Generalized synthesis of NaLn(MoO4)2 nano/microcrystals (Ln = La–Lu and Y): The effects of lanthanide contraction, structure, and down-/up-conversion luminescence

Abstract

Systematic synthesis of scheelite-type double molybdate nano-/microcrystals NaLn (MoO4)2 (Ln = La–Lu lanthanides and Y) was successfully carried out via hydrothermal reaction at 180 °C without using any organic additive, and the intrinsic effects of lanthanide contraction on phase preference and crystallite morphology were unambiguously observed. It was shown that the products of larger Ln3+ (Ln = La–Dy) and smaller Ln3+ (Ln = Ho–Lu and Y) elements crystallized as tetragonal NaLn (MoO4)2 and new orthorhombic NaLnMo2O8·2H2O structures, respectively, with the latter being able to dehydrate to tetragonal NaLn (MoO4)2 upon calcination at ∼300 °C. Y was found to be a demarcation point for the two structures, and phase preference was shown to be influenced by solution pH and MoO42−/Y3+ molar ratio. With Eu3+ as a downconversion (DC) luminescence probe, the effects of Ln type (Ln = La, Gd, Y and Lu) and crystal structure were manifested. Furthermore, the upconversion (UC) luminescence of Yb3+/Ho3+ and Yb3+/Er3+ pairs in NaLu(MoO4)2 was studied for the first time, and strong red and green UC emissions were observed under the 978 nm laser excitation. Besides, the processes/mechanisms of UC were studied via varying the excitation power and were discussed with Yb3+-MoO42- dimer sensitizer.