Structural and Spectroscopic Effects of Li+ Substitution for Na+ in LixNa1-xCaGd0.5Ho0.05Yb0.45(MoO4)3 Scheelite-Type Upconversion Phosphors

Abstract

A set of new triple molybdates, LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45, was successfully manufactured by the microwave-accompanied sol–gel-based process (MAS). Yellow molybdate phosphors LixNa1-xCaGd0.5(MoO4)3:Ho3+0.05/Yb3+0.45 with variation of the LixNa1-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 LixNa1-xCaGd0.5(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 NaCaGd0.5Ho0.05Yb0.45(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 the 5S2/5F4 → 5I8 green emission and the 5F5 → 5I8 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 molybdates LixNa1-xCaGd0.5(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 LixNa1-xCaGd0.5(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.

Upconversion Properties and Temperature-Sensing Behaviors of Alkaline-Earth-Metal Scandate Nanocrystals Doped with Er3+/Yb3+ Ions in the Presence of Alkali Ions (Li+, Na+, and K+)

“…Both CaSc 2 O 4 :Er 3+ /Yb 3+ and SrSc 2 O 4 :Er 3+ /Yb 3+ samples, in the presence of Li + ions, emit dazzling yellow light like the noonday sun, and the UCL in the SrSc 2 O 4 :Er 3+ /Yb 3+ sample is stronger than that in the CaSc 2 O 4 :Er 3+ /Yb 3+ sample. As reported previously, due to the small ionic radii of Li + ions, it can enter into the crystal lattice easily, decreasing the symmetry of the crystal field and increasing crystallinity, compared with Na + and K + ions. , So we can see that Li + ions are conducive not only to crystallinity but also to enhancement of UCL.…”

“…Alkali ions are well known to greatly modify the local crystal field of the host and lead to luminescence enhancement. The effect of alkali ions (Li + , Na + , K + ) on the optical properties of lanthanide-doped oxide materials has been studied by various groups. ,, Thus, codoping with alkali ions should be an effective way to further enhance UCL.…”

Up-Converting K2Gd(PO4)(WO4):20%Yb3+,Ho3+ Phosphors for Temperature Sensing

“…The up-converting luminescent materials are widely used in a variety of applications, such as solar energy [1,2], anti-counterfeiting pigments [3], temperature sensors [4], fluorescence probes [5], bio-imaging [6], cancer therapeutics [7], etc. Usually, the inorganic up-converting luminescent materials contain at least two incorporated lanthanide ions: one as a sensitizer, typically Yb 3+ , and another as an emitter, such as Er 3+ [8,9], Ho 3+ [10][11][12][13], Tm 3+ [14][15][16], etc. Another critical step in preparing up-converting phosphors is the selection of an appropriate host matrix.…”

Exploration of the Crystal Structure and Thermal and Spectroscopic Properties of Monoclinic Praseodymium Sulfate Pr2(SO4)3

“…Rare earth (Ln) containing crystals exhibit exceptional material properties with wideranging technological significance [1][2][3][4]. The materials are widely used in solid-state laser devices, nonlinear optics and electronic and photonic systems because of their unique electron level configuration and specific chemical properties [5][6][7][8][9][10][11][12][13][14][15][16][17]. As to the crystal chemistry of Ln-containing compounds, it is based on the existence of the element range from La to Lu with a continuous variation of effective radius of Ln 3+ ions that, in many cases, governs the boundaries of particular structure types [18].…”