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600 oC. The hydrothermal crystallization of NaLa(WO4)2 experienced the two stages of amorphous mass and crystalline NaLaW2O7(OH)2(H2O), and the latter can be transformed to phase-pure NaLa(WO4)2 by calcination at

350 oC. Upon being excited with their most effective excitation wavelengths, NaEu(WO4)2, NaTb(WO4)2 and NaDy(WO4)2 exhibited red (616 nm), green (546 nm) and yellow (576 nm) emissions, with fluorescence lifetimes of ~0.91 ms, 0.88 ms and 5.42 μs, respectively. The morphology of NaRE(WO4)2 can be regulated with tartrate ions via chelation and surface adsorption, and decreasing RE3+ size induced weaker chelation and stronger surface adsorption, which facilitated the orientated crystallite growth. (3) Hydrothermally reacting Na2WO4 and Lu(NO3)3 and La(NO3)3 produced new types of tungstates, which were analyzed via TG/DTA, FTIR, ICP and the Rietveld technique to be NaLuW2O8?2H2O and NaLaW2O7(OH)2?H2O, respectively. NaLuW2O8?2H2O was found to crystallize in the orthorhombic system (space group: Cmmm), with the crystal structure built up via alternative stacking of the ...-(NaO6)-(NaO6)-... and ...-(LuO4(H2O)2WO5)- (LuO4(H2O)2WO5)-... layers. NaLaW2O7(OH)2?H2O crystallized in the triclinic system (space group: P-1), and the crystal structure presented 2D layers of ―4― alternating ...-(W1O6)-(W1O6)-(W2O6)-(W2O6)-(W1O6)-(W1O6)-... and ...-(LaO9)-(LaO9)-... chains in the ab plane, which were linked together through NaO6 octahedron by edges to form 3D net along the c axis. NaLaW2O7(OH)2?H2O exhibited a higher temperature of thermal decomposition than NaLuW2O8?2H2O due to the presence of hydroxyls. The two compounds can be converted to phase-pure NaRE(WO4)2 via calcination. (4) The bandgap of NaRE(WO4)2 (RE=La-Lu and Y) was found to be related with the type of RE, and compounds of smaller RE3+ (RE=Ho-Lu) show similar and larger values. NaGd(WO4)2, NaY(WO4)2 and NaLu(WO4)2 exhibited similar intrinsic blue emission of tungstate ligands under UV excitation. Taking the advantage that the emission band of tungstate ligand partly overlaps with the excitation band of Eu3+, the emission color of Na(Y1-xEux)(WO4)2 (x=0.001-0.01) was successfully modulated from blue to red under UV excitation, and energy process of which was discussed in detail. Additionally, white emission was achieved when x=0.4. Tetragonal structured NaLu(WO4)2:Yb/Ln (Ln=Ho, Er and Tm) up-/down-conversion phosphors were produced by calcining their orthorhombic NaLuW2O8?2H2O:Yb/Ln precursors at

600 oC. Under

978 nm infrared laser pumping, it was found that Yb3+ can effectively sensitize........