Mild hydrothermal preparations using nitrate and chloride reagents as Gd~(3+) and Yb~(3+) sources lead to the synthesis of NaGd_(1-x)Yb_x(WO4)2 (0.001 < × < 0.5) particles with tetragonal scheelite-like structure phase. Nearly neutral pH ~7.5 conditions ensure the stability of this crystalline phase over a wide range of reaction times. Synthetical routes with both kind of reagents yield basically the same particle morphology sequences, although using Gd(Yb)-chlorides the presence of hanorods is more evident, whereas using Gd(Yb)-nitrates the faster growth rate favors well-defined micrometer-sized octahedral particles. The spectroscopic properties of Yb~(3+) in NaGd_(1-x)Yb_x(WO4)2 synthesized micrometer-sized octahedra are equivalent to those obtained in bulk single crystals, showing a single exponential photoluminescence decay and ~2F_(5/2) lifetime r≈ 330 μs for 0.001 ≤ × ≤ 0.005 doped samples. The nanoparticles and nanorods formed with Gd(Yb)-nitrates and pH < 7 by short time annealing as well as in Gd(Yb)-chloride preparations show a strong reduction of the Yb~(3+) lifetime with regards to that observed in micrometer-sized octahedra or in single crystals, leading to nonexpohential fluorescence decays when both nano- and micrometer-sized particles coexist. The use of raw Yb2O3 with increasing purity level in the hydrothermal process yields materials that show an increase in Yb~(3+) lifetime toward its radiative value.
展开▼
