Structural and spectroscopic effects of li+ substitution for na+ in lix na1-x cagd0.5 ho0.05 yb0.45 (moo4 )3 scheelite-type upconversion phosphors

Abstract

A set of new triple molybdates, Lix Na1-x CaGd0.5 (MoO4 )3:Ho3+ 0.05/Yb3+ 0.45, was success-fully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors Lix Na1-x CaGd0.5 (MoO4 )3:Ho3+ 0.05/Yb3+ 0.45 with variation of the Lix Na1-x (x = 0, 0.05, 0.1, 0.2, 0.3) ratio under constant doping amounts of Ho3+ = 0.05 and Yb3+ = 0.45 were obtained, and the effect of Li+ on their spectroscopic features was investigated. The crystal structures of Lix Na1-x CaGd0.5 (MoO4 )3:Ho3+ 0.05/Yb3+ 0.45 (x = 0, 0.05, 0.1, 0.2, 0.3) at room temperature were determined in space group I41 /a by Rietveld analysis. Pure NaCaGd0.5 Ho0.05 Yb0.45 (MoO4 )3 has a scheelite-type structure with cell parameters a = 5.2077 (2) and c = 11.3657 (5) Å, V = 308.24 (3) Å3, 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 the5 S2 /5 F4 →5 I8 green emission and the5 F5 →5 I8 red emission. The incorporated Li+ ions gave rise to local symmetry distortion (LSD) around the cations in the substituted crystalline structure by the Ho3+ and Yb3+ ions, and they further affected the UC transition probabilities in triple molyb-dates Lix Na1-x CaGd0.5 (MoO4 )3:Ho3+ 0.05/Yb3+ 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 Lix Na1-x CaGd0.5 (MoO4 )3 doped with Ho3+ and Yb3+ ions were totally superimposed with the luminescence signal of Ho3+ ions in the range of Mo–O stretching vibrations, and increasing the Li+ content resulted in a change in the Ho3+ multiplet intensity. The individual chromaticity points (ICP) for the LiNaCaGd(MoO4 )3:Ho3+,Yb3+ phosphors correspond to the equal-energy point in the standard CIE (Commission Internationale de L’Eclairage) coordinates.