Comparison of up-converted emissions in Yb3+, Er3+ co-doped Gd2(WO4)3 and Gd2WO6 phosphors

Yb3+, Er3+ co-doped Gd2(WO4)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 Gd2WO6:Yb3+, Er3+ upon excitation at 980 nm, suggesting that the excited state absorptions from 4I11/2 and 4I13/2 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 4F7/2 through a cross relaxation of 4F7/2→4F9/2 and 4I11/2→4F9/2, yielding the dominating red emission in Gd2WO6. On the other hand, intense near infrared emissions have been observed from two Stark splitting groups of 4I13/2 to 4I15/2 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 4F9/2 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.