Selective Crystallization of Four Tungstates (La2W3O12, La2W2O9, La14W8O45, and La6W2O15) via Hydrothermal Reaction and Comparative Study of Eu3+ Luminescence.

Hydrothermal reaction at 200 °C was systematically undertaken in wide ranges of solution pH (4-13) and W/La molar ratio ( R = 0.5-2), without using any organic additive, to investigate the effect of hydrothermal parameter on product property and the underlying mechanism. Combined analysis by X-ray diffraction (XRD), inductively coupled plasma (ICP) spectroscopy, elemental mapping, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that either a decreasing pH or increasing R value yielded a product richer in W and, conversely, richer in La. The results were interpreted from the solution chemistry of La3+ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstates-La2W3O12, La2W2O9, La14W8O45, and La6W2O15-were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu3+ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at. % Eu3+-doped La2W3O12 phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).