Structural evolution and second harmonic properties of lithium niobate-tantalate nanocrystals embedded in a borate glass

Nano-crystals of LiNbxTa1 − xO3 were evolved by subjecting melt-quenched 1.5Li2O-2B2O3-xNb2O5-(1 − x)Ta2O5 glasses (where x = 0, 0.25, 0.5, 0.75 and 1.00) to a controlled 3-h isothermal heat treatment between 530 and 560 °C. Detailed X-ray diffraction and Raman spectral studies confirmed the formation of nano-crystalline LiNbxTa1 − xO3 along with a minor phase of ferroelectric and non-linear optic Li2B4O7. The sizes of the nanocrystals evolved in the glass were in the range of 19-37 nm for x = 0-0.75 and 23-45 nm for x = 1.00. Electron microscopic studies confirmed a transformation of the morphology of the nano-crystallites from dendritic star-shaped spherulites for x = 0 to rod-shaped structures for x = 1.00 brought about by a coalescence of crystallites. Broad Maker-fringe patterns (recorded at 532 nm) were obtained by subjecting the heat-treated glass plates to 1064 nm fundamental radiation. However, an effective second order non-linear optic coefficient, deff, of 0.45 pm/V, which is nearly 1.2 times the d36 of KDP single crystal, was obtained for a 560 °C/3 h heat-treated glass of the representative composition x = 0.50 comprising 37 nm sized crystallites.