Hydrothermal Yb³⁺-Doped NaGd(WO₄)₂ Nano- and Micrometer-Sized Crystals with Preserved Photoluminescence Properties

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 (WO 4 ) 2 (0.001≤ x ≤ 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 nanorods 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 (WO 4 ) 2 synthesized micrometer-sized octahedra are equivalent to those obtained in bulk single crystals, showing a single exponential photoluminescence decay and 2 F 5/2 lifetime τ ≈ 330 μs for 0.001 ≤ x ≤ 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 nonexponential fluorescence decays when both nano- and micrometer-sized particles coexist. The use of raw Yb 2 O 3 with increasing purity level in the hydrothermal process yields materials that show an increase in Yb 3+ lifetime toward its radiative value.