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.
Highlights
Complex molybdate crystals have become a subject of extensive investigation due to their diverse crystal chemistry, high chemical stability, and specific physical properties, which are promising for applications in such fields as electronics, laser systems, electrochemistry, and electro-photonics [1,2,3,4,5,6,7,8,9]
The crystal structure of CaMoO4 was taken as a starting model for Rietveld refinement
The Ca2+ ion site was considered as the one occupied by a mixture of Li+, Ca2+, Na+, Gd3+, Ho3+, and Yb3+ ions (Figure 2) with fixed partial occupations according to the nominal sample composition
Summary
Complex molybdate crystals have become a subject of extensive investigation due to their diverse crystal chemistry, high chemical stability, and specific physical properties, which are promising for applications in such fields as electronics, laser systems, electrochemistry, and electro-photonics [1,2,3,4,5,6,7,8,9]. Many molybdate crystals are appropriate for the incorporation of rare earth (Ln) ions in their structure, and the materials are considered as potential hosts for the creation of phosphors to be used in photonic structures [2,3,4,7,17,18,19,20,21,22,23] Among such crystals, scheelite-type (ST) molybdates are interesting in terms of the search for new structures, including structure-modulation effects, and promising spectroscopic characteristics [2,21,24,25,26,27,28,29,30]. The spectroscopic characteristics were investigated under the consideration of efficient UC emissions, individual chromaticity points (ICP) according to Commission Internationale de L’Eclairage (CIE), and Raman scattering
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
