Comparison of up-converted emissions in Yb³⁺, Er³⁺ co-doped Gd₂(WO₄)₃ and Gd₂WO ₆ phosphors

Yb³⁺, Er³⁺ co-doped Gd₂(WO ₄)₃ and Gd₂WO₆ phosphors have been prepared using co-precipitation method and characterized by X-ray diffraction and optical and infrared spectroscopy. Different processes of upconverted transitions have been observed in the two hosts. Strong green and red emissions have been respectively detected in Gd₂(WO₄) ₃:Yb³⁺, Er³⁺ and Gd₂WO ₆:Yb³⁺, Er³⁺ upon excitation at 980 nm, suggesting that the excited state absorptions from ⁴I_11/2 and ⁴I_13/2 of Er³⁺ dominate the excitation processes in the two systems, respectively. The different excitation processes of the up-conversion are related to structural difference. The existence of near-neighbor rare earth ion-pair in Gd₂WO₆ may promote the depopulation of ⁴F_7/2 through a cross relaxation of ⁴F_7/2→⁴F_9/2 and ⁴I_11/2→⁴F_9/2, yielding the dominating red emission in Gd₂WO₆. On the other hand, intense near infrared emissions have been observed from two Stark splitting groups of ⁴I_13/2 to ⁴I_15/2 at 1540 nm and 1490 nm, respectively, in both hosts. However, the infrared emission is dominated by the lower state at 1540 nm in Gd₂WO₆, resulting in an efficient ESA, due to the better energy difference matching, from the lower state to ⁴F_9/2 that gives red emission. Concentration dependence of the intensity ratio of green to red emissions is also studied and energy transfer between Yb³⁺ and Er³⁺ ions analyzed.