Structural and Spectroscopic Effects of Li(+) Substitution for Na(+) in Li(x)Na(1-x)CaGd(0.5)Ho(0.05)Yb(0.45)(MoO(4))(3) Scheelite-Type Upconversion Phosphors

A set of new triple molybdates, Li(x)Na(1-x)CaGd(0.5)(MoO(4))(3):Ho(3+)(0.05)/Yb(3+)(0.45), was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors Li(x)Na(1-x)CaGd(0.5)(MoO(4))(3):Ho(3+)(0.05)/Yb(3+)(0.45) with variation of the Li(x)Na(1-x) (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho(3+) = 0.05 and Yb(3+) = 0.45 were obtained, and the effect of Li(+) on their spectroscopic features was investigated. The crystal structures of Li(x)Na(1-x)CaGd(0.5)(MoO(4))(3):Ho(3+)(0.05)/Yb(3+)(0.45) (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I4(1)/a by Rietveld analysis. Pure NaCaGd(0.5)Ho(0.05)Yb(0.45)(MoO(4))(3) has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) Å(3), Z = 4. In Li-doped samples, big cation sites are occupied by a mixture of (Li,Na,Gd,Ho,Yb) ions, and this provides a linear cell volume decrease with increasing Li doping level. The evaluated upconversion (UC) behavior and Raman spectroscopic results of the phosphors are discussed in detail. Under excitation at 980 nm, the phosphors provide yellow color emission based on the (5)S(2)/(5)F(4) → (5)I(8) green emission and the (5)F(5) → (5)I(8) red emission. The incorporated Li(+) ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho(3+) and Yb(3+) ions, and they further affected the UC transition probabilities in triple molybdates Li(x)Na(1-x)CaGd(0.5)(MoO(4))(3):Ho(3+)(0.05)/Yb(3+)(0.45). The complex UC intensity dependence on the Li content is explained by the specificity of unit cell distortion in a disordered large ion system within the scheelite crystal structure. The Raman spectra of Li(x)Na(1-x)CaGd(0.5)(MoO(4))(3) doped with Ho(3+) and Yb(3+) ions were totally superimposed with the luminescence signal of Ho(3+) ions in the range of Mo–O stretching vibrations, and increasing the Li(+) content resulted in a change in the Ho(3+) multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO(4))(3):Ho(3+),Yb(3+) phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.