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 La 3+ and tungstate ions. As an outcome of our 40 well-designed experiments, four La tungstates—La 2 W 3 O 12, La 2 W 2 O 9, La 14 W 8 O 45, and La 6 W 2 O 15 —were successfully obtained in a phase-pure form by calcining their hydrothermal precursors. Phase and morphology evolution, structure features, and properties of Eu 3+ emission were, for the first time, comparatively investigated for the four compounds. Spectral analysis found that the 5 at. % Eu 3+ -doped La 2 W 3 O 12 phosphor exhibits the highest quantum efficiency (∼47%), more red component, and the shortest fluorescence lifetime of luminescence (∼0.72 ms).