale) dependence. The optical band gap increased with the decrease in crystallite size. The refractive indices of the glass nanocrystal composites as determined by Brewster angle method were rationalized using different empirical models. The refractive index dispersion with wavelength of light was analyzed on the basis of the Sellmeier relations. At room temperature under UV excitation (355 nm) these glass nanocrystal composites displayed violet-blue emission which was ascribed to the defects states.