KLn(MoO4)2 micro/nanocrystals (Ln = La-Lu, Y): systematic hydrothermal crystallization, structure, and the performance of doped Eu3+ for optical thermometry

Evidence and Practical Applications of Site Occupancy Theory (SOT) of Eu3+ in Scheelite Compounds

The determination of the site occupancy of activators in phosphors is essential for precise synthesis, understanding the relationship between their luminescence properties and crystal structure, and tailoring their properties by modifying the host composition. Herein, one simple method was proposed to help determine the sites at which the doping of rare earth ions or transition metal ions occupies in the host lattice through site occupancy theory (SOT) for ions doped into the matrix lattice. SOT was established based on the fact that doping ions preferentially occupy the sites with the lowest bonding energy deviations. In order to provide detailed experimental evidence to prove the feasibility of SOT, several scheelite-type compounds were successfully synthesized using a high-temperature solid-phase method. When Eu3+ ions occupy a similar surrounding environment site, the photoluminescence spectra of the activators Eu3+ are similar. Therefore, by comparing the intensity ratio of photoluminescence spectra and the mechanism of all transitions of KEu(WO4)2, KY(WO4)2:Eu3+, Na5Eu(WO4)4, and Na5Y(WO4)4:Eu3+, it was proved that SOT can successfully confirm the site occupation when doped ions enter the matrix lattice. SOT was further applied to the sites occupied by Eu3+ ion-doped LiAl(MoO4)2 and LiLu(MoO4)2.

Multisignal optical temperature-sensing properties of Eu3+ -doped NaYF4 nanoparticles

In this paper, the multisignal (different emissions/colours) temperature sensing of NaYF4 :Eu3+ nanoparticles is investigated, which is based on fluorescence intensity ratios (FIRs) between 5 D0 →7 FJ (J=1-4) and 5 D1 →7 FJ' emissions. The 5 D1 →7 FJ' (J'=0-2) emissions can be clearly observed due to the low photon energy of NaYF4 . Based on the FIRs between different 5 D0 →7 FJ and 5 D1 →7 FJ' emissions, higher absolute/relative temperature sensitivities are obtained. Compared with the FIR between whole 5 D0 →7 FJ and 5 D1 →7 FJ' emissions, the maximum value of Sa was improved from 0.27 K-1 to 5.02 K-1 and that of Sr was improved from 0.89%·K-1 to 1.27%·K-1 . Furthermore, the FIRs between different colours of emissions were investigated for the application of wide-range multicolour temperature sensing.

Systematic Crystallization of NH4Ln(MoO4)2 as a Family of Layered Compounds (Ln = La-Lu and Y), Derivation of Ln2Mo4O15, Crystal Structure, and Photoluminescence

NH₄Ln(MoO₄)₂ (Ln = La-Lu lanthanide, Y) was crystallized via hydrothermal reaction as a new family of layered materials, from which phase-pure Ln₂Mo₄O₁₅ was successfully derived via subsequent annealing at 700 °C for the series of Ln elements excluding Ce and Lu. Detailed structure analysis revealed that the ionic size of Ln³⁺ decisively determined the crystal structure and Mo/Ln coordination for the two families of compounds. NH₄Ln(MoO₄)₂ was analyzed to be orthorhombic (Pbcn space group, no. 60) and monoclinic (P2/c, no. 13) for the larger and smaller Ln³⁺ of Ln = La-Gd and Ln = Tb-Lu (including Y), respectively, where both the crystal structures have a layered topology featured by the alternative stacking of a [Ln(MoO₄)₂]^- three-tier infinite anionic layer and interlayer NH₄⁺. Four types of crystal structures were found for the Ln₂Mo₄O₁₅ series, which are monoclinic (P2₁/a, no. 14) for Ln = La, triclinic (P1̅, no. 2) for Ln = Pr-Sm, triclinic (P1̅, no. 2) for Ln = Eu and Gd, and monoclinic (P2₁/c, no. 14) for Ln = Tb-Yb (including Y). The photoluminescence of NH₄Ln(MoO₄)₂ (Ln = Eu, Tb) and Ln₂Mo₄O₁₅:Eu³⁺ (Ln = La, Gd, Y) was thoroughly investigated in terms of spectral features, quantum efficiency, fluorescence decay, and CIE chromaticity. The thermal stability of luminescence was also studied for Ln₂Mo₄O₁₅:Eu³⁺, and the observed charge-transfer excitation components were successfully correlated with the features of the Mo-O polyhedron/unit.

Extensive tailoring of REPO4 and REVO4 crystallites via solution processing and luminescence

This article highlighted the recent achievements in crystal engineering of REPO4 and REVO4via solution processing, with an emphasis on solution chemistry, the role of chelate ion, crystallization mechanism and luminescence properties.